LinuxLists.cc - [v8 0/4] cgroup-aware OOM killer

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Subject: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware OOM killer

Add a "groupoom" cgroup v2 mount option to enable the cgroup-aware
OOM killer. If not set, the OOM selection is performed in
a "traditional" per-process way.

The behavior can be changed dynamically by remounting the cgroupfs.

Signed-off-by: Roman Gushchin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
include/linux/cgroup-defs.h | 5 +++++
kernel/cgroup/cgroup.c | 10 ++++++++++
mm/memcontrol.c | 3 +++
3 files changed, 18 insertions(+)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index ade4a78a54c2..db4ff3e233a9 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -79,6 +79,11 @@ enum {
* Enable cpuset controller in v1 cgroup to use v2 behavior.
*/
CGRP_ROOT_CPUSET_V2_MODE = (1 << 4),
+
+ /*
+ * Enable cgroup-aware OOM killer.
+ */
+ CGRP_GROUP_OOM = (1 << 5),
};

/* cftype->flags */
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d6551cd45238..5f8a97a233bb 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1699,6 +1699,9 @@ static int parse_cgroup_root_flags(char *data, unsigned int *root_flags)
if (!strcmp(token, "nsdelegate")) {
*root_flags |= CGRP_ROOT_NS_DELEGATE;
continue;
+ } else if (!strcmp(token, "groupoom")) {
+ *root_flags |= CGRP_GROUP_OOM;
+ continue;
}

pr_err("cgroup2: unknown option \"%s\"\n", token);
@@ -1715,6 +1718,11 @@ static void apply_cgroup_root_flags(unsigned int root_flags)
cgrp_dfl_root.flags |= CGRP_ROOT_NS_DELEGATE;
else
cgrp_dfl_root.flags &= ~CGRP_ROOT_NS_DELEGATE;
+
+ if (root_flags & CGRP_GROUP_OOM)
+ cgrp_dfl_root.flags |= CGRP_GROUP_OOM;
+ else
+ cgrp_dfl_root.flags &= ~CGRP_GROUP_OOM;
}
}

@@ -1722,6 +1730,8 @@ static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root
{
if (cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE)
seq_puts(seq, ",nsdelegate");
+ if (cgrp_dfl_root.flags & CGRP_GROUP_OOM)
+ seq_puts(seq, ",groupoom");
return 0;
}

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index da2b12ea4667..d645f70cb3a7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2871,6 +2871,9 @@ bool mem_cgroup_select_oom_victim(struct oom_control *oc)
if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
return false;

+ if (!(cgrp_dfl_root.flags & CGRP_GROUP_OOM))
+ return false;
+
if (oc->memcg)
root = oc->memcg;
else
--
2.13.5

2017-09-11 13:18:26

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Subject: [v8 1/4] mm, oom: refactor the oom_kill_process() function

The oom_kill_process() function consists of two logical parts:
the first one is responsible for considering task's children as
a potential victim and printing the debug information.
The second half is responsible for sending SIGKILL to all
tasks sharing the mm struct with the given victim.

This commit splits the oom_kill_process() function with
an intention to re-use the the second half: __oom_kill_process().

The cgroup-aware OOM killer will kill multiple tasks
belonging to the victim cgroup. We don't need to print
the debug information for the each task, as well as play
with task selection (considering task's children),
so we can't use the existing oom_kill_process().

Signed-off-by: Roman Gushchin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
mm/oom_kill.c | 123 +++++++++++++++++++++++++++++++---------------------------
1 file changed, 65 insertions(+), 58 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 99736e026712..f061b627092c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -804,68 +804,12 @@ static bool task_will_free_mem(struct task_struct *task)
return ret;
}

-static void oom_kill_process(struct oom_control *oc, const char *message)
+static void __oom_kill_process(struct task_struct *victim)
{
- struct task_struct *p = oc->chosen;
- unsigned int points = oc->chosen_points;
- struct task_struct *victim = p;
- struct task_struct *child;
- struct task_struct *t;
+ struct task_struct *p;
struct mm_struct *mm;
- unsigned int victim_points = 0;
- static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
- DEFAULT_RATELIMIT_BURST);
bool can_oom_reap = true;

- /*
- * If the task is already exiting, don't alarm the sysadmin or kill
- * its children or threads, just give it access to memory reserves
- * so it can die quickly
- */
- task_lock(p);
- if (task_will_free_mem(p)) {
- mark_oom_victim(p);
- wake_oom_reaper(p);
- task_unlock(p);
- put_task_struct(p);
- return;
- }
- task_unlock(p);
-
- if (__ratelimit(&oom_rs))
- dump_header(oc, p);
-
- pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
- message, task_pid_nr(p), p->comm, points);
-
- /*
- * If any of p's children has a different mm and is eligible for kill,
- * the one with the highest oom_badness() score is sacrificed for its
- * parent. This attempts to lose the minimal amount of work done while
- * still freeing memory.
- */
- read_lock(&tasklist_lock);
- for_each_thread(p, t) {
- list_for_each_entry(child, &t->children, sibling) {
- unsigned int child_points;
-
- if (process_shares_mm(child, p->mm))
- continue;
- /*
- * oom_badness() returns 0 if the thread is unkillable
- */
- child_points = oom_badness(child,
- oc->memcg, oc->nodemask, oc->totalpages);
- if (child_points > victim_points) {
- put_task_struct(victim);
- victim = child;
- victim_points = child_points;
- get_task_struct(victim);
- }
- }
- }
- read_unlock(&tasklist_lock);
-
p = find_lock_task_mm(victim);
if (!p) {
put_task_struct(victim);
@@ -939,6 +883,69 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
}
#undef K

+static void oom_kill_process(struct oom_control *oc, const char *message)
+{
+ struct task_struct *p = oc->chosen;
+ unsigned int points = oc->chosen_points;
+ struct task_struct *victim = p;
+ struct task_struct *child;
+ struct task_struct *t;
+ unsigned int victim_points = 0;
+ static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
+ DEFAULT_RATELIMIT_BURST);
+
+ /*
+ * If the task is already exiting, don't alarm the sysadmin or kill
+ * its children or threads, just give it access to memory reserves
+ * so it can die quickly
+ */
+ task_lock(p);
+ if (task_will_free_mem(p)) {
+ mark_oom_victim(p);
+ wake_oom_reaper(p);
+ task_unlock(p);
+ put_task_struct(p);
+ return;
+ }
+ task_unlock(p);
+
+ if (__ratelimit(&oom_rs))
+ dump_header(oc, p);
+
+ pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
+ message, task_pid_nr(p), p->comm, points);
+
+ /*
+ * If any of p's children has a different mm and is eligible for kill,
+ * the one with the highest oom_badness() score is sacrificed for its
+ * parent. This attempts to lose the minimal amount of work done while
+ * still freeing memory.
+ */
+ read_lock(&tasklist_lock);
+ for_each_thread(p, t) {
+ list_for_each_entry(child, &t->children, sibling) {
+ unsigned int child_points;
+
+ if (process_shares_mm(child, p->mm))
+ continue;
+ /*
+ * oom_badness() returns 0 if the thread is unkillable
+ */
+ child_points = oom_badness(child,
+ oc->memcg, oc->nodemask, oc->totalpages);
+ if (child_points > victim_points) {
+ put_task_struct(victim);
+ victim = child;
+ victim_points = child_points;
+ get_task_struct(victim);
+ }
+ }
+ }
+ read_unlock(&tasklist_lock);
+
+ __oom_kill_process(victim);
+}
+
/*
* Determines whether the kernel must panic because of the panic_on_oom sysctl.
*/
--
2.13.5

2017-09-11 13:18:40

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Subject: [v8 4/4] mm, oom, docs: describe the cgroup-aware OOM killer

Document the cgroup-aware OOM killer.

Signed-off-by: Roman Gushchin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
Documentation/cgroup-v2.txt | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index dc44785dc0fa..61a2e959e07a 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -48,6 +48,7 @@ v1 is available under Documentation/cgroup-v1/.
5-2-1. Memory Interface Files
5-2-2. Usage Guidelines
5-2-3. Memory Ownership
+ 5-2-4. OOM Killer
5-3. IO
5-3-1. IO Interface Files
5-3-2. Writeback
@@ -1034,6 +1035,18 @@ PAGE_SIZE multiple when read back.
high limit is used and monitored properly, this limit's
utility is limited to providing the final safety net.

+ memory.oom_group
+
+ A read-write single value file which exists on non-root
+ cgroups. The default is "0".
+
+ If set, OOM killer will kill all processes attached to the cgroup
+ if selected as an OOM victim.
+
+ OOM killer respects the /proc/pid/oom_score_adj value -1000,
+ and will never kill the unkillable task, even if memory.oom_group
+ is set.
+
memory.events
A read-only flat-keyed file which exists on non-root cgroups.
The following entries are defined. Unless specified
@@ -1237,6 +1250,32 @@ to be accessed repeatedly by other cgroups, it may make sense to use
POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
belonging to the affected files to ensure correct memory ownership.

+OOM Killer
+~~~~~~~~~~
+
+Cgroup v2 memory controller implements a cgroup-aware OOM killer.
+It means that it treats cgroups as first class OOM entities.
+
+Under OOM conditions the memory controller tries to make the best
+choice of a victim, hierarchically looking for a cgroup with the
+largest memory footprint.
+
+By default, OOM killer will kill the biggest task in the selected
+memory cgroup. A user can change this behavior by enabling
+the per-cgroup oom_group option. If set, it causes the OOM killer
+to kill all processes attached to the cgroup, except processes
+with oom_score_adj set to -1000.
+
+This affects both system- and cgroup-wide OOMs. For a cgroup-wide OOM
+the memory controller considers only cgroups belonging to the sub-tree
+of the OOM'ing cgroup.
+
+The root cgroup is treated as a leaf memory cgroup, so it's compared
+with top-level memory cgroups.
+
+If there are no cgroups with the enabled memory controller,
+the OOM killer is using the "traditional" process-based approach.
+

IO
--
--
2.13.5

2017-09-11 13:19:35

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Subject: [v8 2/4] mm, oom: cgroup-aware OOM killer

Traditionally, the OOM killer is operating on a process level.
Under oom conditions, it finds a process with the highest oom score
and kills it.

This behavior doesn't suit well the system with many running
containers:

1) There is no fairness between containers. A small container with
few large processes will be chosen over a large one with huge
number of small processes.

2) Containers often do not expect that some random process inside
will be killed. In many cases much safer behavior is to kill
all tasks in the container. Traditionally, this was implemented
in userspace, but doing it in the kernel has some advantages,
especially in a case of a system-wide OOM.

To address these issues, the cgroup-aware OOM killer is introduced.

Under OOM conditions, it looks for the biggest memory consumer,
walking down by the memcg tree from root cgroup or OOMing cgroup,
if OOM is caused by reaching memcg memory limit. On each level
the memcg with biggest memory footprint is selected.
By default, a leaf memcg is chosen, and the biggest task
inside is killed.

But a user can change this behavior by enabling the per-cgroup
memory.oom_group option. If set, it causes the OOM killer to treat
the whole cgroup as an indivisible memory consumer. This means
that OOM victim selection will stop at such memcg, the whole
memcg will be selected, and all belonging tasks will be killed.
The only exception is tasks with oom_score_adj set to -1000
are considered as unkillable.

The root cgroup is treated as a leaf memcg, so it's score
is compared with top-level memory cgroups. The oom_group option
is not supported for the root cgroup. Due to memcg statistics
implementation a special algorithm is used for estimating
root cgroup oom_score: we define it as maximum oom_score
of the belonging tasks.

Signed-off-by: Roman Gushchin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
include/linux/memcontrol.h | 33 ++++++
include/linux/oom.h | 12 ++-
mm/memcontrol.c | 258 +++++++++++++++++++++++++++++++++++++++++++++
mm/oom_kill.c | 95 ++++++++++++++---
4 files changed, 383 insertions(+), 15 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 69966c461d1c..5b5c2b89968e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -35,6 +35,7 @@ struct mem_cgroup;
struct page;
struct mm_struct;
struct kmem_cache;
+struct oom_control;

/* Cgroup-specific page state, on top of universal node page state */
enum memcg_stat_item {
@@ -199,6 +200,12 @@ struct mem_cgroup {
/* OOM-Killer disable */
int oom_kill_disable;

+ /* kill all tasks in the subtree in case of OOM */
+ bool oom_group;
+
+ /* cached OOM score */
+ long oom_score;
+
/* handle for "memory.events" */
struct cgroup_file events_file;

@@ -342,6 +349,11 @@ struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *css){
return css ? container_of(css, struct mem_cgroup, css) : NULL;
}

+static inline void mem_cgroup_put(struct mem_cgroup *memcg)
+{
+ css_put(&memcg->css);
+}
+
#define mem_cgroup_from_counter(counter, member) \
container_of(counter, struct mem_cgroup, member)

@@ -480,6 +492,13 @@ static inline bool task_in_memcg_oom(struct task_struct *p)

bool mem_cgroup_oom_synchronize(bool wait);

+bool mem_cgroup_select_oom_victim(struct oom_control *oc);
+
+static inline bool mem_cgroup_oom_group(struct mem_cgroup *memcg)
+{
+ return memcg->oom_group;
+}
+
#ifdef CONFIG_MEMCG_SWAP
extern int do_swap_account;
#endif
@@ -744,6 +763,10 @@ static inline bool task_in_mem_cgroup(struct task_struct *task,
return true;
}

+static inline void mem_cgroup_put(struct mem_cgroup *memcg)
+{
+}
+
static inline struct mem_cgroup *
mem_cgroup_iter(struct mem_cgroup *root,
struct mem_cgroup *prev,
@@ -936,6 +959,16 @@ static inline
void count_memcg_event_mm(struct mm_struct *mm, enum vm_event_item idx)
{
}
+
+static inline bool mem_cgroup_select_oom_victim(struct oom_control *oc)
+{
+ return false;
+}
+
+static inline bool mem_cgroup_oom_group(struct mem_cgroup *memcg)
+{
+ return false;
+}
#endif /* CONFIG_MEMCG */

/* idx can be of type enum memcg_stat_item or node_stat_item */
diff --git a/include/linux/oom.h b/include/linux/oom.h
index 76aac4ce39bc..ca78e2d5956e 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -9,6 +9,13 @@
#include <linux/sched/coredump.h> /* MMF_* */
#include <linux/mm.h> /* VM_FAULT* */

+
+/*
+ * Special value returned by victim selection functions to indicate
+ * that are inflight OOM victims.
+ */
+#define INFLIGHT_VICTIM ((void *)-1UL)
+
struct zonelist;
struct notifier_block;
struct mem_cgroup;
@@ -39,7 +46,8 @@ struct oom_control {

/* Used by oom implementation, do not set */
unsigned long totalpages;
- struct task_struct *chosen;
+ struct task_struct *chosen_task;
+ struct mem_cgroup *chosen_memcg;
unsigned long chosen_points;
};

@@ -101,6 +109,8 @@ extern void oom_killer_enable(void);

extern struct task_struct *find_lock_task_mm(struct task_struct *p);

+extern int oom_evaluate_task(struct task_struct *task, void *arg);
+
/* sysctls */
extern int sysctl_oom_dump_tasks;
extern int sysctl_oom_kill_allocating_task;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 15af3da5af02..da2b12ea4667 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2661,6 +2661,231 @@ static inline bool memcg_has_children(struct mem_cgroup *memcg)
return ret;
}

+static long memcg_oom_badness(struct mem_cgroup *memcg,
+ const nodemask_t *nodemask,
+ unsigned long totalpages)
+{
+ long points = 0;
+ int nid;
+ pg_data_t *pgdat;
+
+ /*
+ * We don't have necessary stats for the root memcg,
+ * so we define it's oom_score as the maximum oom_score
+ * of the belonging tasks.
+ */
+ if (memcg == root_mem_cgroup) {
+ struct css_task_iter it;
+ struct task_struct *task;
+ long score, max_score = 0;
+
+ css_task_iter_start(&memcg->css, 0, &it);
+ while ((task = css_task_iter_next(&it))) {
+ score = oom_badness(task, memcg, nodemask,
+ totalpages);
+ if (max_score > score)
+ max_score = score;
+ }
+ css_task_iter_end(&it);
+
+ return max_score;
+ }
+
+ for_each_node_state(nid, N_MEMORY) {
+ if (nodemask && !node_isset(nid, *nodemask))
+ continue;
+
+ points += mem_cgroup_node_nr_lru_pages(memcg, nid,
+ LRU_ALL_ANON | BIT(LRU_UNEVICTABLE));
+
+ pgdat = NODE_DATA(nid);
+ points += lruvec_page_state(mem_cgroup_lruvec(pgdat, memcg),
+ NR_SLAB_UNRECLAIMABLE);
+ }
+
+ points += memcg_page_state(memcg, MEMCG_KERNEL_STACK_KB) /
+ (PAGE_SIZE / 1024);
+ points += memcg_page_state(memcg, MEMCG_SOCK);
+ points += memcg_page_state(memcg, MEMCG_SWAP);
+
+ return points;
+}
+
+/*
+ * Checks if the given memcg is a valid OOM victim and returns a number,
+ * which means the folowing:
+ * -1: there are inflight OOM victim tasks, belonging to the memcg
+ * 0: memcg is not eligible, e.g. all belonging tasks are protected
+ * by oom_score_adj set to OOM_SCORE_ADJ_MIN
+ * >0: memcg is eligible, and the returned value is an estimation
+ * of the memory footprint
+ */
+static long oom_evaluate_memcg(struct mem_cgroup *memcg,
+ const nodemask_t *nodemask,
+ unsigned long totalpages)
+{
+ struct css_task_iter it;
+ struct task_struct *task;
+ int eligible = 0;
+
+ /*
+ * Memcg is OOM eligible if there are OOM killable tasks inside.
+ *
+ * We treat tasks with oom_score_adj set to OOM_SCORE_ADJ_MIN
+ * as unkillable.
+ *
+ * If there are inflight OOM victim tasks inside the memcg,
+ * we return -1.
+ */
+ css_task_iter_start(&memcg->css, 0, &it);
+ while ((task = css_task_iter_next(&it))) {
+ if (!eligible &&
+ task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN)
+ eligible = 1;
+
+ if (tsk_is_oom_victim(task) &&
+ !test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags)) {
+ eligible = -1;
+ break;
+ }
+ }
+ css_task_iter_end(&it);
+
+ if (eligible <= 0)
+ return eligible;
+
+ return memcg_oom_badness(memcg, nodemask, totalpages);
+}
+
+static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
+{
+ struct mem_cgroup *iter, *parent;
+
+ /*
+ * If OOM is memcg-wide, and the memcg has the oom_group flag,
+ * simple select the memcg as a victim.
+ */
+ if (oc->memcg && oc->memcg->oom_group) {
+ oc->chosen_memcg = oc->memcg;
+ css_get(&oc->chosen_memcg->css);
+ oc->chosen_points = oc->memcg->oom_score;
+ return;
+ }
+
+ /*
+ * The oom_score is calculated for leaf memcgs and propagated upwards
+ * by the tree.
+ *
+ * for_each_mem_cgroup_tree() walks the tree in pre-order,
+ * so we simple reset oom_score for non-lead cgroups before
+ * starting accumulating an actual value from underlying sub-tree.
+ *
+ * Root memcg is treated as a leaf memcg.
+ */
+ for_each_mem_cgroup_tree(iter, root) {
+ if (memcg_has_children(iter) && iter != root_mem_cgroup) {
+ iter->oom_score = 0;
+ continue;
+ }
+
+ iter->oom_score = oom_evaluate_memcg(iter, oc->nodemask,
+ oc->totalpages);
+
+ /*
+ * Ignore empty and non-eligible memory cgroups.
+ */
+ if (iter->oom_score == 0)
+ continue;
+
+ /*
+ * If there are inflight OOM victims, we don't need to look
+ * further for new victims.
+ */
+ if (iter->oom_score == -1) {
+ oc->chosen_memcg = INFLIGHT_VICTIM;
+ mem_cgroup_iter_break(root, iter);
+ return;
+ }
+
+ for (parent = parent_mem_cgroup(iter); parent && parent != root;
+ parent = parent_mem_cgroup(parent))
+ parent->oom_score += iter->oom_score;
+ }
+
+ for (;;) {
+ struct cgroup_subsys_state *css;
+ struct mem_cgroup *memcg = NULL;
+ long score = LONG_MIN;
+
+ /*
+ * Root memcg is compared with top-level memcgs.
+ */
+ if (root == root_mem_cgroup && root->oom_score > 0) {
+ score = root->oom_score;
+ memcg = root_mem_cgroup;
+ }
+
+ css_for_each_child(css, &root->css) {
+ struct mem_cgroup *iter = mem_cgroup_from_css(css);
+
+ /*
+ * Ignore empty and non-eligible memory cgroups.
+ */
+ if (iter->oom_score == 0)
+ continue;
+
+ if (iter->oom_score > score) {
+ memcg = iter;
+ score = iter->oom_score;
+ }
+ }
+
+ if (!memcg) {
+ if (oc->memcg && root == oc->memcg) {
+ oc->chosen_memcg = oc->memcg;
+ css_get(&oc->chosen_memcg->css);
+ oc->chosen_points = oc->memcg->oom_score;
+ }
+ break;
+ }
+
+ if (memcg->oom_group || !memcg_has_children(memcg) ||
+ memcg == root_mem_cgroup) {
+ oc->chosen_memcg = memcg;
+ css_get(&oc->chosen_memcg->css);
+ oc->chosen_points = score;
+ break;
+ }
+
+ root = memcg;
+ }
+}
+
+bool mem_cgroup_select_oom_victim(struct oom_control *oc)
+{
+ struct mem_cgroup *root;
+
+ if (mem_cgroup_disabled())
+ return false;
+
+ if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+ return false;
+
+ if (oc->memcg)
+ root = oc->memcg;
+ else
+ root = root_mem_cgroup;
+
+ oc->chosen_task = NULL;
+ oc->chosen_memcg = NULL;
+
+ rcu_read_lock();
+ select_victim_memcg(root, oc);
+ rcu_read_unlock();
+
+ return oc->chosen_task || oc->chosen_memcg;
+}
+
/*
* Reclaims as many pages from the given memcg as possible.
*
@@ -5258,6 +5483,33 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
return nbytes;
}

