At the moment memcg stats are read in four contexts:
1. memcg stat user interfaces
2. dirty throttling
3. page fault
4. memory reclaim
Currently the kernel flushes the stats for first two cases. Flushing the
stats for remaining two casese may have performance impact. Always
flushing the memcg stats on the page fault code path may negatively
impacts the performance of the applications. In addition flushing in the
memory reclaim code path, though treated as slowpath, can become the
source of contention for the global lock taken for stat flushing because
when system or memcg is under memory pressure, many tasks may enter the
reclaim path.
This patch uses following mechanisms to solve these challenges:
1. Periodically flush the stats from root memcg every 2 seconds. This
will time limit the out of sync stats.
2. Asynchronously flush the stats after fixed number of stat updates.
In the worst case the stat can be out of sync by O(nr_cpus * BATCH) for
2 seconds.
3. For avoiding thundering herd to flush the stats particularly from the
memory reclaim context, introduce memcg local spinlock and let only one
flusher active at a time. This could have been done through
cgroup_rstat_lock lock but that lock is used by other subsystem and for
userspace reading memcg stats. So, it is better to keep flushers
introduced by this patch decoupled from cgroup_rstat_lock.
Signed-off-by: Shakeel Butt <[email protected]>
---
Changes since v3:
- Add back the sigoff
Changes since v2:
- Changed the subject of the patch
- Added mechanism to bound errors to nr_cpus instead of nr_cgroups
- memcg local lock to let one active flusher
Changes since v1:
- use system_unbound_wq for flushing the memcg stats
include/linux/memcontrol.h | 6 ++++++
mm/memcontrol.c | 34 ++++++++++++++++++++++++++++++++++
mm/vmscan.c | 6 ++++++
3 files changed, 46 insertions(+)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 0bfa0409af22..fa095a94ae56 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -991,6 +991,8 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
return x;
}
+void mem_cgroup_flush_stats(void);
+
void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
int val);
void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val);
@@ -1400,6 +1402,10 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
return node_page_state(lruvec_pgdat(lruvec), idx);
}
+static inline void mem_cgroup_flush_stats(void)
+{
+}
+
static inline void __mod_memcg_lruvec_state(struct lruvec *lruvec,
enum node_stat_item idx, int val)
{
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 848d711bf576..39a00991fc80 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -103,6 +103,14 @@ static bool do_memsw_account(void)
return !cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_noswap;
}
+/* memcg and lruvec stats flushing */
+static void flush_memcg_stats_dwork(struct work_struct *w);
+static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork);
+static void flush_memcg_stats_work(struct work_struct *w);
+static DECLARE_WORK(stats_flush_work, flush_memcg_stats_work);
+static DEFINE_PER_CPU(unsigned int, stats_flush_threshold);
+static DEFINE_SPINLOCK(stats_flush_lock);
+
#define THRESHOLDS_EVENTS_TARGET 128
#define SOFTLIMIT_EVENTS_TARGET 1024
@@ -685,6 +693,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
/* Update lruvec */
__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
+ if (!(__this_cpu_inc_return(stats_flush_threshold) % MEMCG_CHARGE_BATCH))
+ queue_work(system_unbound_wq, &stats_flush_work);
}
/**
@@ -5248,6 +5258,10 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
/* Online state pins memcg ID, memcg ID pins CSS */
refcount_set(&memcg->id.ref, 1);
css_get(css);
+
+ if (unlikely(mem_cgroup_is_root(memcg)))
+ queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
+ 2UL*HZ);
return 0;
}
@@ -5339,6 +5353,26 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
memcg_wb_domain_size_changed(memcg);
}
+void mem_cgroup_flush_stats(void)
+{
+ if (!spin_trylock(&stats_flush_lock))
+ return;
+
+ cgroup_rstat_flush(root_mem_cgroup->css.cgroup);
+ spin_unlock(&stats_flush_lock);
+}
+
+static void flush_memcg_stats_dwork(struct work_struct *w)
+{
+ mem_cgroup_flush_stats();
+ queue_delayed_work(system_unbound_wq, &stats_flush_dwork, 2UL*HZ);
+}
+
+static void flush_memcg_stats_work(struct work_struct *w)
+{
+ mem_cgroup_flush_stats();
+}
+
static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
{
struct mem_cgroup *memcg = mem_cgroup_from_css(css);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a7602f71ec04..1cc05ab8ca15 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2893,6 +2893,12 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
again:
+ /*
+ * Flush the memory cgroup stats, so that we read accurate per-memcg
+ * lruvec stats for heuristics.
+ */
+ mem_cgroup_flush_stats();
+
memset(&sc->nr, 0, sizeof(sc->nr));
nr_reclaimed = sc->nr_reclaimed;
--
2.32.0.93.g670b81a890-goog
Hi,
On 14.07.2021 03:39, Shakeel Butt wrote:
> At the moment memcg stats are read in four contexts:
>
> 1. memcg stat user interfaces
> 2. dirty throttling
> 3. page fault
> 4. memory reclaim
>
> Currently the kernel flushes the stats for first two cases. Flushing the
> stats for remaining two casese may have performance impact. Always
> flushing the memcg stats on the page fault code path may negatively
> impacts the performance of the applications. In addition flushing in the
> memory reclaim code path, though treated as slowpath, can become the
> source of contention for the global lock taken for stat flushing because
> when system or memcg is under memory pressure, many tasks may enter the
> reclaim path.