+static int memory_oom_group_show(struct seq_file *m, void *v)
+{
+ struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
+ bool oom_group = memcg->oom_group;
+
+ seq_printf(m, "%d\n", oom_group);
+
+ return 0;
+}
+
+static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
+ char *buf, size_t nbytes,
+ loff_t off)
+{
+ struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+ int oom_group;
+ int err;
+
+ err = kstrtoint(strstrip(buf), 0, &oom_group);
+ if (err)
+ return err;
+
+ memcg->oom_group = oom_group;
+
+ return nbytes;
+}
+
static int memory_events_show(struct seq_file *m, void *v)
{
struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
@@ -5378,6 +5630,12 @@ static struct cftype memory_files[] = {
.write = memory_max_write,
},
{
+ .name = "oom_group",
+ .flags = CFTYPE_NOT_ON_ROOT,
+ .seq_show = memory_oom_group_show,
+ .write = memory_oom_group_write,
+ },
+ {
.name = "events",
.flags = CFTYPE_NOT_ON_ROOT,
.file_offset = offsetof(struct mem_cgroup, events_file),
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f061b627092c..70359a535e62 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -288,7 +288,7 @@ static enum oom_constraint constrained_alloc(struct oom_control *oc)
return CONSTRAINT_NONE;
}

-static int oom_evaluate_task(struct task_struct *task, void *arg)
+int oom_evaluate_task(struct task_struct *task, void *arg)
{
struct oom_control *oc = arg;
unsigned long points;
@@ -322,26 +322,26 @@ static int oom_evaluate_task(struct task_struct *task, void *arg)
goto next;

/* Prefer thread group leaders for display purposes */
- if (points == oc->chosen_points && thread_group_leader(oc->chosen))
+ if (points == oc->chosen_points && thread_group_leader(oc->chosen_task))
goto next;
select:
- if (oc->chosen)
- put_task_struct(oc->chosen);
+ if (oc->chosen_task)
+ put_task_struct(oc->chosen_task);
get_task_struct(task);
- oc->chosen = task;
+ oc->chosen_task = task;
oc->chosen_points = points;
next:
return 0;
abort:
- if (oc->chosen)
- put_task_struct(oc->chosen);
- oc->chosen = (void *)-1UL;
+ if (oc->chosen_task)
+ put_task_struct(oc->chosen_task);
+ oc->chosen_task = INFLIGHT_VICTIM;
return 1;
}

/*
* Simple selection loop. We choose the process with the highest number of
- * 'points'. In case scan was aborted, oc->chosen is set to -1.
+ * 'points'. In case scan was aborted, oc->chosen_task is set to -1.
*/
static void select_bad_process(struct oom_control *oc)
{
@@ -810,6 +810,12 @@ static void __oom_kill_process(struct task_struct *victim)
struct mm_struct *mm;
bool can_oom_reap = true;

+ if (is_global_init(victim) || (victim->flags & PF_KTHREAD) ||
+ victim->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
+ put_task_struct(victim);
+ return;
+ }
+
p = find_lock_task_mm(victim);
if (!p) {
put_task_struct(victim);
@@ -885,7 +891,7 @@ static void __oom_kill_process(struct task_struct *victim)

static void oom_kill_process(struct oom_control *oc, const char *message)
{
- struct task_struct *p = oc->chosen;
+ struct task_struct *p = oc->chosen_task;
unsigned int points = oc->chosen_points;
struct task_struct *victim = p;
struct task_struct *child;
@@ -946,6 +952,64 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
__oom_kill_process(victim);
}

+static int oom_kill_memcg_member(struct task_struct *task, void *unused)
+{
+ if (!tsk_is_oom_victim(task)) {
+ get_task_struct(task);
+ __oom_kill_process(task);
+ }
+ return 0;
+}
+
+static bool oom_kill_memcg_victim(struct oom_control *oc)
+{
+ static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
+ DEFAULT_RATELIMIT_BURST);
+
+ if (oc->chosen_task) {
+ if (oc->chosen_task == INFLIGHT_VICTIM)
+ return true;
+
+ if (__ratelimit(&oom_rs))
+ dump_header(oc, oc->chosen_task);
+
+ __oom_kill_process(oc->chosen_task);
+
+ schedule_timeout_killable(1);
+ return true;
+
+ } else if (oc->chosen_memcg) {
+ if (oc->chosen_memcg == INFLIGHT_VICTIM)
+ return true;
+
+ /* Always begin with the biggest task */
+ oc->chosen_points = 0;
+ oc->chosen_task = NULL;
+ mem_cgroup_scan_tasks(oc->chosen_memcg, oom_evaluate_task, oc);
+
+ if (oc->chosen_task && oc->chosen_task != INFLIGHT_VICTIM) {
+ if (__ratelimit(&oom_rs))
+ dump_header(oc, oc->chosen_task);
+
+ __oom_kill_process(oc->chosen_task);
+
+ if (mem_cgroup_oom_group(oc->chosen_memcg))
+ mem_cgroup_scan_tasks(oc->chosen_memcg,
+ oom_kill_memcg_member,
+ NULL);
+ schedule_timeout_killable(1);
+ }
+
+ mem_cgroup_put(oc->chosen_memcg);
+ oc->chosen_memcg = NULL;
+ return oc->chosen_task;
+
+ } else {
+ oc->chosen_points = 0;
+ return false;
+ }
+}
+
/*
* Determines whether the kernel must panic because of the panic_on_oom sysctl.
*/
@@ -1042,18 +1106,21 @@ bool out_of_memory(struct oom_control *oc)
current->mm && !oom_unkillable_task(current, NULL, oc->nodemask) &&
current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
get_task_struct(current);
- oc->chosen = current;
+ oc->chosen_task = current;
oom_kill_process(oc, "Out of memory (oom_kill_allocating_task)");
return true;
}

+ if (mem_cgroup_select_oom_victim(oc) && oom_kill_memcg_victim(oc))
+ return true;
+
select_bad_process(oc);
/* Found nothing?!?! Either we hang forever, or we panic. */
- if (!oc->chosen && !is_sysrq_oom(oc) && !is_memcg_oom(oc)) {
+ if (!oc->chosen_task && !is_sysrq_oom(oc) && !is_memcg_oom(oc)) {
dump_header(oc, NULL);
panic("Out of memory and no killable processes...\n");
}
- if (oc->chosen && oc->chosen != (void *)-1UL) {
+ if (oc->chosen_task && oc->chosen_task != INFLIGHT_VICTIM) {
oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
"Memory cgroup out of memory");
/*
@@ -1062,7 +1129,7 @@ bool out_of_memory(struct oom_control *oc)
*/
schedule_timeout_killable(1);
}
- return !!oc->chosen;
+ return !!oc->chosen_task;
}

/*
--
2.13.5

2017-09-11 20:44:43

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, 11 Sep 2017, Roman Gushchin wrote:

> This patchset makes the OOM killer cgroup-aware.
>
> v8:
> - Do not kill tasks with OOM_SCORE_ADJ -1000
> - Make the whole thing opt-in with cgroup mount option control
> - Drop oom_priority for further discussions

Nack, we specifically require oom_priority for this to function correctly,
otherwise we cannot prefer to kill from low priority leaf memcgs as
required. v8 appears to implement new functionality that we want, to
compare two memcgs based on usage, but without the ability to influence
that decision to protect important userspace, so now I'm in a position
where (1) nothing has changed if I don't use the new mount option or (2) I
get completely different oom kill selection with the new mount option but
not the ability to influence it. I was much happier with the direction
that v7 was taking, but since v8 causes us to regress without the ability
to change memcg priority, this has to be nacked.

> - Kill the whole cgroup if oom_group is set and it's
> memory.max is reached
> - Update docs and commit messages

2017-09-11 20:48:43

[permalink] [raw]

Subject: Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware OOM killer

On Mon, 11 Sep 2017, Roman Gushchin wrote:

> Add a "groupoom" cgroup v2 mount option to enable the cgroup-aware
> OOM killer. If not set, the OOM selection is performed in
> a "traditional" per-process way.
>
> The behavior can be changed dynamically by remounting the cgroupfs.

I can't imagine that Tejun would be happy with a new mount option,
especially when it's not required.

OOM behavior does not need to be defined at mount time and for the entire
hierarchy. It's possible to very easily implement a tunable as part of
mem cgroup that is propagated to descendants and controls the oom scoring
behavior for that hierarchy. It does not need to be system wide and
affect scoring of all processes based on which mem cgroup they are
attached to at any given time.

2017-09-11 20:51:21

[permalink] [raw]

Subject: Re: [v8 1/4] mm, oom: refactor the oom_kill_process() function

On Mon, 11 Sep 2017, Roman Gushchin wrote:

> The oom_kill_process() function consists of two logical parts:
> the first one is responsible for considering task's children as
> a potential victim and printing the debug information.
> The second half is responsible for sending SIGKILL to all
> tasks sharing the mm struct with the given victim.
>
> This commit splits the oom_kill_process() function with
> an intention to re-use the the second half: __oom_kill_process().
>
> The cgroup-aware OOM killer will kill multiple tasks
> belonging to the victim cgroup. We don't need to print
> the debug information for the each task, as well as play
> with task selection (considering task's children),
> so we can't use the existing oom_kill_process().
>
> Signed-off-by: Roman Gushchin <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Vladimir Davydov <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Tetsuo Handa <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]

Acked-by: David Rientjes <[email protected]>

https://marc.info/?l=linux-kernel&m=150274805412752

2017-09-12 20:01:45

[permalink] [raw]

Subject: Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware OOM killer

On Mon, Sep 11, 2017 at 01:48:39PM -0700, David Rientjes wrote:
> On Mon, 11 Sep 2017, Roman Gushchin wrote:
>
> > Add a "groupoom" cgroup v2 mount option to enable the cgroup-aware
> > OOM killer. If not set, the OOM selection is performed in
> > a "traditional" per-process way.
> >
> > The behavior can be changed dynamically by remounting the cgroupfs.
>
> I can't imagine that Tejun would be happy with a new mount option,
> especially when it's not required.
>
> OOM behavior does not need to be defined at mount time and for the entire
> hierarchy. It's possible to very easily implement a tunable as part of
> mem cgroup that is propagated to descendants and controls the oom scoring
> behavior for that hierarchy. It does not need to be system wide and
> affect scoring of all processes based on which mem cgroup they are
> attached to at any given time.

No, I don't think that mixing per-cgroup and per-process OOM selection
algorithms is a good idea.

So, there are 3 reasonable options:
1) boot option
2) sysctl
3) cgroup mount option

I believe, 3) is better, because it allows changing the behavior dynamically,
and explicitly depends on v2 (what sysctl lacks).

So, the only question is should it be opt-in or opt-out option.
Personally, I would prefer opt-out, but Michal has a very strong opinion here.

Thanks!

2017-09-12 20:23:05

[permalink] [raw]

Subject: Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware OOM killer

On Tue, 12 Sep 2017, Roman Gushchin wrote:

> > I can't imagine that Tejun would be happy with a new mount option,
> > especially when it's not required.
> >
> > OOM behavior does not need to be defined at mount time and for the entire
> > hierarchy. It's possible to very easily implement a tunable as part of
> > mem cgroup that is propagated to descendants and controls the oom scoring
> > behavior for that hierarchy. It does not need to be system wide and
> > affect scoring of all processes based on which mem cgroup they are
> > attached to at any given time.
>
> No, I don't think that mixing per-cgroup and per-process OOM selection
> algorithms is a good idea.
>
> So, there are 3 reasonable options:
> 1) boot option
> 2) sysctl
> 3) cgroup mount option
>
> I believe, 3) is better, because it allows changing the behavior dynamically,
> and explicitly depends on v2 (what sysctl lacks).
>
> So, the only question is should it be opt-in or opt-out option.
> Personally, I would prefer opt-out, but Michal has a very strong opinion here.
>

If it absolutely must be a mount option, then I would agree it should be
opt-in so that it's known what is being changed rather than changing how
selection was done in the past and requiring legacy users to now mount in
a new way.

I'd be interested to hear Tejun's comments, however, about whether we want
to add controller specific mount options like this instead of a tunable at
the root level, for instance, that controls victim selection and would be
isolated to the memory cgroup controller as opposed to polluting mount
options.

2017-09-13 12:23:19

[permalink] [raw]

Subject: Re: [v8 3/4] mm, oom: add cgroup v2 mount option for cgroup-aware OOM killer

On Tue 12-09-17 21:01:15, Roman Gushchin wrote:
> On Mon, Sep 11, 2017 at 01:48:39PM -0700, David Rientjes wrote:
> > On Mon, 11 Sep 2017, Roman Gushchin wrote:
> >
> > > Add a "groupoom" cgroup v2 mount option to enable the cgroup-aware
> > > OOM killer. If not set, the OOM selection is performed in
> > > a "traditional" per-process way.
> > >
> > > The behavior can be changed dynamically by remounting the cgroupfs.
> >
> > I can't imagine that Tejun would be happy with a new mount option,
> > especially when it's not required.
> >
> > OOM behavior does not need to be defined at mount time and for the entire
> > hierarchy. It's possible to very easily implement a tunable as part of
> > mem cgroup that is propagated to descendants and controls the oom scoring
> > behavior for that hierarchy. It does not need to be system wide and
> > affect scoring of all processes based on which mem cgroup they are
> > attached to at any given time.
>
> No, I don't think that mixing per-cgroup and per-process OOM selection
> algorithms is a good idea.
>
> So, there are 3 reasonable options:
> 1) boot option
> 2) sysctl
> 3) cgroup mount option
>
> I believe, 3) is better, because it allows changing the behavior dynamically,
> and explicitly depends on v2 (what sysctl lacks).

I see your argument here. I would just be worried that we end up really
needing more oom strategies in future and those wouldn't fit into memcg
mount option scope. So 1/2 sounds more exensible to me long term. Boot
time would be easier because we do not have to bother dynamic selection
in that case.

> So, the only question is should it be opt-in or opt-out option.
> Personally, I would prefer opt-out, but Michal has a very strong opinion here.

Yes I still strongly believe this has to be opt-in.
--
Michal Hocko
SUSE Labs

2017-09-13 12:29:20

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon 11-09-17 13:44:39, David Rientjes wrote:
> On Mon, 11 Sep 2017, Roman Gushchin wrote:
>
> > This patchset makes the OOM killer cgroup-aware.
> >
> > v8:
> > - Do not kill tasks with OOM_SCORE_ADJ -1000
> > - Make the whole thing opt-in with cgroup mount option control
> > - Drop oom_priority for further discussions
>
> Nack, we specifically require oom_priority for this to function correctly,
> otherwise we cannot prefer to kill from low priority leaf memcgs as
> required.

While I understand that your usecase might require priorities I do not
think this part missing is a reason to nack the cgroup based selection
and kill-all parts. This can be done on top. The only important part
right now is the current selection semantic - only leaf memcgs vs. size
of the hierarchy). I strongly believe that comparing only leaf memcgs
is more straightforward and it doesn't lead to unexpected results as
mentioned before (kill a small memcg which is a part of the larger
sub-hierarchy).

I didn't get to read the new version of this series yet and hope to get
to it soon.
--
Michal Hocko
SUSE Labs

2017-09-13 20:46:12

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed, 13 Sep 2017, Michal Hocko wrote:

> > > This patchset makes the OOM killer cgroup-aware.
> > >
> > > v8:
> > > - Do not kill tasks with OOM_SCORE_ADJ -1000
> > > - Make the whole thing opt-in with cgroup mount option control
> > > - Drop oom_priority for further discussions
> >
> > Nack, we specifically require oom_priority for this to function correctly,
> > otherwise we cannot prefer to kill from low priority leaf memcgs as
> > required.
>
> While I understand that your usecase might require priorities I do not
> think this part missing is a reason to nack the cgroup based selection
> and kill-all parts. This can be done on top. The only important part
> right now is the current selection semantic - only leaf memcgs vs. size
> of the hierarchy). I strongly believe that comparing only leaf memcgs
> is more straightforward and it doesn't lead to unexpected results as
> mentioned before (kill a small memcg which is a part of the larger
> sub-hierarchy).
>

The problem is that we cannot enable the cgroup-aware oom killer and
oom_group behavior because, without oom priorities, we have no ability to
influence the cgroup that it chooses. It is doing two things: providing
more fairness amongst cgroups by selecting based on cumulative usage
rather than single large process (good!), and effectively is removing all
userspace control of oom selection (bad). We want the former, but it
needs to be coupled with support so that we can protect vital cgroups,
regardless of their usage.

It is certainly possible to add oom priorities on top before it is merged,
but I don't see why it isn't part of the patchset. We need it before its
merged to avoid users playing with /proc/pid/oom_score_adj to prevent any
killing in the most preferable memcg when they could have simply changed
the oom priority.

2017-09-13 20:46:56

[permalink] [raw]

Subject: Re: [v8 2/4] mm, oom: cgroup-aware OOM killer

On Mon, 11 Sep 2017, Roman Gushchin wrote:

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 15af3da5af02..da2b12ea4667 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2661,6 +2661,231 @@ static inline bool memcg_has_children(struct mem_cgroup *memcg)
> return ret;
> }
>
> +static long memcg_oom_badness(struct mem_cgroup *memcg,
> + const nodemask_t *nodemask,
> + unsigned long totalpages)
> +{
> + long points = 0;
> + int nid;
> + pg_data_t *pgdat;
> +
> + /*
> + * We don't have necessary stats for the root memcg,
> + * so we define it's oom_score as the maximum oom_score
> + * of the belonging tasks.
> + */
> + if (memcg == root_mem_cgroup) {
> + struct css_task_iter it;
> + struct task_struct *task;
> + long score, max_score = 0;
> +
> + css_task_iter_start(&memcg->css, 0, &it);
> + while ((task = css_task_iter_next(&it))) {
> + score = oom_badness(task, memcg, nodemask,
> + totalpages);
> + if (max_score > score)

score > max_score

> + max_score = score;
> + }
> + css_task_iter_end(&it);
> +
> + return max_score;
> + }
> +
> + for_each_node_state(nid, N_MEMORY) {
> + if (nodemask && !node_isset(nid, *nodemask))
> + continue;
> +
> + points += mem_cgroup_node_nr_lru_pages(memcg, nid,
> + LRU_ALL_ANON | BIT(LRU_UNEVICTABLE));
> +
> + pgdat = NODE_DATA(nid);
> + points += lruvec_page_state(mem_cgroup_lruvec(pgdat, memcg),
> + NR_SLAB_UNRECLAIMABLE);
> + }
> +
> + points += memcg_page_state(memcg, MEMCG_KERNEL_STACK_KB) /
> + (PAGE_SIZE / 1024);
> + points += memcg_page_state(memcg, MEMCG_SOCK);
> + points += memcg_page_state(memcg, MEMCG_SWAP);
> +
> + return points;
> +}

2017-09-13 21:56:55

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> On Mon 11-09-17 13:44:39, David Rientjes wrote:
> > On Mon, 11 Sep 2017, Roman Gushchin wrote:
> >
> > > This patchset makes the OOM killer cgroup-aware.
> > >
> > > v8:
> > > - Do not kill tasks with OOM_SCORE_ADJ -1000
> > > - Make the whole thing opt-in with cgroup mount option control
> > > - Drop oom_priority for further discussions
> >
> > Nack, we specifically require oom_priority for this to function correctly,
> > otherwise we cannot prefer to kill from low priority leaf memcgs as
> > required.
>
> While I understand that your usecase might require priorities I do not
> think this part missing is a reason to nack the cgroup based selection
> and kill-all parts. This can be done on top. The only important part
> right now is the current selection semantic - only leaf memcgs vs. size
> of the hierarchy).

I agree.

> I strongly believe that comparing only leaf memcgs
> is more straightforward and it doesn't lead to unexpected results as
> mentioned before (kill a small memcg which is a part of the larger
> sub-hierarchy).

One of two main goals of this patchset is to introduce cgroup-level
fairness: bigger cgroups should be affected more than smaller,
despite the size of tasks inside. I believe the same principle
should be used for cgroups.

Also, the opposite will make oom_semantics more weird: it will mean
kill all tasks, but also treat memcg as a leaf cgroup.

>
> I didn't get to read the new version of this series yet and hope to get
> to it soon.

Thanks!

2017-09-13 22:00:32

[permalink] [raw]

Subject: Re: [v8 2/4] mm, oom: cgroup-aware OOM killer

On Wed, Sep 13, 2017 at 01:46:51PM -0700, David Rientjes wrote:
> On Mon, 11 Sep 2017, Roman Gushchin wrote:
>
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 15af3da5af02..da2b12ea4667 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2661,6 +2661,231 @@ static inline bool memcg_has_children(struct mem_cgroup *memcg)
> > return ret;
> > }
> >
> > +static long memcg_oom_badness(struct mem_cgroup *memcg,
> > + const nodemask_t *nodemask,
> > + unsigned long totalpages)
> > +{
> > + long points = 0;
> > + int nid;
> > + pg_data_t *pgdat;
> > +
> > + /*
> > + * We don't have necessary stats for the root memcg,
> > + * so we define it's oom_score as the maximum oom_score
> > + * of the belonging tasks.
> > + */
> > + if (memcg == root_mem_cgroup) {
> > + struct css_task_iter it;
> > + struct task_struct *task;
> > + long score, max_score = 0;
> > +
> > + css_task_iter_start(&memcg->css, 0, &it);
> > + while ((task = css_task_iter_next(&it))) {
> > + score = oom_badness(task, memcg, nodemask,
> > + totalpages);
> > + if (max_score > score)
>
> score > max_score

Ups. Fixed. Thanks!

2017-09-14 13:34:14

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed 13-09-17 13:46:08, David Rientjes wrote:
> On Wed, 13 Sep 2017, Michal Hocko wrote:
>
> > > > This patchset makes the OOM killer cgroup-aware.
> > > >
> > > > v8:
> > > > - Do not kill tasks with OOM_SCORE_ADJ -1000
> > > > - Make the whole thing opt-in with cgroup mount option control
> > > > - Drop oom_priority for further discussions
> > >
> > > Nack, we specifically require oom_priority for this to function correctly,
> > > otherwise we cannot prefer to kill from low priority leaf memcgs as
> > > required.
> >
> > While I understand that your usecase might require priorities I do not
> > think this part missing is a reason to nack the cgroup based selection
> > and kill-all parts. This can be done on top. The only important part
> > right now is the current selection semantic - only leaf memcgs vs. size
> > of the hierarchy). I strongly believe that comparing only leaf memcgs
> > is more straightforward and it doesn't lead to unexpected results as
> > mentioned before (kill a small memcg which is a part of the larger
> > sub-hierarchy).
> >
>
> The problem is that we cannot enable the cgroup-aware oom killer and
> oom_group behavior because, without oom priorities, we have no ability to
> influence the cgroup that it chooses. It is doing two things: providing
> more fairness amongst cgroups by selecting based on cumulative usage
> rather than single large process (good!), and effectively is removing all
> userspace control of oom selection (bad). We want the former, but it
> needs to be coupled with support so that we can protect vital cgroups,
> regardless of their usage.

I understand that your usecase needs a more fine grained control over
the selection but that alone is not a reason to nack the implementation
which doesn't provide it (yet).

> It is certainly possible to add oom priorities on top before it is merged,
> but I don't see why it isn't part of the patchset.

Because the semantic of the priority for non-leaf memcgs is not fully
clear and I would rather have the core of the functionality merged
before this is sorted out.

> We need it before its
> merged to avoid users playing with /proc/pid/oom_score_adj to prevent any
> killing in the most preferable memcg when they could have simply changed
> the oom priority.

I am sorry but I do not really understand your concern. Are you
suggesting that users would start oom disable all tasks in a memcg to
give it a higher priority? Even if that was the case why should such an
abuse be a blocker for generic memcg aware oom killer being merged?
--
Michal Hocko
SUSE Labs

2017-09-14 13:40:20

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
[...]
> > I strongly believe that comparing only leaf memcgs
> > is more straightforward and it doesn't lead to unexpected results as
> > mentioned before (kill a small memcg which is a part of the larger
> > sub-hierarchy).
>
> One of two main goals of this patchset is to introduce cgroup-level
> fairness: bigger cgroups should be affected more than smaller,
> despite the size of tasks inside. I believe the same principle
> should be used for cgroups.

Yes bigger cgroups should be preferred but I fail to see why bigger
hierarchies should be considered as well if they are not kill-all. And
whether non-leaf memcgs should allow kill-all is not entirely clear to
me. What would be the usecase?
Consider that it might be not your choice (as a user) how deep is your
leaf memcg. I can already see how people complain that their memcg has
been killed just because it was one level deeper in the hierarchy...

I would really start simple and only allow kill-all on leaf memcgs and
only compare leaf memcgs & root. If we ever need to kill whole
hierarchies then allow kill-all on intermediate memcgs as well and then
consider cumulative consumptions only on those that have kill-all
enabled.

Or do I miss any reasonable usecase that would suffer from such a
semantic?
--
Michal Hocko
SUSE Labs

2017-09-14 13:42:23

[permalink] [raw]

Subject: Re: [v8 1/4] mm, oom: refactor the oom_kill_process() function

On Mon 11-09-17 14:17:39, Roman Gushchin wrote:
> The oom_kill_process() function consists of two logical parts:
> the first one is responsible for considering task's children as
> a potential victim and printing the debug information.
> The second half is responsible for sending SIGKILL to all
> tasks sharing the mm struct with the given victim.
>
> This commit splits the oom_kill_process() function with
> an intention to re-use the the second half: __oom_kill_process().
>
> The cgroup-aware OOM killer will kill multiple tasks
> belonging to the victim cgroup. We don't need to print
> the debug information for the each task, as well as play
> with task selection (considering task's children),
> so we can't use the existing oom_kill_process().
>
> Signed-off-by: Roman Gushchin <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Vladimir Davydov <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Tetsuo Handa <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]

Acked-by: Michal Hocko <[email protected]>

> ---
> mm/oom_kill.c | 123 +++++++++++++++++++++++++++++++---------------------------
> 1 file changed, 65 insertions(+), 58 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 99736e026712..f061b627092c 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -804,68 +804,12 @@ static bool task_will_free_mem(struct task_struct *task)
> return ret;
> }
>
> -static void oom_kill_process(struct oom_control *oc, const char *message)
> +static void __oom_kill_process(struct task_struct *victim)
> {
> - struct task_struct *p = oc->chosen;
> - unsigned int points = oc->chosen_points;
> - struct task_struct *victim = p;
> - struct task_struct *child;
> - struct task_struct *t;
> + struct task_struct *p;
> struct mm_struct *mm;
> - unsigned int victim_points = 0;
> - static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> - DEFAULT_RATELIMIT_BURST);
> bool can_oom_reap = true;
>
> - /*
> - * If the task is already exiting, don't alarm the sysadmin or kill
> - * its children or threads, just give it access to memory reserves
> - * so it can die quickly
> - */
> - task_lock(p);
> - if (task_will_free_mem(p)) {
> - mark_oom_victim(p);
> - wake_oom_reaper(p);
> - task_unlock(p);
> - put_task_struct(p);
> - return;
> - }
> - task_unlock(p);
> -
> - if (__ratelimit(&oom_rs))
> - dump_header(oc, p);
> -
> - pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
> - message, task_pid_nr(p), p->comm, points);
> -
> - /*
> - * If any of p's children has a different mm and is eligible for kill,
> - * the one with the highest oom_badness() score is sacrificed for its
> - * parent. This attempts to lose the minimal amount of work done while
> - * still freeing memory.
> - */
> - read_lock(&tasklist_lock);
> - for_each_thread(p, t) {
> - list_for_each_entry(child, &t->children, sibling) {
> - unsigned int child_points;
> -
> - if (process_shares_mm(child, p->mm))
> - continue;
> - /*
> - * oom_badness() returns 0 if the thread is unkillable
> - */
> - child_points = oom_badness(child,
> - oc->memcg, oc->nodemask, oc->totalpages);
> - if (child_points > victim_points) {
> - put_task_struct(victim);
> - victim = child;
> - victim_points = child_points;
> - get_task_struct(victim);
> - }
> - }
> - }
> - read_unlock(&tasklist_lock);
> -
> p = find_lock_task_mm(victim);
> if (!p) {
> put_task_struct(victim);
> @@ -939,6 +883,69 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> }
> #undef K
>
> +static void oom_kill_process(struct oom_control *oc, const char *message)
> +{
> + struct task_struct *p = oc->chosen;
> + unsigned int points = oc->chosen_points;
> + struct task_struct *victim = p;
> + struct task_struct *child;
> + struct task_struct *t;
> + unsigned int victim_points = 0;
> + static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> + DEFAULT_RATELIMIT_BURST);
> +
> + /*
> + * If the task is already exiting, don't alarm the sysadmin or kill
> + * its children or threads, just give it access to memory reserves
> + * so it can die quickly
> + */
> + task_lock(p);
> + if (task_will_free_mem(p)) {
> + mark_oom_victim(p);
> + wake_oom_reaper(p);
> + task_unlock(p);
> + put_task_struct(p);
> + return;
> + }
> + task_unlock(p);
> +
> + if (__ratelimit(&oom_rs))
> + dump_header(oc, p);
> +
> + pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
> + message, task_pid_nr(p), p->comm, points);
> +
> + /*
> + * If any of p's children has a different mm and is eligible for kill,
> + * the one with the highest oom_badness() score is sacrificed for its
> + * parent. This attempts to lose the minimal amount of work done while
> + * still freeing memory.
> + */
> + read_lock(&tasklist_lock);
> + for_each_thread(p, t) {
> + list_for_each_entry(child, &t->children, sibling) {
> + unsigned int child_points;
> +
> + if (process_shares_mm(child, p->mm))
> + continue;
> + /*
> + * oom_badness() returns 0 if the thread is unkillable
> + */
> + child_points = oom_badness(child,
> + oc->memcg, oc->nodemask, oc->totalpages);
> + if (child_points > victim_points) {
> + put_task_struct(victim);
> + victim = child;
> + victim_points = child_points;
> + get_task_struct(victim);
> + }
> + }
> + }
> + read_unlock(&tasklist_lock);
> +
> + __oom_kill_process(victim);
> +}
> +
> /*
> * Determines whether the kernel must panic because of the panic_on_oom sysctl.
> */
> --
> 2.13.5

--
Michal Hocko
SUSE Labs

2017-09-14 16:06:22

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Thu, Sep 14, 2017 at 03:40:14PM +0200, Michal Hocko wrote:
> On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> > On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> [...]
> > > I strongly believe that comparing only leaf memcgs
> > > is more straightforward and it doesn't lead to unexpected results as
> > > mentioned before (kill a small memcg which is a part of the larger
> > > sub-hierarchy).
> >
> > One of two main goals of this patchset is to introduce cgroup-level
> > fairness: bigger cgroups should be affected more than smaller,
> > despite the size of tasks inside. I believe the same principle
> > should be used for cgroups.
>
> Yes bigger cgroups should be preferred but I fail to see why bigger
> hierarchies should be considered as well if they are not kill-all. And
> whether non-leaf memcgs should allow kill-all is not entirely clear to
> me. What would be the usecase?

We definitely want to support kill-all for non-leaf cgroups.
A workload can consist of several cgroups and we want to clean up
the whole thing on OOM. I don't see any reasons to limit
this functionality to leaf cgroups only.

Hierarchies are memory consumers, we do account their usage,
we do apply limits and guarantees for the hierarchies. The same is
with OOM victim selection: we are reclaiming memory from the
biggest consumer. Kill-all knob only defines the way _how_ we do that:
by killing one or all processes.

Just for example, we might want to take memory.low into account at
some point: prefer cgroups which are above their guarantees, avoid
killing those who fit. It would be hard if we're comparing cgroups
from different hierarchies. The same will be with introducing
oom_priorities, which is much more required functionality.

> Consider that it might be not your choice (as a user) how deep is your
> leaf memcg. I can already see how people complain that their memcg has
> been killed just because it was one level deeper in the hierarchy...

The kill-all functionality is enforced by parent, and it seems to be
following the overall memcg design. The parent cgroup enforces memory
limit, memory low limit, etc.

I don't know why OOM control should be different.

>
> I would really start simple and only allow kill-all on leaf memcgs and
> only compare leaf memcgs & root. If we ever need to kill whole
> hierarchies then allow kill-all on intermediate memcgs as well and then
> consider cumulative consumptions only on those that have kill-all
> enabled.

This sounds hacky to me: the whole thing is depending on cgroup v2 and
is additionally explicitly opt-in.

Why do we need to introduce such incomplete functionality first,
and then suffer trying to extend it and provide backward compatibility?

Also, I think we should compare root cgroup with top-level cgroups,
rather than leaf cgroups. A process in the root cgroup is definitely
system-level entity, and we should compare it with other top-level
entities (other containerized workloads), rather then some random
leaf cgroup deep inside the tree. If we decided, that we're not comparing
random tasks from different cgroups, why should we do this for leaf
cgroups? Is sounds like making only one step towards right direction,
while we can do more.

>
> Or do I miss any reasonable usecase that would suffer from such a
> semantic?

Kill-all for sub-trees is definitely required.
Enforcing oom_priorities for sub-trees is something that I would expect
very useful too. Comparing leaf cgroups system-wide instead of processes
doesn't sound good for me, we're lacking hierarchical fairness, which
was one of two goals of this patchset.

Thanks!

2017-09-14 20:07:37

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Thu, 14 Sep 2017, Michal Hocko wrote:

> > It is certainly possible to add oom priorities on top before it is merged,
> > but I don't see why it isn't part of the patchset.
>
> Because the semantic of the priority for non-leaf memcgs is not fully
> clear and I would rather have the core of the functionality merged
> before this is sorted out.
>

We can't merge the core of the feature before this is sorted out because
then users start to depend on behavior and we must be backwards
compatible. We need a full patchset that introduces the new selection
heuristic and a way for userspace to control it to either bias or prefer
one cgroup over another. The kill-all mechanism is a more orthogonal
feature for the cgroup-aware oom killer than oom priorities.

I have a usecase for both the cgroup-aware oom killer and its oom
priorities from previous versions of this patchset, I assume that Roman
does as well, and would like to see it merged bacause there are real-world
usecases for it rather than hypothetical usecases that would want to do
something different.

> > We need it before its
> > merged to avoid users playing with /proc/pid/oom_score_adj to prevent any
> > killing in the most preferable memcg when they could have simply changed
> > the oom priority.
>
> I am sorry but I do not really understand your concern. Are you
> suggesting that users would start oom disable all tasks in a memcg to
> give it a higher priority? Even if that was the case why should such an
> abuse be a blocker for generic memcg aware oom killer being merged?

If users do not have any way to control victim selection because of a
shortcoming in the kernel implementation, they will be required to oom
disable processes and let that be inherited by children they fork in the
memcg hierarchy to protect cgroups that they do not want to be oom killed,
regardless of their size. They simply are left with no other alternative
if they want to use the cgroup-aware oom killer and/or the kill-all
mechanism.

2017-09-15 10:58:34

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Thu 14-09-17 09:05:48, Roman Gushchin wrote:
> On Thu, Sep 14, 2017 at 03:40:14PM +0200, Michal Hocko wrote:
> > On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> > > On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> > [...]
> > > > I strongly believe that comparing only leaf memcgs
> > > > is more straightforward and it doesn't lead to unexpected results as
> > > > mentioned before (kill a small memcg which is a part of the larger
> > > > sub-hierarchy).
> > >
> > > One of two main goals of this patchset is to introduce cgroup-level
> > > fairness: bigger cgroups should be affected more than smaller,
> > > despite the size of tasks inside. I believe the same principle
> > > should be used for cgroups.
> >
> > Yes bigger cgroups should be preferred but I fail to see why bigger
> > hierarchies should be considered as well if they are not kill-all. And
> > whether non-leaf memcgs should allow kill-all is not entirely clear to
> > me. What would be the usecase?
>
> We definitely want to support kill-all for non-leaf cgroups.
> A workload can consist of several cgroups and we want to clean up
> the whole thing on OOM.

Could you be more specific about such a workload? E.g. how can be such a
hierarchy handled consistently when its sub-tree gets killed due to
internal memory pressure? Or do you expect that none of the subtree will
have hard limit configured?

> I don't see any reasons to limit this functionality to leaf cgroups
> only.

Well, I wanted to start simple first and extend on top. Memcg v1 is full
of seemingly interesting and very generic concepts which turned out
being a headache long term.

> Hierarchies are memory consumers, we do account their usage,
> we do apply limits and guarantees for the hierarchies. The same is
> with OOM victim selection: we are reclaiming memory from the
> biggest consumer. Kill-all knob only defines the way _how_ we do that:
> by killing one or all processes.

But then you just enforce a structural restriction on your configuration
because
root
/ \
A D
/\
B C

is a different thing than
root
/ | \
B C D

And consider that the sole purpose of A might be a control over
a non-memory resource (e.g. a cpu share distribution). Why should we
discriminate B and C in such a case?

> Just for example, we might want to take memory.low into account at
> some point: prefer cgroups which are above their guarantees, avoid
> killing those who fit. It would be hard if we're comparing cgroups
> from different hierarchies.

This can be reflected in the memcg oom score calculation I believe. We
already do something similar during the reclaim.

> The same will be with introducing oom_priorities, which is much more
> required functionality.

More on that below.

> > Consider that it might be not your choice (as a user) how deep is your
> > leaf memcg. I can already see how people complain that their memcg has
> > been killed just because it was one level deeper in the hierarchy...
>
> The kill-all functionality is enforced by parent, and it seems to be
> following the overall memcg design. The parent cgroup enforces memory
> limit, memory low limit, etc.

And the same is true for the memcg oom killer. It enforces the selection
to the out-of-memory subtree. We are trying to be proportional on the
size of the _reclaimable_ memory in that scope. Same way as with the
LRU reclaim. We do not prefer larger hierarchies over smaller. We just
iterate over those that have pages on LRUs (leaf memcgs with v2) and
scan/reclaim proportionally to their size. Why should the oom killer
decision be any different in that regards?

> I don't know why OOM control should be different.

I am not arguing that kill-all functionality on non-leaf is wrong. I
just haven't heard the usecase for it yet. I am also not opposed to
consider the cumulative size of non-leaf memcg if it is kill-all as the
cumulative size will be reclaimed then. But I fail to see why we should
prefer larger hierarchies when the resulting memcg victim is much
smaller in the end.

> > I would really start simple and only allow kill-all on leaf memcgs and
> > only compare leaf memcgs & root. If we ever need to kill whole
> > hierarchies then allow kill-all on intermediate memcgs as well and then
> > consider cumulative consumptions only on those that have kill-all
> > enabled.
>
> This sounds hacky to me: the whole thing is depending on cgroup v2 and
> is additionally explicitly opt-in.
>
> Why do we need to introduce such incomplete functionality first,
> and then suffer trying to extend it and provide backward compatibility?

Why would a backward compatibility be a problem? kill-all on non-leaf
memcgs should be seamless. We would simply allow setting the knob. Adding
a priority shouldn't be a problem either. A new knob would be added. Any
memcg with a non-zero priority would be considered during selection for
example (cumulative size would be considered as a tie-breaker for
non-leaf memcgs and a victim selected from the largest hierarchy/leaf
memcg - but that really needs to be thought through and hear about
specific usecases).

> Also, I think we should compare root cgroup with top-level cgroups,
> rather than leaf cgroups. A process in the root cgroup is definitely
> system-level entity, and we should compare it with other top-level
> entities (other containerized workloads), rather then some random
> leaf cgroup deep inside the tree. If we decided, that we're not comparing
> random tasks from different cgroups, why should we do this for leaf
> cgroups? Is sounds like making only one step towards right direction,
> while we can do more.

The main problem I have with that is mentioned above. A single hierarchy
enforces some structural constrains when multiple controllers are in
place.
--
Michal Hocko
SUSE Labs

2017-09-15 15:23:33

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
> On Thu 14-09-17 09:05:48, Roman Gushchin wrote:
> > On Thu, Sep 14, 2017 at 03:40:14PM +0200, Michal Hocko wrote:
> > > On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> > > > On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> > > [...]
> > > > > I strongly believe that comparing only leaf memcgs
> > > > > is more straightforward and it doesn't lead to unexpected results as
> > > > > mentioned before (kill a small memcg which is a part of the larger
> > > > > sub-hierarchy).
> > > >
> > > > One of two main goals of this patchset is to introduce cgroup-level
> > > > fairness: bigger cgroups should be affected more than smaller,
> > > > despite the size of tasks inside. I believe the same principle
> > > > should be used for cgroups.
> > >
> > > Yes bigger cgroups should be preferred but I fail to see why bigger
> > > hierarchies should be considered as well if they are not kill-all. And
> > > whether non-leaf memcgs should allow kill-all is not entirely clear to
> > > me. What would be the usecase?
> >
> > We definitely want to support kill-all for non-leaf cgroups.
> > A workload can consist of several cgroups and we want to clean up
> > the whole thing on OOM.
>
> Could you be more specific about such a workload? E.g. how can be such a
> hierarchy handled consistently when its sub-tree gets killed due to
> internal memory pressure?

Or just system-wide OOM.

> Or do you expect that none of the subtree will
> have hard limit configured?

And this can also be a case: the whole workload may have hard limit
configured, while internal memcgs have only memory.low set for "soft"
prioritization.

>
> But then you just enforce a structural restriction on your configuration
> because
> root
> / \
> A D
> /\
> B C
>
> is a different thing than
> root
> / | \
> B C D
>

I actually don't have a strong argument against an approach to select
largest leaf or kill-all-set memcg. I think, in practice there will be
no much difference.

The only real concern I have is that then we have to do the same with
oom_priorities (select largest priority tree-wide), and this will limit
an ability to enforce the priority by parent cgroup.

Thanks!

2017-09-15 19:55:59

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri, 15 Sep 2017, Roman Gushchin wrote:

> > But then you just enforce a structural restriction on your configuration
> > because
> > root
> > / \
> > A D
> > /\
> > B C
> >
> > is a different thing than
> > root
> > / | \
> > B C D
> >
>
> I actually don't have a strong argument against an approach to select
> largest leaf or kill-all-set memcg. I think, in practice there will be
> no much difference.
>
> The only real concern I have is that then we have to do the same with
> oom_priorities (select largest priority tree-wide), and this will limit
> an ability to enforce the priority by parent cgroup.
>

Yes, oom_priority cannot select the largest priority tree-wide for exactly
that reason. We need the ability to control from which subtree the kill
occurs in ancestor cgroups. If multiple jobs are allocated their own
cgroups and they can own memory.oom_priority for their own subcontainers,
this becomes quite powerful so they can define their own oom priorities.
Otherwise, they can easily override the oom priorities of other cgroups.

2017-09-15 21:08:35

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri, Sep 15, 2017 at 12:55:55PM -0700, David Rientjes wrote:
> On Fri, 15 Sep 2017, Roman Gushchin wrote:
>
> > > But then you just enforce a structural restriction on your configuration
> > > because
> > > root
> > > / \
> > > A D
> > > /\
> > > B C
> > >
> > > is a different thing than
> > > root
> > > / | \
> > > B C D
> > >
> >
> > I actually don't have a strong argument against an approach to select
> > largest leaf or kill-all-set memcg. I think, in practice there will be
> > no much difference.
> >
> > The only real concern I have is that then we have to do the same with
> > oom_priorities (select largest priority tree-wide), and this will limit
> > an ability to enforce the priority by parent cgroup.
> >
>
> Yes, oom_priority cannot select the largest priority tree-wide for exactly
> that reason. We need the ability to control from which subtree the kill
> occurs in ancestor cgroups. If multiple jobs are allocated their own
> cgroups and they can own memory.oom_priority for their own subcontainers,
> this becomes quite powerful so they can define their own oom priorities.
> Otherwise, they can easily override the oom priorities of other cgroups.

I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
the oom_priority below parent's value, or something like this.

But it looks more complex, and I'm not sure there are real examples,
when we have to compare memcgs, which are on different levels
(or in different subtrees).

In any case, oom_priorities and size-based comparison should share the
same tree-walking policy. And I still would prefer comparing sizes and
priorities independently on each level.

Thanks!

2017-09-18 06:14:15

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri 15-09-17 08:23:01, Roman Gushchin wrote:
> On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
> > On Thu 14-09-17 09:05:48, Roman Gushchin wrote:
> > > On Thu, Sep 14, 2017 at 03:40:14PM +0200, Michal Hocko wrote:
> > > > On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> > > > > On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> > > > [...]
> > > > > > I strongly believe that comparing only leaf memcgs
> > > > > > is more straightforward and it doesn't lead to unexpected results as
> > > > > > mentioned before (kill a small memcg which is a part of the larger
> > > > > > sub-hierarchy).
> > > > >
> > > > > One of two main goals of this patchset is to introduce cgroup-level
> > > > > fairness: bigger cgroups should be affected more than smaller,
> > > > > despite the size of tasks inside. I believe the same principle
> > > > > should be used for cgroups.
> > > >
> > > > Yes bigger cgroups should be preferred but I fail to see why bigger
> > > > hierarchies should be considered as well if they are not kill-all. And
> > > > whether non-leaf memcgs should allow kill-all is not entirely clear to
> > > > me. What would be the usecase?
> > >
> > > We definitely want to support kill-all for non-leaf cgroups.
> > > A workload can consist of several cgroups and we want to clean up
> > > the whole thing on OOM.
> >
> > Could you be more specific about such a workload? E.g. how can be such a
> > hierarchy handled consistently when its sub-tree gets killed due to
> > internal memory pressure?
>
> Or just system-wide OOM.
>
> > Or do you expect that none of the subtree will
> > have hard limit configured?
>
> And this can also be a case: the whole workload may have hard limit
> configured, while internal memcgs have only memory.low set for "soft"
> prioritization.
>
> >
> > But then you just enforce a structural restriction on your configuration
> > because
> > root
> > / \
> > A D
> > /\
> > B C
> >
> > is a different thing than
> > root
> > / | \
> > B C D
> >
>
> I actually don't have a strong argument against an approach to select
> largest leaf or kill-all-set memcg. I think, in practice there will be
> no much difference.

Well, I am worried that the difference will come unexpected when a
deeper hierarchy is needed because of the structural needs.

--
Michal Hocko
SUSE Labs

2017-09-18 06:16:09

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri 15-09-17 12:55:55, David Rientjes wrote:
> On Fri, 15 Sep 2017, Roman Gushchin wrote:
>
> > > But then you just enforce a structural restriction on your configuration
> > > because
> > > root
> > > / \
> > > A D
> > > /\
> > > B C
> > >
> > > is a different thing than
> > > root
> > > / | \
> > > B C D
> > >
> >
> > I actually don't have a strong argument against an approach to select
> > largest leaf or kill-all-set memcg. I think, in practice there will be
> > no much difference.
> >
> > The only real concern I have is that then we have to do the same with
> > oom_priorities (select largest priority tree-wide), and this will limit
> > an ability to enforce the priority by parent cgroup.
> >
>
> Yes, oom_priority cannot select the largest priority tree-wide for exactly
> that reason. We need the ability to control from which subtree the kill
> occurs in ancestor cgroups. If multiple jobs are allocated their own
> cgroups and they can own memory.oom_priority for their own subcontainers,
> this becomes quite powerful so they can define their own oom priorities.
> Otherwise, they can easily override the oom priorities of other cgroups.

Could you be more speicific about your usecase? What would be a
problem If we allow to only increase priority in children (like other
hierarchical controls).

--
Michal Hocko
SUSE Labs

2017-09-18 06:20:58

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri 15-09-17 14:08:07, Roman Gushchin wrote:
> On Fri, Sep 15, 2017 at 12:55:55PM -0700, David Rientjes wrote:
> > On Fri, 15 Sep 2017, Roman Gushchin wrote:
> >
> > > > But then you just enforce a structural restriction on your configuration
> > > > because
> > > > root
> > > > / \
> > > > A D
> > > > /\
> > > > B C
> > > >
> > > > is a different thing than
> > > > root
> > > > / | \
> > > > B C D
> > > >
> > >
> > > I actually don't have a strong argument against an approach to select
> > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > no much difference.
> > >
> > > The only real concern I have is that then we have to do the same with
> > > oom_priorities (select largest priority tree-wide), and this will limit
> > > an ability to enforce the priority by parent cgroup.
> > >
> >
> > Yes, oom_priority cannot select the largest priority tree-wide for exactly
> > that reason. We need the ability to control from which subtree the kill
> > occurs in ancestor cgroups. If multiple jobs are allocated their own
> > cgroups and they can own memory.oom_priority for their own subcontainers,
> > this becomes quite powerful so they can define their own oom priorities.
> > Otherwise, they can easily override the oom priorities of other cgroups.
>
> I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
> the oom_priority below parent's value, or something like this.

As said in other email. We can make priorities hierarchical (in the same
sense as hard limit or others) so that children cannot override their
parent.

> But it looks more complex, and I'm not sure there are real examples,
> when we have to compare memcgs, which are on different levels
> (or in different subtrees).

Well, I have given you one that doesn't sounds completely insane to me
in other email. You may need an intermediate level for other than memcg
controller. The whole concept of significance of the hierarchy level
seems really odd to me. Or am I wrong here?
--
Michal Hocko
SUSE Labs

2017-09-18 15:03:26

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, Sep 18, 2017 at 08:20:45AM +0200, Michal Hocko wrote:
> On Fri 15-09-17 14:08:07, Roman Gushchin wrote:
> > On Fri, Sep 15, 2017 at 12:55:55PM -0700, David Rientjes wrote:
> > > On Fri, 15 Sep 2017, Roman Gushchin wrote:
> > >
> > > > > But then you just enforce a structural restriction on your configuration
> > > > > because
> > > > > root
> > > > > / \
> > > > > A D
> > > > > /\
> > > > > B C
> > > > >
> > > > > is a different thing than
> > > > > root
> > > > > / | \
> > > > > B C D
> > > > >
> > > >
> > > > I actually don't have a strong argument against an approach to select
> > > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > > no much difference.
> > > >
> > > > The only real concern I have is that then we have to do the same with
> > > > oom_priorities (select largest priority tree-wide), and this will limit
> > > > an ability to enforce the priority by parent cgroup.
> > > >
> > >
> > > Yes, oom_priority cannot select the largest priority tree-wide for exactly
> > > that reason. We need the ability to control from which subtree the kill
> > > occurs in ancestor cgroups. If multiple jobs are allocated their own
> > > cgroups and they can own memory.oom_priority for their own subcontainers,
> > > this becomes quite powerful so they can define their own oom priorities.
> > > Otherwise, they can easily override the oom priorities of other cgroups.
> >
> > I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
> > the oom_priority below parent's value, or something like this.
>
> As said in other email. We can make priorities hierarchical (in the same
> sense as hard limit or others) so that children cannot override their
> parent.

You mean they can set the knob to any value, but parent's value is enforced,
if it's greater than child's value?

If so, this sounds logical to me. Then we have size-based comparison and
priority-based comparison with similar rules, and all use cases are covered.

Ok, can we stick with this design?
Then I'll return oom_priorities in place, and post a (hopefully) final version.

Thanks!

2017-09-19 20:51:31

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, 18 Sep 2017, Michal Hocko wrote:

> > > > But then you just enforce a structural restriction on your configuration
> > > > because
> > > > root
> > > > / \
> > > > A D
> > > > /\
> > > > B C
> > > >
> > > > is a different thing than
> > > > root
> > > > / | \
> > > > B C D
> > > >
> > >
> > > I actually don't have a strong argument against an approach to select
> > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > no much difference.
> > >
> > > The only real concern I have is that then we have to do the same with
> > > oom_priorities (select largest priority tree-wide), and this will limit
> > > an ability to enforce the priority by parent cgroup.
> > >
> >
> > Yes, oom_priority cannot select the largest priority tree-wide for exactly
> > that reason. We need the ability to control from which subtree the kill
> > occurs in ancestor cgroups. If multiple jobs are allocated their own
> > cgroups and they can own memory.oom_priority for their own subcontainers,
> > this becomes quite powerful so they can define their own oom priorities.
> > Otherwise, they can easily override the oom priorities of other cgroups.
>
> Could you be more speicific about your usecase? What would be a
> problem If we allow to only increase priority in children (like other
> hierarchical controls).
>

For memcg constrained oom conditions, there is only a theoretical issue if
the subtree is not under the control of a single user and various users
can alter their priorities without knowledge of the priorities of other
children in the same subtree that is oom, or those values change without
knowledge of a child. I don't know of anybody that configures memory
cgroup hierarchies that way, though.

The problem is more obvious in system oom conditions. If we have two
top-level memory cgroups with the same "job" priority, they get the same
oom priority. The user who configures subcontainers is now always
targeted for oom kill in an "increase priority in children" policy.

The hierarchy becomes this:

root
/ \
A D
/ \ / | \
B C E F G

where A/memory.oom_priority == D/memory.oom_priority.

D wants to kill in order of E -> F -> G, but can't configure that if
B = A - 1 and C = B - 1. It also shouldn't need to adjust its own oom
priorities based on a hierarchy outside its control and which can change
at any time at the discretion of the user (with namespaces you may not
even be able to access it).

But also if A/memory.oom_priority = D/memory.oom_priority - 100, A is
preferred unless its subcontainers configure themselves in a way where
they have higher oom priority values than E, F, and G. That may yield
very different results when additional jobs get scheduled on the system
(and H tree) where the user has full control over their own oom
priorities, even when the value must only increase.

2017-09-19 20:54:56

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri, 15 Sep 2017, Roman Gushchin wrote:

> > > > But then you just enforce a structural restriction on your configuration
> > > > because
> > > > root
> > > > / \
> > > > A D
> > > > /\
> > > > B C
> > > >
> > > > is a different thing than
> > > > root
> > > > / | \
> > > > B C D
> > > >
> > >
> > > I actually don't have a strong argument against an approach to select
> > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > no much difference.
> > >
> > > The only real concern I have is that then we have to do the same with
> > > oom_priorities (select largest priority tree-wide), and this will limit
> > > an ability to enforce the priority by parent cgroup.
> > >
> >
> > Yes, oom_priority cannot select the largest priority tree-wide for exactly
> > that reason. We need the ability to control from which subtree the kill
> > occurs in ancestor cgroups. If multiple jobs are allocated their own
> > cgroups and they can own memory.oom_priority for their own subcontainers,
> > this becomes quite powerful so they can define their own oom priorities.
> > Otherwise, they can easily override the oom priorities of other cgroups.
>
> I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
> the oom_priority below parent's value, or something like this.
>
> But it looks more complex, and I'm not sure there are real examples,
> when we have to compare memcgs, which are on different levels
> (or in different subtrees).
>

It's actually much more complex because in our environment we'd need an
"activity manager" with CAP_SYS_RESOURCE to control oom priorities of user
subcontainers when today it need only be concerned with top-level memory
cgroups. Users can create their own hierarchies with their own oom
priorities at will, it doesn't alter the selection heuristic for another
other user running on the same system and gives them full control over the
selection in their own subtree. We shouldn't need to have a system-wide
daemon with CAP_SYS_RESOURCE be required to manage subcontainers when
nothing else requires it. I believe it's also much easier to document:
oom_priority is considered for all sibling cgroups at each level of the
hierarchy and the cgroup with the lowest priority value gets iterated.

2017-09-20 21:54:11

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, Sep 18, 2017 at 08:14:05AM +0200, Michal Hocko wrote:
> On Fri 15-09-17 08:23:01, Roman Gushchin wrote:
> > On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
> > > On Thu 14-09-17 09:05:48, Roman Gushchin wrote:
> > > > On Thu, Sep 14, 2017 at 03:40:14PM +0200, Michal Hocko wrote:
> > > > > On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> > > > > > On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> > > > > [...]
> > > > > > > I strongly believe that comparing only leaf memcgs
> > > > > > > is more straightforward and it doesn't lead to unexpected results as
> > > > > > > mentioned before (kill a small memcg which is a part of the larger
> > > > > > > sub-hierarchy).
> > > > > >
> > > > > > One of two main goals of this patchset is to introduce cgroup-level
> > > > > > fairness: bigger cgroups should be affected more than smaller,
> > > > > > despite the size of tasks inside. I believe the same principle
> > > > > > should be used for cgroups.
> > > > >
> > > > > Yes bigger cgroups should be preferred but I fail to see why bigger
> > > > > hierarchies should be considered as well if they are not kill-all. And
> > > > > whether non-leaf memcgs should allow kill-all is not entirely clear to
> > > > > me. What would be the usecase?
> > > >
> > > > We definitely want to support kill-all for non-leaf cgroups.
> > > > A workload can consist of several cgroups and we want to clean up
> > > > the whole thing on OOM.
> > >
> > > Could you be more specific about such a workload? E.g. how can be such a
> > > hierarchy handled consistently when its sub-tree gets killed due to
> > > internal memory pressure?
> >
> > Or just system-wide OOM.
> >
> > > Or do you expect that none of the subtree will
> > > have hard limit configured?
> >
> > And this can also be a case: the whole workload may have hard limit
> > configured, while internal memcgs have only memory.low set for "soft"
> > prioritization.
> >
> > >
> > > But then you just enforce a structural restriction on your configuration
> > > because
> > > root
> > > / \
> > > A D
> > > /\
> > > B C
> > >
> > > is a different thing than
> > > root
> > > / | \
> > > B C D
> > >
> >
> > I actually don't have a strong argument against an approach to select
> > largest leaf or kill-all-set memcg. I think, in practice there will be
> > no much difference.

I've tried to implement this approach, and it's really arguable.
Although your example looks reasonable, the opposite example is also valid:
you might want to compare whole hierarchies, and it's a quite typical usecase.

Assume, you have several containerized workloads on a machine (probably,
each will be contained in a memcg with memory.max set), with some hierarchy
of cgroups inside. Then in case of global memory shortage we want to reclaim
some memory from the biggest workload, and the selection should not depend
on group_oom settings. It would be really strange, if setting group_oom will
higher the chances to be killed.

In other words, let's imagine processes as leaf nodes in memcg tree. We decided
to select the biggest memcg and kill one or more processes inside (depending
on group_oom setting), but the memcg selection doesn't depend on it.
We do not compare processes from different cgroups, as well as cgroups with
processes. The same should apply to cgroups: why do we want to compare cgroups
from different sub-trees?

While size-based comparison can be implemented with this approach,
the priority-based is really weird (as David mentioned).
If priorities have no hierarchical meaning at all, we lack the very important
ability to enforce hierarchy oom_priority. Otherwise we have to invent some
complex rules of oom_priority propagation (e.g. is someone is raising
the oom_priority in parent, should it be applied to children immediately, etc).

The oom_group knob meaning also becoms more complex. It affects both
the victim selection and OOM action. _ANY_ mechanism which allows to affect
OOM victim selection (either priorities, either bpf-based approach) should
not have global system-wide meaning, it breaks everything.

I do understand your point, but the same is true for other stuff, right?
E.g. cpu time distribution (and io, etc) depends on hierarchy configuration.
It's a limitation, but it's ok, as user should create a hierarchy which
reflects some logical relations between processes and groups of processes.
Otherwise we're going to the configuration hell.

In any case, OOM is a last resort mechanism. The goal is to reclaim some memory
and do not crash the system or do not leave it in totally broken state.
Any really complex mm in userspace should be applied _before_ OOM happens.
So, I don't think we have to support all possible configurations here,
if we're able to achieve the main goal (kill some processes and do not leave
broken systems/containers).

2017-09-20 22:24:37

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue, Sep 19, 2017 at 01:54:48PM -0700, David Rientjes wrote:
> On Fri, 15 Sep 2017, Roman Gushchin wrote:
>
> > > > > But then you just enforce a structural restriction on your configuration
> > > > > because
> > > > > root
> > > > > / \
> > > > > A D
> > > > > /\
> > > > > B C
> > > > >
> > > > > is a different thing than
> > > > > root
> > > > > / | \
> > > > > B C D
> > > > >
> > > >
> > > > I actually don't have a strong argument against an approach to select
> > > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > > no much difference.
> > > >
> > > > The only real concern I have is that then we have to do the same with
> > > > oom_priorities (select largest priority tree-wide), and this will limit
> > > > an ability to enforce the priority by parent cgroup.
> > > >
> > >
> > > Yes, oom_priority cannot select the largest priority tree-wide for exactly
> > > that reason. We need the ability to control from which subtree the kill
> > > occurs in ancestor cgroups. If multiple jobs are allocated their own
> > > cgroups and they can own memory.oom_priority for their own subcontainers,
> > > this becomes quite powerful so they can define their own oom priorities.
> > > Otherwise, they can easily override the oom priorities of other cgroups.
> >
> > I believe, it's a solvable problem: we can require CAP_SYS_RESOURCE to set
> > the oom_priority below parent's value, or something like this.
> >
> > But it looks more complex, and I'm not sure there are real examples,
> > when we have to compare memcgs, which are on different levels
> > (or in different subtrees).
> >
>
> It's actually much more complex because in our environment we'd need an
> "activity manager" with CAP_SYS_RESOURCE to control oom priorities of user
> subcontainers when today it need only be concerned with top-level memory
> cgroups. Users can create their own hierarchies with their own oom
> priorities at will, it doesn't alter the selection heuristic for another
> other user running on the same system and gives them full control over the
> selection in their own subtree. We shouldn't need to have a system-wide
> daemon with CAP_SYS_RESOURCE be required to manage subcontainers when
> nothing else requires it. I believe it's also much easier to document:
> oom_priority is considered for all sibling cgroups at each level of the
> hierarchy and the cgroup with the lowest priority value gets iterated.

I do agree actually. System-wide OOM priorities make no sense.

Always compare sibling cgroups, either by priority or size, seems to be
simple, clear and powerful enough for all reasonable use cases. Am I right,
that it's exactly what you've used internally? This is a perfect confirmation,
I believe.

Thanks!

2017-09-21 08:27:35

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed, 20 Sep 2017, Roman Gushchin wrote:

> > It's actually much more complex because in our environment we'd need an
> > "activity manager" with CAP_SYS_RESOURCE to control oom priorities of user
> > subcontainers when today it need only be concerned with top-level memory
> > cgroups. Users can create their own hierarchies with their own oom
> > priorities at will, it doesn't alter the selection heuristic for another
> > other user running on the same system and gives them full control over the
> > selection in their own subtree. We shouldn't need to have a system-wide
> > daemon with CAP_SYS_RESOURCE be required to manage subcontainers when
> > nothing else requires it. I believe it's also much easier to document:
> > oom_priority is considered for all sibling cgroups at each level of the
> > hierarchy and the cgroup with the lowest priority value gets iterated.
>
> I do agree actually. System-wide OOM priorities make no sense.
>
> Always compare sibling cgroups, either by priority or size, seems to be
> simple, clear and powerful enough for all reasonable use cases. Am I right,
> that it's exactly what you've used internally? This is a perfect confirmation,
> I believe.
>

We've used it for at least four years, I added my Tested-by to your patch,
we would convert to your implementation if it is merged upstream, and I
would enthusiastically support your patch if you would integrate it back
into your series.

2017-09-21 08:30:49

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, 18 Sep 2017, Roman Gushchin wrote:

> > As said in other email. We can make priorities hierarchical (in the same
> > sense as hard limit or others) so that children cannot override their
> > parent.
>
> You mean they can set the knob to any value, but parent's value is enforced,
> if it's greater than child's value?
>
> If so, this sounds logical to me. Then we have size-based comparison and
> priority-based comparison with similar rules, and all use cases are covered.
>
> Ok, can we stick with this design?
> Then I'll return oom_priorities in place, and post a (hopefully) final version.
>

I just want to make sure that we are going with your original
implementation here: that oom_priority is only effective for compare
sibling memory cgroups and nothing beyond that. The value alone has no
relationship to any ancestor. We can't set oom_priority based on the
priorities of any other memory cgroups other than our own siblings because
we have no control over how those change.

2017-09-21 14:21:24

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, Sep 11, 2017 at 01:44:39PM -0700, David Rientjes wrote:
> On Mon, 11 Sep 2017, Roman Gushchin wrote:
>
> > This patchset makes the OOM killer cgroup-aware.
> >
> > v8:
> > - Do not kill tasks with OOM_SCORE_ADJ -1000
> > - Make the whole thing opt-in with cgroup mount option control
> > - Drop oom_priority for further discussions
>
> Nack, we specifically require oom_priority for this to function correctly,
> otherwise we cannot prefer to kill from low priority leaf memcgs as
> required. v8 appears to implement new functionality that we want, to
> compare two memcgs based on usage, but without the ability to influence
> that decision to protect important userspace, so now I'm in a position
> where (1) nothing has changed if I don't use the new mount option or (2) I
> get completely different oom kill selection with the new mount option but
> not the ability to influence it. I was much happier with the direction
> that v7 was taking, but since v8 causes us to regress without the ability
> to change memcg priority, this has to be nacked.

That's a ridiculous nak.

The fact that this patch series doesn't solve your particular problem
is not a technical argument to *reject* somebody else's work to solve
a different problem. It's not a regression when behavior is completely
unchanged unless you explicitly opt into a new functionality.

So let's stay reasonable here.

The patch series has merit as it currently stands. It makes OOM
killing in a cgrouped system fairer and less surprising. Whether you
have the ability to influence this in a new way is an entirely
separate discussion. It's one that involves ABI and user guarantees.

Right now Roman's patches make no guarantees on how the cgroup tree is
descended. But once we define an interface for prioritization, it
locks the victim algorithm into place to a certain extent.

It also involves a discussion about how much control userspace should
have over OOM killing in the first place. It's a last-minute effort to
save the kernel from deadlocking on memory. Whether that is the time
and place to have userspace make clever resource management decisions
is an entirely different thing than what Roman is doing.

But this patch series doesn't prevent any such future discussion and
implementations, and it's not useless without it. So let's not
conflate these two things, and hold the priority patch for now.

Thanks.

2017-09-21 21:17:30

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Thu, 21 Sep 2017, Johannes Weiner wrote:

> That's a ridiculous nak.
>
> The fact that this patch series doesn't solve your particular problem
> is not a technical argument to *reject* somebody else's work to solve
> a different problem. It's not a regression when behavior is completely
> unchanged unless you explicitly opt into a new functionality.
>
> So let's stay reasonable here.
>

The issue is that if you opt-in to the new feature, then you are forced to
change /proc/pid/oom_score_adj of all processes attached to a cgroup that
you do not want oom killed based on size to be oom disabled. The kernel
provides no other remedy without oom priorities since the new feature
would otherwise disregard oom_score_adj. In that case, userspace is
racing in two ways: (1) attach of process to a memcg you want to protect
from oom kill (first class, vital, large memory hog job) to set to oom
disable and (2) adjustment of other cgroups to make them eligible after
first oom kill.

It doesn't have anything to do with my particular usecase, but rather the
ability of userspace to influence the decisions of the kernel. Previous
to this patchset, when selection is done based on process size, userspace
has full control over selection. After this patchset, userspace has no
control other than setting all processes to be oom disabled if the largest
memory consumer is to be protected. Roman's memory.oom_priority provides
a perfect solution for userspace to be able to influence this decision
making and causes no change in behavior for users who choose not to tune
memory.oom_priority. The nack originates from the general need for
userspace influence over oom victim selection and to avoid userspace
needing to take the rather drastic measure of setting all processes to be
oom disabled to prevent oom kill in kernels before oom priorities are
introduced.

> The patch series has merit as it currently stands. It makes OOM
> killing in a cgrouped system fairer and less surprising. Whether you
> have the ability to influence this in a new way is an entirely
> separate discussion. It's one that involves ABI and user guarantees.
>
> Right now Roman's patches make no guarantees on how the cgroup tree is
> descended. But once we define an interface for prioritization, it
> locks the victim algorithm into place to a certain extent.
>

The patchset compares memory cgroup size relative to sibling cgroups only,
the same comparison for memory.oom_priority. There is a guarantee
provided on how cgroup size is compared in select_victim_memcg(), it
hierarchically accumulates the "size" from leaf nodes up to the root memcg
and then iterates the tree comparing sizes between sibling cgroups to
choose a victim memcg. That algorithm could be more elaborately described
in the documentation, but we simply cannot change the implementation of
select_victim_memcg() later even without oom priorities since users cannot
get inconsistent results after opting into a feature between kernel
versions. I believe the selection criteria should be implemented to be
deterministic, as select_victim_memcg() does, and the documentation should
fully describe what the selection criteria is, and then allow the user to
decide.

> It also involves a discussion about how much control userspace should
> have over OOM killing in the first place. It's a last-minute effort to
> save the kernel from deadlocking on memory. Whether that is the time
> and place to have userspace make clever resource management decisions
> is an entirely different thing than what Roman is doing.
>
> But this patch series doesn't prevent any such future discussion and
> implementations, and it's not useless without it. So let's not
> conflate these two things, and hold the priority patch for now.
>

Roman is planning on introducing memory.oom_priority back into the
patchset per https://marc.info/?l=linux-kernel&m=150574701126877 and I
agree with the very clear semantic that it introduces: to have the
size-based comparison use the same rules as the userspace priority
comparison. It's very powerful and I'm happy to ack the final version
that he plans on posting.

2017-09-21 21:51:17

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Thu, Sep 21, 2017 at 02:17:25PM -0700, David Rientjes wrote:
> On Thu, 21 Sep 2017, Johannes Weiner wrote:
>
> > That's a ridiculous nak.
> >
> > The fact that this patch series doesn't solve your particular problem
> > is not a technical argument to *reject* somebody else's work to solve
> > a different problem. It's not a regression when behavior is completely
> > unchanged unless you explicitly opt into a new functionality.
> >
> > So let's stay reasonable here.
> >
>
> The issue is that if you opt-in to the new feature, then you are forced to
> change /proc/pid/oom_score_adj of all processes attached to a cgroup that
> you do not want oom killed based on size to be oom disabled.

You're assuming that most people would want to influence the oom
behavior in the first place. I think the opposite is the case: most
people don't care as long as the OOM killer takes the intent the user
has expressed wrt runtime containerization/grouping into account.

> The kernel provides no other remedy without oom priorities since the
> new feature would otherwise disregard oom_score_adj.

As of v8, it respects this setting and doesn't kill min score tasks.

> The nack originates from the general need for userspace influence
> over oom victim selection and to avoid userspace needing to take the
> rather drastic measure of setting all processes to be oom disabled
> to prevent oom kill in kernels before oom priorities are introduced.

As I said, we can discuss this in a separate context. Because again, I
really don't see how the lack of configurability in an opt-in feature
would diminish its value for many people who don't even care to adjust
and influence this behavior.

> > The patch series has merit as it currently stands. It makes OOM
> > killing in a cgrouped system fairer and less surprising. Whether you
> > have the ability to influence this in a new way is an entirely
> > separate discussion. It's one that involves ABI and user guarantees.
> >
> > Right now Roman's patches make no guarantees on how the cgroup tree is
> > descended. But once we define an interface for prioritization, it
> > locks the victim algorithm into place to a certain extent.
> >
>
> The patchset compares memory cgroup size relative to sibling cgroups only,
> the same comparison for memory.oom_priority. There is a guarantee
> provided on how cgroup size is compared in select_victim_memcg(), it
> hierarchically accumulates the "size" from leaf nodes up to the root memcg
> and then iterates the tree comparing sizes between sibling cgroups to
> choose a victim memcg. That algorithm could be more elaborately described
> in the documentation, but we simply cannot change the implementation of
> select_victim_memcg() later even without oom priorities since users cannot
> get inconsistent results after opting into a feature between kernel
> versions. I believe the selection criteria should be implemented to be
> deterministic, as select_victim_memcg() does, and the documentation should
> fully describe what the selection criteria is, and then allow the user to
> decide.

I wholeheartedly disagree. We have changed the behavior multiple times
in the past. In fact, you have arguably done the most drastic changes
to the algorithm since the OOM killer was first introduced. E.g.

a63d83f427fb oom: badness heuristic rewrite

And that's completely fine. Because this thing is not a resource
management tool for userspace, it's the kernel saving itself. At best
in a manner that's not too surprising to userspace.

To me, your argument behind the NAK still boils down to "this doesn't
support my highly specialized usecase." But since it doesn't prohibit
your usecase - which isn't even supported upstream, btw - this really
doesn't carry much weight.

I'd say if you want configurability on top of Roman's code, please
submit patches and push the case for these in a separate effort.

Thanks

2017-09-22 15:44:34

by Tejun Heo

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

Hello, David.

On Thu, Sep 21, 2017 at 02:17:25PM -0700, David Rientjes wrote:
> It doesn't have anything to do with my particular usecase, but rather the
> ability of userspace to influence the decisions of the kernel. Previous
> to this patchset, when selection is done based on process size, userspace
> has full control over selection. After this patchset, userspace has no
> control other than setting all processes to be oom disabled if the largest
> memory consumer is to be protected. Roman's memory.oom_priority provides
> a perfect solution for userspace to be able to influence this decision
> making and causes no change in behavior for users who choose not to tune
> memory.oom_priority. The nack originates from the general need for
> userspace influence over oom victim selection and to avoid userspace
> needing to take the rather drastic measure of setting all processes to be
> oom disabled to prevent oom kill in kernels before oom priorities are
> introduced.

Overall, I think that OOM killing is the wrong place to implement
sophisticated intelligence in. It's too late to be smart - the
workload already has suffered significantly and there's only very
limited amount of computing which can be performed. That said, if
there's a useful and general enough mechanism to configure OOM killer
behavior from userland, that can definitely be useful.

> The patchset compares memory cgroup size relative to sibling cgroups only,
> the same comparison for memory.oom_priority. There is a guarantee
> provided on how cgroup size is compared in select_victim_memcg(), it
> hierarchically accumulates the "size" from leaf nodes up to the root memcg
> and then iterates the tree comparing sizes between sibling cgroups to
> choose a victim memcg. That algorithm could be more elaborately described
> in the documentation, but we simply cannot change the implementation of
> select_victim_memcg() later even without oom priorities since users cannot
> get inconsistent results after opting into a feature between kernel
> versions. I believe the selection criteria should be implemented to be
> deterministic, as select_victim_memcg() does, and the documentation should
> fully describe what the selection criteria is, and then allow the user to
> decide.

We even change the whole scheduling behaviors and try really hard to
not get locked into specific implementation details which exclude
future improvements. Guaranteeing OOM killing selection would be
crazy. Why would we prevent ourselves from doing things better in the
future? We aren't talking about the semantics of read(2) here. This
is a kernel emergency mechanism to avoid deadlock at the last moment.

> Roman is planning on introducing memory.oom_priority back into the
> patchset per https://marc.info/?l=linux-kernel&m=150574701126877 and I
> agree with the very clear semantic that it introduces: to have the
> size-based comparison use the same rules as the userspace priority
> comparison. It's very powerful and I'm happy to ack the final version
> that he plans on posting.

To me, the proposed oom_priority mechanism seems too limited and makes
the error of tightly coupling the hierarchical behavior of resource
distribution with OOM victim selection. They can be related but are
not the same and coupling them together in the kernel interface is
likely a mistake which will lead to long term pains that we can't
easily get out of.

Here's a really simple use case. Imagine a system which hosts two
containers of services and one is somewhat favored over the other and
wants to set up cgroup hierarchy so that resources are split at the
top level between the two containers. oom_priority is set accordingly
too. Let's say a low priority maintenance job in higher priority
container goes berserk, as they oftne do, and pushing the system into
OOM.

With the proposed static oom_priority mechanism, the only
configuration which can be expressed is "kill all of the lower top
level subtree before any of the higher one", which is a silly
restriction leading to silly behavior and a direct result of
conflating resource distribution network with level-by-level OOM
killing decsion.

If we want to allow users to steer OOM killing, I suspect that it
should be aligned at delegation boundaries rather than on cgroup
hierarchy itself. We can discuss that but it is a separate
discussion.

The mechanism being proposed is fundamentally flawed. You can't push
that in by nacking other improvements.

Thanks.

--
tejun

2017-09-22 20:40:00

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri, 22 Sep 2017, Tejun Heo wrote:

> > It doesn't have anything to do with my particular usecase, but rather the
> > ability of userspace to influence the decisions of the kernel. Previous
> > to this patchset, when selection is done based on process size, userspace
> > has full control over selection. After this patchset, userspace has no
> > control other than setting all processes to be oom disabled if the largest
> > memory consumer is to be protected. Roman's memory.oom_priority provides
> > a perfect solution for userspace to be able to influence this decision
> > making and causes no change in behavior for users who choose not to tune
> > memory.oom_priority. The nack originates from the general need for
> > userspace influence over oom victim selection and to avoid userspace
> > needing to take the rather drastic measure of setting all processes to be
> > oom disabled to prevent oom kill in kernels before oom priorities are
> > introduced.
>
> Overall, I think that OOM killing is the wrong place to implement
> sophisticated intelligence in. It's too late to be smart - the
> workload already has suffered significantly and there's only very
> limited amount of computing which can be performed. That said, if
> there's a useful and general enough mechanism to configure OOM killer
> behavior from userland, that can definitely be useful.
>

What is under discussion is a new way to compare sibling cgroups when
selecting a victim for oom kill. It's a new heuristic based on a
characteristic of the memory cgroup rather than the individual process.
We want this behavior that the patchset implements. The only desire is a
way for userspace to influence that decision making in the same way that
/proc/pid/oom_score_adj allows userspace to influence the current
heuristic.

Current heuristic based on processes is coupled with per-process
/proc/pid/oom_score_adj. The proposed
heuristic has no ability to be influenced by userspace, and it needs one.
The proposed heuristic based on memory cgroups coupled with Roman's
per-memcg memory.oom_priority is appropriate and needed. It is not
"sophisticated intelligence," it merely allows userspace to protect vital
memory cgroups when opting into the new features (cgroups compared based
on size and memory.oom_group) that we very much want.

> We even change the whole scheduling behaviors and try really hard to
> not get locked into specific implementation details which exclude
> future improvements. Guaranteeing OOM killing selection would be
> crazy. Why would we prevent ourselves from doing things better in the
> future? We aren't talking about the semantics of read(2) here. This
> is a kernel emergency mechanism to avoid deadlock at the last moment.
>

We merely want to prefer other memory cgroups are oom killed on system oom
conditions before important ones, regardless if the important one is using
more memory than the others because of the new heuristic this patchset
introduces. This is exactly the same as /proc/pid/oom_score_adj for the
current heuristic.

> Here's a really simple use case. Imagine a system which hosts two
> containers of services and one is somewhat favored over the other and
> wants to set up cgroup hierarchy so that resources are split at the
> top level between the two containers. oom_priority is set accordingly
> too. Let's say a low priority maintenance job in higher priority
> container goes berserk, as they oftne do, and pushing the system into
> OOM.
>
> With the proposed static oom_priority mechanism, the only
> configuration which can be expressed is "kill all of the lower top
> level subtree before any of the higher one", which is a silly
> restriction leading to silly behavior and a direct result of
> conflating resource distribution network with level-by-level OOM
> killing decsion.
>

The problem you're describing is an issue with the top-level limits after
this patchset is merged, not memory.oom_priority at all.

If they are truly split evenly, this patchset kills the largest process
from the hierarchy with the most charged memory. That's unchanged if the
two priorities are equal. By changing the priority to be more preferred
for a hierarchy, you indeed prefer oom kills from the lower priority
hierarchy. You've opted in. One hierarchy is more important than the
other, regardless of any hypothetical low priority maintenance job going
berserk.

If you have this low priority maintenance job charging memory to the high
priority hierarchy, you're already misconfigured unless you adjust
/proc/pid/oom_score_adj because it will oom kill any larger process than
itself in today's kernels anyway.

A better configuration would be attach this hypothetical low priority
maintenance job to its own sibling cgroup with its own memory limit to
avoid exactly that problem: it going berserk and charging too much memory
to the high priority container that results in one of its processes
getting oom killed.

2017-09-22 20:53:52

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Thu, 21 Sep 2017, Johannes Weiner wrote:

> > The issue is that if you opt-in to the new feature, then you are forced to
> > change /proc/pid/oom_score_adj of all processes attached to a cgroup that
> > you do not want oom killed based on size to be oom disabled.
>
> You're assuming that most people would want to influence the oom
> behavior in the first place. I think the opposite is the case: most
> people don't care as long as the OOM killer takes the intent the user
> has expressed wrt runtime containerization/grouping into account.
>

If you do not want to influence the oom behavior, do not change
memory.oom_priority from its default. It's that simple.

> > The kernel provides no other remedy without oom priorities since the
> > new feature would otherwise disregard oom_score_adj.
>
> As of v8, it respects this setting and doesn't kill min score tasks.
>

That's the issue. To protect a memory cgroup from being oom killed in a
system oom condition, you need to change oom_score_adj of *all* processes
attached to be oom disabled. Then, you have a huge problem in memory
cgroup oom conditions because nothing can be killed in that hierarchy
itself.

> > The patchset compares memory cgroup size relative to sibling cgroups only,
> > the same comparison for memory.oom_priority. There is a guarantee
> > provided on how cgroup size is compared in select_victim_memcg(), it
> > hierarchically accumulates the "size" from leaf nodes up to the root memcg
> > and then iterates the tree comparing sizes between sibling cgroups to
> > choose a victim memcg. That algorithm could be more elaborately described
> > in the documentation, but we simply cannot change the implementation of
> > select_victim_memcg() later even without oom priorities since users cannot
> > get inconsistent results after opting into a feature between kernel
> > versions. I believe the selection criteria should be implemented to be
> > deterministic, as select_victim_memcg() does, and the documentation should
> > fully describe what the selection criteria is, and then allow the user to
> > decide.
>
> I wholeheartedly disagree. We have changed the behavior multiple times
> in the past. In fact, you have arguably done the most drastic changes
> to the algorithm since the OOM killer was first introduced. E.g.
>
> a63d83f427fb oom: badness heuristic rewrite
>
> And that's completely fine. Because this thing is not a resource
> management tool for userspace, it's the kernel saving itself. At best
> in a manner that's not too surprising to userspace.
>

When I did that, I had to add /proc/pid/oom_score_adj to allow userspace
to influence selection. We came up with /proc/pid/oom_score_adj when
working with kde, openssh, chromium, and udev because they cared about the
ability to influence the decisionmaking. I'm perfectly happy with the new
heuristic presented in this patchset, I simply want userspace to be able
to influence it, if it desires. Requiring userspace to set all processes
to be oom disabled to protect a hierarchy is totally and completely
broken. It livelocks the memory cgroup if it is oom itself.

> To me, your argument behind the NAK still boils down to "this doesn't
> support my highly specialized usecase." But since it doesn't prohibit
> your usecase - which isn't even supported upstream, btw - this really
> doesn't carry much weight.
>
> I'd say if you want configurability on top of Roman's code, please
> submit patches and push the case for these in a separate effort.
>

Roman implemented memory.oom_priority himself, it has my Tested-by, and it
allows users who want to protect high priority memory cgroups from using
the size based comparison for all other cgroups that we very much desire.

2017-09-22 21:05:26

by Tejun Heo

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

Hello,

On Fri, Sep 22, 2017 at 01:39:55PM -0700, David Rientjes wrote:
> Current heuristic based on processes is coupled with per-process
> /proc/pid/oom_score_adj. The proposed
> heuristic has no ability to be influenced by userspace, and it needs one.
> The proposed heuristic based on memory cgroups coupled with Roman's
> per-memcg memory.oom_priority is appropriate and needed. It is not

So, this is where we disagree. I don't think it's a good design.

> "sophisticated intelligence," it merely allows userspace to protect vital
> memory cgroups when opting into the new features (cgroups compared based
> on size and memory.oom_group) that we very much want.

which can't achieve that goal very well for wide variety of users.

> > We even change the whole scheduling behaviors and try really hard to
> > not get locked into specific implementation details which exclude
> > future improvements. Guaranteeing OOM killing selection would be
> > crazy. Why would we prevent ourselves from doing things better in the
> > future? We aren't talking about the semantics of read(2) here. This
> > is a kernel emergency mechanism to avoid deadlock at the last moment.
>
> We merely want to prefer other memory cgroups are oom killed on system oom
> conditions before important ones, regardless if the important one is using
> more memory than the others because of the new heuristic this patchset
> introduces. This is exactly the same as /proc/pid/oom_score_adj for the
> current heuristic.

You were arguing that we should lock into a specific heuristics and
guarantee the same behavior. We shouldn't.

When we introduce a user visible interface, we're making a lot of
promises. My point is that we need to be really careful when making
those promises.

> If you have this low priority maintenance job charging memory to the high
> priority hierarchy, you're already misconfigured unless you adjust
> /proc/pid/oom_score_adj because it will oom kill any larger process than
> itself in today's kernels anyway.
>
> A better configuration would be attach this hypothetical low priority
> maintenance job to its own sibling cgroup with its own memory limit to
> avoid exactly that problem: it going berserk and charging too much memory
> to the high priority container that results in one of its processes
> getting oom killed.

And how do you guarantee that across delegation boundaries? The
points you raise on why the priority should be applied level-by-level
are exactly the same points why this doesn't really work. OOM killing
priority isn't something which can be distributed across cgroup
hierarchy level-by-level. The resulting decision tree doesn't make
any sense.

I'm not against adding something which works but strict level-by-level
comparison isn't the solution.

Thanks.

--
tejun

2017-09-23 08:16:36

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri, 22 Sep 2017, Tejun Heo wrote:

> > If you have this low priority maintenance job charging memory to the high
> > priority hierarchy, you're already misconfigured unless you adjust
> > /proc/pid/oom_score_adj because it will oom kill any larger process than
> > itself in today's kernels anyway.
> >
> > A better configuration would be attach this hypothetical low priority
> > maintenance job to its own sibling cgroup with its own memory limit to
> > avoid exactly that problem: it going berserk and charging too much memory
> > to the high priority container that results in one of its processes
> > getting oom killed.
>
> And how do you guarantee that across delegation boundaries? The
> points you raise on why the priority should be applied level-by-level
> are exactly the same points why this doesn't really work. OOM killing
> priority isn't something which can be distributed across cgroup
> hierarchy level-by-level. The resulting decision tree doesn't make
> any sense.
>

It works very well in practice with real world usecases, and Roman has
developed the same design independently that we have used for the past
four years. Saying it doesn't make any sense doesn't hold a lot of weight
when we both independently designed and implemented the same solution to
address our usecases.

> I'm not against adding something which works but strict level-by-level
> comparison isn't the solution.
>

Each of the eight versions of Roman's cgroup aware oom killer has done
comparisons between siblings at each level. Userspace influence on that
comparison would thus also need to be done at each level. It's a very
powerful combination in practice.

Thanks.

2017-09-25 12:24:05

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

I would really appreciate some feedback from Tejun, Johannes here.

On Wed 20-09-17 14:53:41, Roman Gushchin wrote:
> On Mon, Sep 18, 2017 at 08:14:05AM +0200, Michal Hocko wrote:
> > On Fri 15-09-17 08:23:01, Roman Gushchin wrote:
> > > On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
[...]
> > > > But then you just enforce a structural restriction on your configuration
> > > > because
> > > > root
> > > > / \
> > > > A D
> > > > /\
> > > > B C
> > > >
> > > > is a different thing than
> > > > root
> > > > / | \
> > > > B C D
> > > >
> > >
> > > I actually don't have a strong argument against an approach to select
> > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > no much difference.
>
> I've tried to implement this approach, and it's really arguable.
> Although your example looks reasonable, the opposite example is also valid:
> you might want to compare whole hierarchies, and it's a quite typical usecase.
>
> Assume, you have several containerized workloads on a machine (probably,
> each will be contained in a memcg with memory.max set), with some hierarchy
> of cgroups inside. Then in case of global memory shortage we want to reclaim
> some memory from the biggest workload, and the selection should not depend
> on group_oom settings. It would be really strange, if setting group_oom will
> higher the chances to be killed.
>
> In other words, let's imagine processes as leaf nodes in memcg tree. We decided
> to select the biggest memcg and kill one or more processes inside (depending
> on group_oom setting), but the memcg selection doesn't depend on it.
> We do not compare processes from different cgroups, as well as cgroups with
> processes. The same should apply to cgroups: why do we want to compare cgroups
> from different sub-trees?
>
> While size-based comparison can be implemented with this approach,
> the priority-based is really weird (as David mentioned).
> If priorities have no hierarchical meaning at all, we lack the very important
> ability to enforce hierarchy oom_priority. Otherwise we have to invent some
> complex rules of oom_priority propagation (e.g. is someone is raising
> the oom_priority in parent, should it be applied to children immediately, etc).

I would really forget about the priority at this stage. This needs
really much more thinking and I consider the David's usecase very
specialized to use it as a template for a general purpose oom
prioritization. I might be wrong here of course...

> The oom_group knob meaning also becoms more complex. It affects both
> the victim selection and OOM action. _ANY_ mechanism which allows to affect
> OOM victim selection (either priorities, either bpf-based approach) should
> not have global system-wide meaning, it breaks everything.
>
> I do understand your point, but the same is true for other stuff, right?
> E.g. cpu time distribution (and io, etc) depends on hierarchy configuration.
> It's a limitation, but it's ok, as user should create a hierarchy which
> reflects some logical relations between processes and groups of processes.
> Otherwise we're going to the configuration hell.

And that is _exactly_ my concern. We surely do not want tell people that
they have to consider their cgroup tree structure to control the global
oom behavior. You simply do not have that constrain with leaf-only
semantic and if kill-all intermediate nodes are used then there is an
explicit opt-in for the hierarchy considerations.

> In any case, OOM is a last resort mechanism. The goal is to reclaim some memory
> and do not crash the system or do not leave it in totally broken state.
> Any really complex mm in userspace should be applied _before_ OOM happens.
> So, I don't think we have to support all possible configurations here,
> if we're able to achieve the main goal (kill some processes and do not leave
> broken systems/containers).

True but we want to have the semantic reasonably understandable. And it
is quite hard to explain that the oom killer hasn't selected the largest
memcg just because it happened to be in a deeper hierarchy which has
been configured to cover a different resource.

I am sorry to repeat my self and I will not argue if there is a
prevalent agreement that level-by-level comparison is considered
desirable and documented behavior but, by all means, do not define this
semantic based on a priority requirements and/or implementation details.
--
Michal Hocko
SUSE Labs

2017-09-25 17:00:20

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, Sep 25, 2017 at 02:24:00PM +0200, Michal Hocko wrote:
> I would really appreciate some feedback from Tejun, Johannes here.
>
> On Wed 20-09-17 14:53:41, Roman Gushchin wrote:
> > On Mon, Sep 18, 2017 at 08:14:05AM +0200, Michal Hocko wrote:
> > > On Fri 15-09-17 08:23:01, Roman Gushchin wrote:
> > > > On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
> [...]
> > > > > But then you just enforce a structural restriction on your configuration
> > > > > because
> > > > > root
> > > > > / \
> > > > > A D
> > > > > /\
> > > > > B C
> > > > >
> > > > > is a different thing than
> > > > > root
> > > > > / | \
> > > > > B C D
> > > > >
> > > >
> > > > I actually don't have a strong argument against an approach to select
> > > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > > no much difference.
> >
> > I've tried to implement this approach, and it's really arguable.
> > Although your example looks reasonable, the opposite example is also valid:
> > you might want to compare whole hierarchies, and it's a quite typical usecase.
> >
> > Assume, you have several containerized workloads on a machine (probably,
> > each will be contained in a memcg with memory.max set), with some hierarchy
> > of cgroups inside. Then in case of global memory shortage we want to reclaim
> > some memory from the biggest workload, and the selection should not depend
> > on group_oom settings. It would be really strange, if setting group_oom will
> > higher the chances to be killed.
> >
> > In other words, let's imagine processes as leaf nodes in memcg tree. We decided
> > to select the biggest memcg and kill one or more processes inside (depending
> > on group_oom setting), but the memcg selection doesn't depend on it.
> > We do not compare processes from different cgroups, as well as cgroups with
> > processes. The same should apply to cgroups: why do we want to compare cgroups
> > from different sub-trees?
> >
> > While size-based comparison can be implemented with this approach,
> > the priority-based is really weird (as David mentioned).
> > If priorities have no hierarchical meaning at all, we lack the very important
> > ability to enforce hierarchy oom_priority. Otherwise we have to invent some
> > complex rules of oom_priority propagation (e.g. is someone is raising
> > the oom_priority in parent, should it be applied to children immediately, etc).
>
> I would really forget about the priority at this stage. This needs
> really much more thinking and I consider the David's usecase very
> specialized to use it as a template for a general purpose oom
> prioritization. I might be wrong here of course...

No, I agree.

> > In any case, OOM is a last resort mechanism. The goal is to reclaim some memory
> > and do not crash the system or do not leave it in totally broken state.
> > Any really complex mm in userspace should be applied _before_ OOM happens.
> > So, I don't think we have to support all possible configurations here,
> > if we're able to achieve the main goal (kill some processes and do not leave
> > broken systems/containers).
>
> True but we want to have the semantic reasonably understandable. And it
> is quite hard to explain that the oom killer hasn't selected the largest
> memcg just because it happened to be in a deeper hierarchy which has
> been configured to cover a different resource.

Going back to Michal's example, say the user configured the following:

root
/ \
A D
/ \
B C

A global OOM event happens and we find this:
- A > D
- B, C, D are oomgroups

What the user is telling us is that B, C, and D are compound memory
consumers. They cannot be divided into their task parts from a memory
point of view.

However, the user doesn't say the same for A: the A subtree summarizes
and controls aggregate consumption of B and C, but without groupoom
set on A, the user says that A is in fact divisible into independent
memory consumers B and C.

If we don't have to kill all of A, but we'd have to kill all of D,
does it make sense to compare the two?

Let's consider an extreme case of this conundrum:

root
/ \
A B
/|\ |
A1-A1000 B1

Again we find:
- A > B
- A1 to A1000 and B1 are oomgroups
But:
- A1 to A1000 individually are tiny, B1 is huge

Going level by level, we'd pick A as the bigger hierarchy in the
system, and then kill off one of the tiny groups A1 to A1000.

Conversely, going for biggest consumer regardless of hierarchy, we'd
compare A1 to A1000 and B1, then pick B1 as the biggest single atomic
memory consumer in the system and kill all its tasks.

Which one of these two fits both the purpose and our historic approach
to OOM killing better?

As was noted in this thread, OOM is the last resort to avoid a memory
deadlock. Killing the biggest consumer is most likely to resolve this
precarious situation. It is also most likely to catch buggy software
with memory leaks or runaway allocations, which is a nice bonus.

Killing a potentially tiny consumer inside the biggest top-level
hierarchy doesn't achieve this. I think we can all agree on this.

But also, global OOM in particular means that the hierarchical
approach to allocating the system's memory among cgroups has
failed. The user expressed control over memory in a way that wasn't
sufficient to isolate memory consumption between the different
hierarchies. IMO what follows from that is that the hierarchy itself
is a questionable guide to finding a culprit.

So I'm leaning toward the second model: compare all oomgroups and
standalone tasks in the system with each other, independent of the
failed hierarchical control structure. Then kill the biggest of them.

2017-09-25 18:16:10

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, Sep 25, 2017 at 01:00:04PM -0400, Johannes Weiner wrote:
> On Mon, Sep 25, 2017 at 02:24:00PM +0200, Michal Hocko wrote:
> > I would really appreciate some feedback from Tejun, Johannes here.
> >
> > On Wed 20-09-17 14:53:41, Roman Gushchin wrote:
> > > On Mon, Sep 18, 2017 at 08:14:05AM +0200, Michal Hocko wrote:
> > > > On Fri 15-09-17 08:23:01, Roman Gushchin wrote:
> > > > > On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
> > [...]
> > > > > > But then you just enforce a structural restriction on your configuration
> > > > > > because
> > > > > > root
> > > > > > / \
> > > > > > A D
> > > > > > /\
> > > > > > B C
> > > > > >
> > > > > > is a different thing than
> > > > > > root
> > > > > > / | \
> > > > > > B C D
> > > > > >
> > > > >
> > > > > I actually don't have a strong argument against an approach to select
> > > > > largest leaf or kill-all-set memcg. I think, in practice there will be
> > > > > no much difference.
> > >
> > > I've tried to implement this approach, and it's really arguable.
> > > Although your example looks reasonable, the opposite example is also valid:
> > > you might want to compare whole hierarchies, and it's a quite typical usecase.
> > >
> > > Assume, you have several containerized workloads on a machine (probably,
> > > each will be contained in a memcg with memory.max set), with some hierarchy
> > > of cgroups inside. Then in case of global memory shortage we want to reclaim
> > > some memory from the biggest workload, and the selection should not depend
> > > on group_oom settings. It would be really strange, if setting group_oom will
> > > higher the chances to be killed.
> > >
> > > In other words, let's imagine processes as leaf nodes in memcg tree. We decided
> > > to select the biggest memcg and kill one or more processes inside (depending
> > > on group_oom setting), but the memcg selection doesn't depend on it.
> > > We do not compare processes from different cgroups, as well as cgroups with
> > > processes. The same should apply to cgroups: why do we want to compare cgroups
> > > from different sub-trees?
> > >
> > > While size-based comparison can be implemented with this approach,
> > > the priority-based is really weird (as David mentioned).
> > > If priorities have no hierarchical meaning at all, we lack the very important
> > > ability to enforce hierarchy oom_priority. Otherwise we have to invent some
> > > complex rules of oom_priority propagation (e.g. is someone is raising
> > > the oom_priority in parent, should it be applied to children immediately, etc).
> >
> > I would really forget about the priority at this stage. This needs
> > really much more thinking and I consider the David's usecase very
> > specialized to use it as a template for a general purpose oom
> > prioritization. I might be wrong here of course...
>
> No, I agree.
>
> > > In any case, OOM is a last resort mechanism. The goal is to reclaim some memory
> > > and do not crash the system or do not leave it in totally broken state.
> > > Any really complex mm in userspace should be applied _before_ OOM happens.
> > > So, I don't think we have to support all possible configurations here,
> > > if we're able to achieve the main goal (kill some processes and do not leave
> > > broken systems/containers).
> >
> > True but we want to have the semantic reasonably understandable. And it
> > is quite hard to explain that the oom killer hasn't selected the largest
> > memcg just because it happened to be in a deeper hierarchy which has
> > been configured to cover a different resource.
>
> Going back to Michal's example, say the user configured the following:
>
> root
> / \
> A D
> / \
> B C
>
> A global OOM event happens and we find this:
> - A > D
> - B, C, D are oomgroups
>
> What the user is telling us is that B, C, and D are compound memory
> consumers. They cannot be divided into their task parts from a memory
> point of view.
>
> However, the user doesn't say the same for A: the A subtree summarizes
> and controls aggregate consumption of B and C, but without groupoom
> set on A, the user says that A is in fact divisible into independent
> memory consumers B and C.
>
> If we don't have to kill all of A, but we'd have to kill all of D,
> does it make sense to compare the two?
>
> Let's consider an extreme case of this conundrum:
>
> root
> / \
> A B
> /|\ |
> A1-A1000 B1
>
> Again we find:
> - A > B
> - A1 to A1000 and B1 are oomgroups
> But:
> - A1 to A1000 individually are tiny, B1 is huge
>
> Going level by level, we'd pick A as the bigger hierarchy in the
> system, and then kill off one of the tiny groups A1 to A1000.
>
> Conversely, going for biggest consumer regardless of hierarchy, we'd
> compare A1 to A1000 and B1, then pick B1 as the biggest single atomic
> memory consumer in the system and kill all its tasks.
>
> Which one of these two fits both the purpose and our historic approach
> to OOM killing better?
>
> As was noted in this thread, OOM is the last resort to avoid a memory
> deadlock. Killing the biggest consumer is most likely to resolve this
> precarious situation. It is also most likely to catch buggy software
> with memory leaks or runaway allocations, which is a nice bonus.
>
> Killing a potentially tiny consumer inside the biggest top-level
> hierarchy doesn't achieve this. I think we can all agree on this.
>
> But also, global OOM in particular means that the hierarchical
> approach to allocating the system's memory among cgroups has
> failed. The user expressed control over memory in a way that wasn't
> sufficient to isolate memory consumption between the different
> hierarchies. IMO what follows from that is that the hierarchy itself
> is a questionable guide to finding a culprit.
>
> So I'm leaning toward the second model: compare all oomgroups and
> standalone tasks in the system with each other, independent of the
> failed hierarchical control structure. Then kill the biggest of them.

I'm not against this model, as I've said before. It feels logical,
and will work fine in most cases.

In this case we can drop any mount/boot options, because it preserves
the existing behavior in the default configuration. A big advantage.

The only thing, I'm slightly concerned, that due to the way how we calculate
the memory footprint for tasks and memory cgroups, we will have a number
of weird edge cases. For instance, when putting a single process into
the group_oom memcg will alter the oom_score significantly and result
in significantly different chances to be killed. An obvious example will
be a task with oom_score_adj set to any non-extreme (other than 0 and -1000)
value, but it can also happen in case of constrained alloc, for instance.

If it considered to be a minor issue, we can choose this approach.

Thanks!

2017-09-25 20:25:27

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
[...]
> I'm not against this model, as I've said before. It feels logical,
> and will work fine in most cases.
>
> In this case we can drop any mount/boot options, because it preserves
> the existing behavior in the default configuration. A big advantage.

I am not sure about this. We still need an opt-in, ragardless, because
selecting the largest process from the largest memcg != selecting the
largest task (just consider memcgs with many processes example).

> The only thing, I'm slightly concerned, that due to the way how we calculate
> the memory footprint for tasks and memory cgroups, we will have a number
> of weird edge cases. For instance, when putting a single process into
> the group_oom memcg will alter the oom_score significantly and result
> in significantly different chances to be killed. An obvious example will
> be a task with oom_score_adj set to any non-extreme (other than 0 and -1000)
> value, but it can also happen in case of constrained alloc, for instance.

I am not sure I understand. Are you talking about root memcg comparing
to other memcgs?
--
Michal Hocko
SUSE Labs

2017-09-25 22:21:14

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, 25 Sep 2017, Johannes Weiner wrote:

> > True but we want to have the semantic reasonably understandable. And it
> > is quite hard to explain that the oom killer hasn't selected the largest
> > memcg just because it happened to be in a deeper hierarchy which has
> > been configured to cover a different resource.
>
> Going back to Michal's example, say the user configured the following:
>
> root
> / \
> A D
> / \
> B C
>
> A global OOM event happens and we find this:
> - A > D
> - B, C, D are oomgroups
>
> What the user is telling us is that B, C, and D are compound memory
> consumers. They cannot be divided into their task parts from a memory
> point of view.
>
> However, the user doesn't say the same for A: the A subtree summarizes
> and controls aggregate consumption of B and C, but without groupoom
> set on A, the user says that A is in fact divisible into independent
> memory consumers B and C.
>
> If we don't have to kill all of A, but we'd have to kill all of D,
> does it make sense to compare the two?
>

No, I agree that we shouldn't compare sibling memory cgroups based on
different criteria depending on whether group_oom is set or not.

I think it would be better to compare siblings based on the same criteria
independent of group_oom if the user has mounted the hierarchy with the
new mode (I think we all agree that the mount option is needed). It's
very easy to describe to the user and the selection is simple to
understand. Then, once a cgroup has been chosen as the victim cgroup,
kill the process with the highest badness, allowing the user to influence
that with /proc/pid/oom_score_adj just as today, if group_oom is disabled;
otherwise, kill all eligible processes if enabled.

That, to me, is a very clear semantic and I believe it addresses Roman's
usecase. My desire to have oom priorities amongst siblings is so that
userspace can influence which cgroup is chosen, just as it can influence
which process is chosen.

I see group_oom as a mechanism to be used when victim selection has
already been done instead of something that should be considered in the
policy of victim selection.

> Let's consider an extreme case of this conundrum:
>
> root
> / \
> A B
> /|\ |
> A1-A1000 B1
>
> Again we find:
> - A > B
> - A1 to A1000 and B1 are oomgroups
> But:
> - A1 to A1000 individually are tiny, B1 is huge
>
> Going level by level, we'd pick A as the bigger hierarchy in the
> system, and then kill off one of the tiny groups A1 to A1000.
>
> Conversely, going for biggest consumer regardless of hierarchy, we'd
> compare A1 to A1000 and B1, then pick B1 as the biggest single atomic
> memory consumer in the system and kill all its tasks.
>

If we compare sibling memcgs independent of group_oom, we don't
necessarily pick A unless it really is larger than B.

2017-09-26 08:46:12

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon 25-09-17 15:21:03, David Rientjes wrote:
> On Mon, 25 Sep 2017, Johannes Weiner wrote:
>
> > > True but we want to have the semantic reasonably understandable. And it
> > > is quite hard to explain that the oom killer hasn't selected the largest
> > > memcg just because it happened to be in a deeper hierarchy which has
> > > been configured to cover a different resource.
> >
> > Going back to Michal's example, say the user configured the following:
> >
> > root
> > / \
> > A D
> > / \
> > B C
> >
> > A global OOM event happens and we find this:
> > - A > D
> > - B, C, D are oomgroups
> >
> > What the user is telling us is that B, C, and D are compound memory
> > consumers. They cannot be divided into their task parts from a memory
> > point of view.
> >
> > However, the user doesn't say the same for A: the A subtree summarizes
> > and controls aggregate consumption of B and C, but without groupoom
> > set on A, the user says that A is in fact divisible into independent
> > memory consumers B and C.
> >
> > If we don't have to kill all of A, but we'd have to kill all of D,
> > does it make sense to compare the two?
> >
>
> No, I agree that we shouldn't compare sibling memory cgroups based on
> different criteria depending on whether group_oom is set or not.
>
> I think it would be better to compare siblings based on the same criteria
> independent of group_oom if the user has mounted the hierarchy with the
> new mode (I think we all agree that the mount option is needed). It's
> very easy to describe to the user and the selection is simple to
> understand.

I disagree. Just take the most simplistic example when cgroups reflect
some other higher level organization - e.g. school with teachers,
students and admins as the top level cgroups to control the proper cpu
share load. Now you want to have a fair OOM selection between different
entities. Do you consider selecting students all the time as an expected
behavior just because their are the largest group? This just doesn't
make any sense to me.

> Then, once a cgroup has been chosen as the victim cgroup,
> kill the process with the highest badness, allowing the user to influence
> that with /proc/pid/oom_score_adj just as today, if group_oom is disabled;
> otherwise, kill all eligible processes if enabled.

And now, what should be the semantic of group_oom on an intermediate
(non-leaf) memcg? Why should we compare it to other killable entities?
Roman was mentioning a setup where a _single_ workload consists of a
deeper hierarchy which has to be shut down at once. It absolutely makes
sense to consider the cumulative memory of that hierarchy when we are
going to kill it all.

> That, to me, is a very clear semantic and I believe it addresses Roman's
> usecase. My desire to have oom priorities amongst siblings is so that
> userspace can influence which cgroup is chosen, just as it can influence
> which process is chosen.

But what you are proposing is something different from oom_score_adj.
That only sets bias to the killable entities while priorities on
intermediate non-killable memcgs controls how the whole oom hierarchy
is traversed. So a non-killable intermediate memcg can hugely influence
what gets killed in the end. This is IMHO a tricky and I would even dare
to claim a wrong semantic. I can see priorities being very useful on
killable entities for sure. I am not entirely sure what would be the
best approach yet and that is why I've suggested that to postpone to
after we settle with a simple approach first. Bringing priorities back
to the discussion again will not help to move that forward I am afraid.

--
Michal Hocko
SUSE Labs

2017-09-26 11:00:28

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
> On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
> [...]
> > I'm not against this model, as I've said before. It feels logical,
> > and will work fine in most cases.
> >
> > In this case we can drop any mount/boot options, because it preserves
> > the existing behavior in the default configuration. A big advantage.
>
> I am not sure about this. We still need an opt-in, ragardless, because
> selecting the largest process from the largest memcg != selecting the
> largest task (just consider memcgs with many processes example).

As I understand Johannes, he suggested to compare individual processes with
group_oom mem cgroups. In other words, always select a killable entity with
the biggest memory footprint.

This is slightly different from my v8 approach, where I treat leaf memcgs
as indivisible memory consumers independent on group_oom setting, so
by default I'm selecting the biggest task in the biggest memcg.

While the approach suggested by Johannes looks clear and reasonable,
I'm slightly concerned about possible implementation issues,
which I've described below:

>
> > The only thing, I'm slightly concerned, that due to the way how we calculate
> > the memory footprint for tasks and memory cgroups, we will have a number
> > of weird edge cases. For instance, when putting a single process into
> > the group_oom memcg will alter the oom_score significantly and result
> > in significantly different chances to be killed. An obvious example will
> > be a task with oom_score_adj set to any non-extreme (other than 0 and -1000)
> > value, but it can also happen in case of constrained alloc, for instance.
>
> I am not sure I understand. Are you talking about root memcg comparing
> to other memcgs?

Not only, but root memcg in this case will be another complication. We can
also use the same trick for all memcg (define memcg oom_score as maximum oom_score
of the belonging tasks), it will turn group_oom into pure container cleanup
solution, without changing victim selection algorithm

But, again, I'm not against approach suggested by Johannes. I think that overall
it's the best possible semantics, if we're not taking some implementation details
into account.

2017-09-26 11:21:39

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue 26-09-17 11:59:25, Roman Gushchin wrote:
> On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
> > On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
> > [...]
> > > I'm not against this model, as I've said before. It feels logical,
> > > and will work fine in most cases.
> > >
> > > In this case we can drop any mount/boot options, because it preserves
> > > the existing behavior in the default configuration. A big advantage.
> >
> > I am not sure about this. We still need an opt-in, ragardless, because
> > selecting the largest process from the largest memcg != selecting the
> > largest task (just consider memcgs with many processes example).
>
> As I understand Johannes, he suggested to compare individual processes with
> group_oom mem cgroups. In other words, always select a killable entity with
> the biggest memory footprint.
>
> This is slightly different from my v8 approach, where I treat leaf memcgs
> as indivisible memory consumers independent on group_oom setting, so
> by default I'm selecting the biggest task in the biggest memcg.

My reading is that he is actually proposing the same thing I've been
mentioning. Simply select the biggest killable entity (leaf memcg or
group_oom hierarchy) and either kill the largest task in that entity
(for !group_oom) or the whole memcg/hierarchy otherwise.

> While the approach suggested by Johannes looks clear and reasonable,
> I'm slightly concerned about possible implementation issues,
> which I've described below:
>
> >
> > > The only thing, I'm slightly concerned, that due to the way how we calculate
> > > the memory footprint for tasks and memory cgroups, we will have a number
> > > of weird edge cases. For instance, when putting a single process into
> > > the group_oom memcg will alter the oom_score significantly and result
> > > in significantly different chances to be killed. An obvious example will
> > > be a task with oom_score_adj set to any non-extreme (other than 0 and -1000)
> > > value, but it can also happen in case of constrained alloc, for instance.
> >
> > I am not sure I understand. Are you talking about root memcg comparing
> > to other memcgs?
>
> Not only, but root memcg in this case will be another complication. We can
> also use the same trick for all memcg (define memcg oom_score as maximum oom_score
> of the belonging tasks), it will turn group_oom into pure container cleanup
> solution, without changing victim selection algorithm

I fail to see the problem to be honest. Simply evaluate the memcg_score
you have so far with one minor detail. You only check memcgs which have
tasks (rather than check for leaf node check) or it is group_oom. An
intermediate memcg will get a cumulative size of the whole subhierarchy
and then you know you can skip the subtree because any subtree can be larger.

> But, again, I'm not against approach suggested by Johannes. I think that overall
> it's the best possible semantics, if we're not taking some implementation details
> into account.

I do not see those implementation details issues and let me repeat do
not develop a semantic based on implementation details.
--
Michal Hocko
SUSE Labs

2017-09-26 12:13:44

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue, Sep 26, 2017 at 01:21:34PM +0200, Michal Hocko wrote:
> On Tue 26-09-17 11:59:25, Roman Gushchin wrote:
> > On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
> > > On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
> > > [...]
> > > > I'm not against this model, as I've said before. It feels logical,
> > > > and will work fine in most cases.
> > > >
> > > > In this case we can drop any mount/boot options, because it preserves
> > > > the existing behavior in the default configuration. A big advantage.
> > >
> > > I am not sure about this. We still need an opt-in, ragardless, because
> > > selecting the largest process from the largest memcg != selecting the
> > > largest task (just consider memcgs with many processes example).
> >
> > As I understand Johannes, he suggested to compare individual processes with
> > group_oom mem cgroups. In other words, always select a killable entity with
> > the biggest memory footprint.
> >
> > This is slightly different from my v8 approach, where I treat leaf memcgs
> > as indivisible memory consumers independent on group_oom setting, so
> > by default I'm selecting the biggest task in the biggest memcg.
>
> My reading is that he is actually proposing the same thing I've been
> mentioning. Simply select the biggest killable entity (leaf memcg or
> group_oom hierarchy) and either kill the largest task in that entity
> (for !group_oom) or the whole memcg/hierarchy otherwise.

He wrote the following:
"So I'm leaning toward the second model: compare all oomgroups and
standalone tasks in the system with each other, independent of the
failed hierarchical control structure. Then kill the biggest of them."

>
> > While the approach suggested by Johannes looks clear and reasonable,
> > I'm slightly concerned about possible implementation issues,
> > which I've described below:
> >
> > >
> > > > The only thing, I'm slightly concerned, that due to the way how we calculate
> > > > the memory footprint for tasks and memory cgroups, we will have a number
> > > > of weird edge cases. For instance, when putting a single process into
> > > > the group_oom memcg will alter the oom_score significantly and result
> > > > in significantly different chances to be killed. An obvious example will
> > > > be a task with oom_score_adj set to any non-extreme (other than 0 and -1000)
> > > > value, but it can also happen in case of constrained alloc, for instance.
> > >
> > > I am not sure I understand. Are you talking about root memcg comparing
> > > to other memcgs?
> >
> > Not only, but root memcg in this case will be another complication. We can
> > also use the same trick for all memcg (define memcg oom_score as maximum oom_score
> > of the belonging tasks), it will turn group_oom into pure container cleanup
> > solution, without changing victim selection algorithm
>
> I fail to see the problem to be honest. Simply evaluate the memcg_score
> you have so far with one minor detail. You only check memcgs which have
> tasks (rather than check for leaf node check) or it is group_oom. An
> intermediate memcg will get a cumulative size of the whole subhierarchy
> and then you know you can skip the subtree because any subtree can be larger.
>
> > But, again, I'm not against approach suggested by Johannes. I think that overall
> > it's the best possible semantics, if we're not taking some implementation details
> > into account.
>
> I do not see those implementation details issues and let me repeat do
> not develop a semantic based on implementation details.

There are no problems in "select the biggest leaf or group_oom memcg, then
kill the biggest task or all tasks depending on group_oom" approach,
which you're describing. Comparing tasks and memcgs (what Johannes is suggesting)
may have some issues.

Thanks!

2017-09-26 13:30:44

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue 26-09-17 13:13:00, Roman Gushchin wrote:
> On Tue, Sep 26, 2017 at 01:21:34PM +0200, Michal Hocko wrote:
> > On Tue 26-09-17 11:59:25, Roman Gushchin wrote:
> > > On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
> > > > On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
> > > > [...]
> > > > > I'm not against this model, as I've said before. It feels logical,
> > > > > and will work fine in most cases.
> > > > >
> > > > > In this case we can drop any mount/boot options, because it preserves
> > > > > the existing behavior in the default configuration. A big advantage.
> > > >
> > > > I am not sure about this. We still need an opt-in, ragardless, because
> > > > selecting the largest process from the largest memcg != selecting the
> > > > largest task (just consider memcgs with many processes example).
> > >
> > > As I understand Johannes, he suggested to compare individual processes with
> > > group_oom mem cgroups. In other words, always select a killable entity with
> > > the biggest memory footprint.
> > >
> > > This is slightly different from my v8 approach, where I treat leaf memcgs
> > > as indivisible memory consumers independent on group_oom setting, so
> > > by default I'm selecting the biggest task in the biggest memcg.
> >
> > My reading is that he is actually proposing the same thing I've been
> > mentioning. Simply select the biggest killable entity (leaf memcg or
> > group_oom hierarchy) and either kill the largest task in that entity
> > (for !group_oom) or the whole memcg/hierarchy otherwise.
>
> He wrote the following:
> "So I'm leaning toward the second model: compare all oomgroups and
> standalone tasks in the system with each other, independent of the
> failed hierarchical control structure. Then kill the biggest of them."

I will let Johannes to comment but I believe this is just a
misunderstanding. If we compared only the biggest task from each memcg
then we are basically losing our fairness objective, aren't we?
--
Michal Hocko
SUSE Labs

2017-09-26 17:26:25

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue, Sep 26, 2017 at 03:30:40PM +0200, Michal Hocko wrote:
> On Tue 26-09-17 13:13:00, Roman Gushchin wrote:
> > On Tue, Sep 26, 2017 at 01:21:34PM +0200, Michal Hocko wrote:
> > > On Tue 26-09-17 11:59:25, Roman Gushchin wrote:
> > > > On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
> > > > > On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
> > > > > [...]
> > > > > > I'm not against this model, as I've said before. It feels logical,
> > > > > > and will work fine in most cases.
> > > > > >
> > > > > > In this case we can drop any mount/boot options, because it preserves
> > > > > > the existing behavior in the default configuration. A big advantage.
> > > > >
> > > > > I am not sure about this. We still need an opt-in, ragardless, because
> > > > > selecting the largest process from the largest memcg != selecting the
> > > > > largest task (just consider memcgs with many processes example).
> > > >
> > > > As I understand Johannes, he suggested to compare individual processes with
> > > > group_oom mem cgroups. In other words, always select a killable entity with
> > > > the biggest memory footprint.
> > > >
> > > > This is slightly different from my v8 approach, where I treat leaf memcgs
> > > > as indivisible memory consumers independent on group_oom setting, so
> > > > by default I'm selecting the biggest task in the biggest memcg.
> > >
> > > My reading is that he is actually proposing the same thing I've been
> > > mentioning. Simply select the biggest killable entity (leaf memcg or
> > > group_oom hierarchy) and either kill the largest task in that entity
> > > (for !group_oom) or the whole memcg/hierarchy otherwise.
> >
> > He wrote the following:
> > "So I'm leaning toward the second model: compare all oomgroups and
> > standalone tasks in the system with each other, independent of the
> > failed hierarchical control structure. Then kill the biggest of them."
>
> I will let Johannes to comment but I believe this is just a
> misunderstanding. If we compared only the biggest task from each memcg
> then we are basically losing our fairness objective, aren't we?

Sorry about the confusion.

Yeah I was making the case for what Michal proposed, to kill the
biggest terminal consumer, which is either a task or an oomgroup.

You'd basically iterate through all the tasks and cgroups in the
system and pick the biggest task that isn't in an oom group or the
biggest oom group and then kill that.

Yeah, you'd have to compare the memory footprints of tasks with the
memory footprints of cgroups. These aren't defined identically, and
tasks don't get attributed every type of allocation that a cgroup
would. But it should get us in the ballpark, and I cannot picture a
scenario where this would lead to a completely undesirable outcome.

2017-09-26 21:04:47

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue, 26 Sep 2017, Michal Hocko wrote:

> > No, I agree that we shouldn't compare sibling memory cgroups based on
> > different criteria depending on whether group_oom is set or not.
> >
> > I think it would be better to compare siblings based on the same criteria
> > independent of group_oom if the user has mounted the hierarchy with the
> > new mode (I think we all agree that the mount option is needed). It's
> > very easy to describe to the user and the selection is simple to
> > understand.
>
> I disagree. Just take the most simplistic example when cgroups reflect
> some other higher level organization - e.g. school with teachers,
> students and admins as the top level cgroups to control the proper cpu
> share load. Now you want to have a fair OOM selection between different
> entities. Do you consider selecting students all the time as an expected
> behavior just because their are the largest group? This just doesn't
> make any sense to me.
>

Are you referring to this?

root
/ \
students admins
/ \ / \
A B C D

If the cumulative usage of all students exceeds the cumulative usage of
all admins, yes, the choice is to kill from the /students tree. This has
been Roman's design from the very beginning. If the preference is to kill
the single largest process, which may be attached to either subtree, you
would not have opted-in to the new heuristic.

> > Then, once a cgroup has been chosen as the victim cgroup,
> > kill the process with the highest badness, allowing the user to influence
> > that with /proc/pid/oom_score_adj just as today, if group_oom is disabled;
> > otherwise, kill all eligible processes if enabled.
>
> And now, what should be the semantic of group_oom on an intermediate
> (non-leaf) memcg? Why should we compare it to other killable entities?
> Roman was mentioning a setup where a _single_ workload consists of a
> deeper hierarchy which has to be shut down at once. It absolutely makes
> sense to consider the cumulative memory of that hierarchy when we are
> going to kill it all.
>

If group_oom is enabled on an intermediate memcg, I think the intuitive
way to handle it would be that all descendants are also implicitly or
explicitly group_oom. It is compared to sibling cgroups based on
cumulative usage at the time of oom and the largest is chosen and
iterated. The point is to separate out the selection heuristic (policy)
from group_oom (mechanism) so that we don't bias or prefer subtrees based
on group_oom, which makes this much more complex.

> But what you are proposing is something different from oom_score_adj.
> That only sets bias to the killable entities while priorities on
> intermediate non-killable memcgs controls how the whole oom hierarchy
> is traversed. So a non-killable intermediate memcg can hugely influence
> what gets killed in the end.

Why is there an intermediate non-killable memcg allowed? Cgroup oom
priorities should not be allowed to disable oom killing, it should only
set a priority. The only reason an intermediate cgroup should be
non-killable is if there are no processes attached, but I don't think
anyone is arguing we should just do nothing in that scenario. The point
is that the user has infleunce over the decisionmaking with a per-process
heuristic with oom_score_adj and should also have influence over the
decisionmaking with a per-cgroup heuristic.

> This is IMHO a tricky and I would even dare
> to claim a wrong semantic. I can see priorities being very useful on
> killable entities for sure. I am not entirely sure what would be the
> best approach yet and that is why I've suggested that to postpone to
> after we settle with a simple approach first. Bringing priorities back
> to the discussion again will not help to move that forward I am afraid.
>

I agree to keep it as simple as possible, especially since some users want
specific victim selection, it should be clear to document, and it
shouldn't be influenced by some excessive amount of usage in another
subtree the user has no control over (/admins over /students) to prevent
the user from defining that it really wants to be the first oom victim or
the admin from defining it really prefers something else killed first.

My suggestion is that Roman's implementation is clear, well defined, and
has real-world usecases and it should be the direction that this moves in.
I think victim selection and group_oom are distinct and should not
influence the decisionmaking. I think that oom_priority should influence
the decisionmaking.

When mounted with the new option, as the oom hierarchy is iterated,
compare all sibling cgroups regarding cumulative size unless an oom
priority overrides that (either user specifying it wants to be oom killed
or admin specifying it prefers something else). When a victim memcg is
chosen, use group_oom to determine what should be killed, otherwise choose
by oom_score_adj. I can't imagine how this can be any simpler.

2017-09-27 03:38:01

by Tim Hockin

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

I'm excited to see this being discussed again - it's been years since
the last attempt. I've tried to stay out of the conversation, but I
feel obligated say something and then go back to lurking.

On Tue, Sep 26, 2017 at 10:26 AM, Johannes Weiner <[email protected]> wrote:
> On Tue, Sep 26, 2017 at 03:30:40PM +0200, Michal Hocko wrote:
>> On Tue 26-09-17 13:13:00, Roman Gushchin wrote:
>> > On Tue, Sep 26, 2017 at 01:21:34PM +0200, Michal Hocko wrote:
>> > > On Tue 26-09-17 11:59:25, Roman Gushchin wrote:
>> > > > On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
>> > > > > On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
>> > > > > [...]
>> > > > > > I'm not against this model, as I've said before. It feels logical,
>> > > > > > and will work fine in most cases.
>> > > > > >
>> > > > > > In this case we can drop any mount/boot options, because it preserves
>> > > > > > the existing behavior in the default configuration. A big advantage.
>> > > > >
>> > > > > I am not sure about this. We still need an opt-in, ragardless, because
>> > > > > selecting the largest process from the largest memcg != selecting the
>> > > > > largest task (just consider memcgs with many processes example).
>> > > >
>> > > > As I understand Johannes, he suggested to compare individual processes with
>> > > > group_oom mem cgroups. In other words, always select a killable entity with
>> > > > the biggest memory footprint.
>> > > >
>> > > > This is slightly different from my v8 approach, where I treat leaf memcgs
>> > > > as indivisible memory consumers independent on group_oom setting, so
>> > > > by default I'm selecting the biggest task in the biggest memcg.
>> > >
>> > > My reading is that he is actually proposing the same thing I've been
>> > > mentioning. Simply select the biggest killable entity (leaf memcg or
>> > > group_oom hierarchy) and either kill the largest task in that entity
>> > > (for !group_oom) or the whole memcg/hierarchy otherwise.
>> >
>> > He wrote the following:
>> > "So I'm leaning toward the second model: compare all oomgroups and
>> > standalone tasks in the system with each other, independent of the
>> > failed hierarchical control structure. Then kill the biggest of them."
>>
>> I will let Johannes to comment but I believe this is just a
>> misunderstanding. If we compared only the biggest task from each memcg
>> then we are basically losing our fairness objective, aren't we?
>
> Sorry about the confusion.
>
> Yeah I was making the case for what Michal proposed, to kill the
> biggest terminal consumer, which is either a task or an oomgroup.
>
> You'd basically iterate through all the tasks and cgroups in the
> system and pick the biggest task that isn't in an oom group or the
> biggest oom group and then kill that.
>
> Yeah, you'd have to compare the memory footprints of tasks with the
> memory footprints of cgroups. These aren't defined identically, and
> tasks don't get attributed every type of allocation that a cgroup
> would. But it should get us in the ballpark, and I cannot picture a
> scenario where this would lead to a completely undesirable outcome.

That last sentence:

> I cannot picture a scenario where this would lead to a completely undesirable outcome.

I feel like David has offered examples here, and many of us at Google
have offered examples as long ago as 2013 (if I recall) of cases where
the proposed heuristic is EXACTLY WRONG. We need OOM behavior to kill
in a deterministic order configured by policy. Sometimes, I would
literally prefer to kill every other cgroup before killing "the big
one". The policy is *all* that matters for shared clusters of varying
users and priorities.

We did this in Borg, and it works REALLY well. Has for years. Now
that the world is adopting Kubernetes we need it again, only it's much
harder to carry a kernel patch in this case.

2017-09-27 07:37:52

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue 26-09-17 14:04:41, David Rientjes wrote:
> On Tue, 26 Sep 2017, Michal Hocko wrote:
>
> > > No, I agree that we shouldn't compare sibling memory cgroups based on
> > > different criteria depending on whether group_oom is set or not.
> > >
> > > I think it would be better to compare siblings based on the same criteria
> > > independent of group_oom if the user has mounted the hierarchy with the
> > > new mode (I think we all agree that the mount option is needed). It's
> > > very easy to describe to the user and the selection is simple to
> > > understand.
> >
> > I disagree. Just take the most simplistic example when cgroups reflect
> > some other higher level organization - e.g. school with teachers,
> > students and admins as the top level cgroups to control the proper cpu
> > share load. Now you want to have a fair OOM selection between different
> > entities. Do you consider selecting students all the time as an expected
> > behavior just because their are the largest group? This just doesn't
> > make any sense to me.
> >
>
> Are you referring to this?
>
> root
> / \
> students admins
> / \ / \
> A B C D
>
> If the cumulative usage of all students exceeds the cumulative usage of
> all admins, yes, the choice is to kill from the /students tree.

Which is wrong IMHO because the number of stutends is likely much more
larger than admins (or teachers) yet it might be the admins one to run
away. This example simply shows how comparing siblinks highly depends
on the way you organize the hierarchy rather than the actual memory
consumer runaways which is the primary goal of the OOM killer to handle.

> This has been Roman's design from the very beginning.

I suspect this was the case because deeper hierarchies for
organizational purposes haven't been considered.

> If the preference is to kill
> the single largest process, which may be attached to either subtree, you
> would not have opted-in to the new heuristic.

I believe you are making a wrong assumption here. The container cleanup
is sound reason to opt in and deeper hierarchies are simply required in
the cgroup v2 world where you do not have separate hierarchies.

> > > Then, once a cgroup has been chosen as the victim cgroup,
> > > kill the process with the highest badness, allowing the user to influence
> > > that with /proc/pid/oom_score_adj just as today, if group_oom is disabled;
> > > otherwise, kill all eligible processes if enabled.
> >
> > And now, what should be the semantic of group_oom on an intermediate
> > (non-leaf) memcg? Why should we compare it to other killable entities?
> > Roman was mentioning a setup where a _single_ workload consists of a
> > deeper hierarchy which has to be shut down at once. It absolutely makes
> > sense to consider the cumulative memory of that hierarchy when we are
> > going to kill it all.
> >
>
> If group_oom is enabled on an intermediate memcg, I think the intuitive
> way to handle it would be that all descendants are also implicitly or
> explicitly group_oom.

This is an interesting point. I would tend to agree here. If somebody
requires all-in clean up up the hierarchy it feels strange that a
subtree would disagree (e.g. during memcg oom on the subtree). I can
hardly see a usecase that would really need a different group_oom policy
depending on where in the hierarchy the oom happened to be honest.
Roman?

> It is compared to sibling cgroups based on
> cumulative usage at the time of oom and the largest is chosen and
> iterated. The point is to separate out the selection heuristic (policy)
> from group_oom (mechanism) so that we don't bias or prefer subtrees based
> on group_oom, which makes this much more complex.

I disagree. group_oom determines killable entity and making a decision
based on a non-killable entities is weird as already pointed out.
--
Michal Hocko
SUSE Labs

2017-09-27 07:43:23

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue 26-09-17 20:37:37, Tim Hockin wrote:
[...]
> I feel like David has offered examples here, and many of us at Google
> have offered examples as long ago as 2013 (if I recall) of cases where
> the proposed heuristic is EXACTLY WRONG.

I do not think we have discussed anything resembling the current
approach. And I would really appreciate some more examples where
decisions based on leaf nodes would be EXACTLY WRONG.

> We need OOM behavior to kill in a deterministic order configured by
> policy.

And nobody is objecting to this usecase. I think we can build a priority
policy on top of leaf-based decision as well. The main point we are
trying to sort out here is a reasonable semantic that would work for
most workloads. Sibling based selection will simply not work on those
that have to use deeper hierarchies for organizational purposes. I
haven't heard a counter argument for that example yet.
--
Michal Hocko
SUSE Labs

2017-09-27 09:58:44

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed, Sep 27, 2017 at 09:37:44AM +0200, Michal Hocko wrote:
> On Tue 26-09-17 14:04:41, David Rientjes wrote:
> > On Tue, 26 Sep 2017, Michal Hocko wrote:
> >
> > > > No, I agree that we shouldn't compare sibling memory cgroups based on
> > > > different criteria depending on whether group_oom is set or not.
> > > >
> > > > I think it would be better to compare siblings based on the same criteria
> > > > independent of group_oom if the user has mounted the hierarchy with the
> > > > new mode (I think we all agree that the mount option is needed). It's
> > > > very easy to describe to the user and the selection is simple to
> > > > understand.
> > >
> > > I disagree. Just take the most simplistic example when cgroups reflect
> > > some other higher level organization - e.g. school with teachers,
> > > students and admins as the top level cgroups to control the proper cpu
> > > share load. Now you want to have a fair OOM selection between different
> > > entities. Do you consider selecting students all the time as an expected
> > > behavior just because their are the largest group? This just doesn't
> > > make any sense to me.
> > >
> >
> > Are you referring to this?
> >
> > root
> > / \
> > students admins
> > / \ / \
> > A B C D
> >
> > If the cumulative usage of all students exceeds the cumulative usage of
> > all admins, yes, the choice is to kill from the /students tree.
>
> Which is wrong IMHO because the number of stutends is likely much more
> larger than admins (or teachers) yet it might be the admins one to run
> away. This example simply shows how comparing siblinks highly depends
> on the way you organize the hierarchy rather than the actual memory
> consumer runaways which is the primary goal of the OOM killer to handle.
>
> > This has been Roman's design from the very beginning.
>
> I suspect this was the case because deeper hierarchies for
> organizational purposes haven't been considered.
>
> > If the preference is to kill
> > the single largest process, which may be attached to either subtree, you
> > would not have opted-in to the new heuristic.
>
> I believe you are making a wrong assumption here. The container cleanup
> is sound reason to opt in and deeper hierarchies are simply required in
> the cgroup v2 world where you do not have separate hierarchies.
>
> > > > Then, once a cgroup has been chosen as the victim cgroup,
> > > > kill the process with the highest badness, allowing the user to influence
> > > > that with /proc/pid/oom_score_adj just as today, if group_oom is disabled;
> > > > otherwise, kill all eligible processes if enabled.
> > >
> > > And now, what should be the semantic of group_oom on an intermediate
> > > (non-leaf) memcg? Why should we compare it to other killable entities?
> > > Roman was mentioning a setup where a _single_ workload consists of a
> > > deeper hierarchy which has to be shut down at once. It absolutely makes
> > > sense to consider the cumulative memory of that hierarchy when we are
> > > going to kill it all.
> > >
> >
> > If group_oom is enabled on an intermediate memcg, I think the intuitive
> > way to handle it would be that all descendants are also implicitly or
> > explicitly group_oom.
>
> This is an interesting point. I would tend to agree here. If somebody
> requires all-in clean up up the hierarchy it feels strange that a
> subtree would disagree (e.g. during memcg oom on the subtree). I can
> hardly see a usecase that would really need a different group_oom policy
> depending on where in the hierarchy the oom happened to be honest.
> Roman?

Yes, I'd say that it's strange to apply settings from outside the OOMing
cgroup to the subtree, but actually it's not. The oom_group setting should
basically mean that the OOM killer will not kill a random task in the subtree.
And it doesn't matter if it was global or memcg-wide OOM.

Applied to v9. Thanks!

2017-09-27 10:19:51

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed, Sep 27, 2017 at 09:43:19AM +0200, Michal Hocko wrote:
> On Tue 26-09-17 20:37:37, Tim Hockin wrote:
> [...]
> > I feel like David has offered examples here, and many of us at Google
> > have offered examples as long ago as 2013 (if I recall) of cases where
> > the proposed heuristic is EXACTLY WRONG.
>
> I do not think we have discussed anything resembling the current
> approach. And I would really appreciate some more examples where
> decisions based on leaf nodes would be EXACTLY WRONG.
>

I would agree here.

The discussing two-step approach (select biggest leaf or oom_group memcg,
then select largest process inside) does really look as a way to go.

It should work well in practice and it allows further development.
It will catch workloads which are leaking child processes by default,
which is an advantage in comparison to the existing algorithm.

Both strong hierarchical approach (as in v8) and pure flat (by Johannes)
are more limiting. In first case, deep hierarchies are affected (as Michal
mentioned) and we stick with tree traverse policy (Tejun's point).

In second case, the further development is under a question: any new idea
(say, oom_priorities, or, for example, if we will have a new useful memcg
metric) should be applied to processes and memcgs simultaneously.
Also, We drop any idea of memcg-level fairness and obtain some implementation
issues (which I mentioned earlier). The idea of mixing tasks and memcgs
leads to a much more hairy code, and the OOM code is already quite hairy.
The idea of comparing killable entities is a leaking abstraction,
as we can't predict how much memory killing a single process will release
(say, for example, the process is the init in a pid namespace).

> > We need OOM behavior to kill in a deterministic order configured by
> > policy.
>
> And nobody is objecting to this usecase. I think we can build a priority
> policy on top of leaf-based decision as well. The main point we are
> trying to sort out here is a reasonable semantic that would work for
> most workloads. Sibling based selection will simply not work on those
> that have to use deeper hierarchies for organizational purposes. I
> haven't heard a counter argument for that example yet.

Yes, implementing oom_priorities is a ~15 lines patch on top of
the discussing approach. David can use this small off-stream patch
for now, in any case it's a step forward in comparison to the existing state.

Overall, do we have any open question left? Does anyone has any strong
arguments against the discussing design?

Thanks!

2017-09-27 15:36:15

by Tim Hockin

[permalink] [raw]

Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Wed, Sep 27, 2017 at 12:43 AM, Michal Hocko <[email protected]> wrote:
> On Tue 26-09-17 20:37:37, Tim Hockin wrote:
> [...]
>> I feel like David has offered examples here, and many of us at Google
>> have offered examples as long ago as 2013 (if I recall) of cases where
>> the proposed heuristic is EXACTLY WRONG.
>
> I do not think we have discussed anything resembling the current
> approach. And I would really appreciate some more examples where
> decisions based on leaf nodes would be EXACTLY WRONG.
>
>> We need OOM behavior to kill in a deterministic order configured by
>> policy.
>
> And nobody is objecting to this usecase. I think we can build a priority
> policy on top of leaf-based decision as well. The main point we are
> trying to sort out here is a reasonable semantic that would work for
> most workloads. Sibling based selection will simply not work on those
> that have to use deeper hierarchies for organizational purposes. I
> haven't heard a counter argument for that example yet.

We have a priority-based, multi-user cluster. That cluster runs a
variety of work, including critical things like search and gmail, as
well as non-critical things like batch work. We try to offer our
users an SLA around how often they will be killed by factors outside
themselves, but we also want to get higher utilization. We know for a
fact (data, lots of data) that most jobs have spare memory capacity,
set aside for spikes or simply because accurate sizing is hard. We
can sell "guaranteed" resources to critical jobs, with a high SLA. We
can sell "best effort" resources to non-critical jobs with a low SLA.
We achieve much better overall utilization this way.

I need to represent the priority of these tasks in a way that gives me
a very strong promise that, in case of system OOM, the non-critical
jobs will be chosen before the critical jobs. Regardless of size.
Regardless of how many non-critical jobs have to die. I'd rather kill
*all* of the non-critical jobs than a single critical job. Size of
the process or cgroup is simply not a factor, and honestly given 2
options of equal priority I'd say age matters more than size.

So concretely I have 2 first-level cgroups, one for "guaranteed" and
one for "best effort" classes. I always want to kill from "best
effort", even if that means killing 100 small cgroups, before touching
"guaranteed".

I apologize if this is not as thorough as the rest of the thread - I
am somewhat out of touch with the guts of it all these days. I just
feel compelled to indicate that, as a historical user (via Google
systems) and current user (via Kubernetes), some of the assertions
being made here do not ring true for our very real use cases. I
desperately want cgroup-aware OOM handing, but it has to be
policy-based or it is just not useful to us.

Thanks.

Tim

2017-09-27 16:23:46