>
> This patch uses following mechanisms to solve these challenges:
>
> 1. Periodically flush the stats from root memcg every 2 seconds. This
> will time limit the out of sync stats.
>
> 2. Asynchronously flush the stats after fixed number of stat updates.
> In the worst case the stat can be out of sync by O(nr_cpus * BATCH) for
> 2 seconds.
>
> 3. For avoiding thundering herd to flush the stats particularly from the
> memory reclaim context, introduce memcg local spinlock and let only one
> flusher active at a time. This could have been done through
> cgroup_rstat_lock lock but that lock is used by other subsystem and for
> userspace reading memcg stats. So, it is better to keep flushers
> introduced by this patch decoupled from cgroup_rstat_lock.
>
> Signed-off-by: Shakeel Butt <[email protected]>
This patch landed in today's linux-next (next-20210716) as commit
42265e014ac7 ("memcg: infrastructure to flush memcg stats"). On my test
system's I found that it triggers a kernel BUG on all ARM64 boards:
BUG: sleeping function called from invalid context at
kernel/cgroup/rstat.c:200
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 7, name:
kworker/u8:0
3 locks held by kworker/u8:0/7:
#0: ffff00004000c938 ((wq_completion)events_unbound){+.+.}-{0:0}, at:
process_one_work+0x200/0x718
#1: ffff80001334bdd0 ((stats_flush_dwork).work){+.+.}-{0:0}, at:
process_one_work+0x200/0x718
#2: ffff8000124f6d40 (stats_flush_lock){+.+.}-{2:2}, at:
mem_cgroup_flush_stats+0x20/0x48
CPU: 2 PID: 7 Comm: kworker/u8:0 Tainted: G W 5.14.0-rc1+ #3713
Hardware name: Raspberry Pi 4 Model B (DT)
Workqueue: events_unbound flush_memcg_stats_dwork
Call trace:
dump_backtrace+0x0/0x1d0
show_stack+0x14/0x20
dump_stack_lvl+0x88/0xb0
dump_stack+0x14/0x2c
___might_sleep+0x1dc/0x200
__might_sleep+0x4c/0x88
cgroup_rstat_flush+0x2c/0x58
mem_cgroup_flush_stats+0x34/0x48
flush_memcg_stats_dwork+0xc/0x38
process_one_work+0x2a8/0x718
worker_thread+0x48/0x460
kthread+0x12c/0x160
ret_from_fork+0x10/0x18
This can be also reproduced with QEmu. Please let me know if I can help
fixing this issue.
> ---
> Changes since v3:
> - Add back the sigoff
>
> Changes since v2:
> - Changed the subject of the patch
> - Added mechanism to bound errors to nr_cpus instead of nr_cgroups
> - memcg local lock to let one active flusher
>
> Changes since v1:
> - use system_unbound_wq for flushing the memcg stats
>
> include/linux/memcontrol.h | 6 ++++++
> mm/memcontrol.c | 34 ++++++++++++++++++++++++++++++++++
> mm/vmscan.c | 6 ++++++
> 3 files changed, 46 insertions(+)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0bfa0409af22..fa095a94ae56 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -991,6 +991,8 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> return x;
> }
>
> +void mem_cgroup_flush_stats(void);
> +
> void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> int val);
> void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val);
> @@ -1400,6 +1402,10 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> return node_page_state(lruvec_pgdat(lruvec), idx);
> }
>
> +static inline void mem_cgroup_flush_stats(void)
> +{
> +}
> +
> static inline void __mod_memcg_lruvec_state(struct lruvec *lruvec,
> enum node_stat_item idx, int val)
> {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 848d711bf576..39a00991fc80 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -103,6 +103,14 @@ static bool do_memsw_account(void)
> return !cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_noswap;
> }
>
> +/* memcg and lruvec stats flushing */
> +static void flush_memcg_stats_dwork(struct work_struct *w);
> +static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork);
> +static void flush_memcg_stats_work(struct work_struct *w);
> +static DECLARE_WORK(stats_flush_work, flush_memcg_stats_work);
> +static DEFINE_PER_CPU(unsigned int, stats_flush_threshold);
> +static DEFINE_SPINLOCK(stats_flush_lock);
> +
> #define THRESHOLDS_EVENTS_TARGET 128
> #define SOFTLIMIT_EVENTS_TARGET 1024
>
> @@ -685,6 +693,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>
> /* Update lruvec */
> __this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> + if (!(__this_cpu_inc_return(stats_flush_threshold) % MEMCG_CHARGE_BATCH))
> + queue_work(system_unbound_wq, &stats_flush_work);
> }
>
> /**
> @@ -5248,6 +5258,10 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
> /* Online state pins memcg ID, memcg ID pins CSS */
> refcount_set(&memcg->id.ref, 1);
> css_get(css);
> +
> + if (unlikely(mem_cgroup_is_root(memcg)))
> + queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
> + 2UL*HZ);
> return 0;
> }
>
> @@ -5339,6 +5353,26 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
> memcg_wb_domain_size_changed(memcg);
> }
>
> +void mem_cgroup_flush_stats(void)
> +{
> + if (!spin_trylock(&stats_flush_lock))
> + return;
> +
> + cgroup_rstat_flush(root_mem_cgroup->css.cgroup);
> + spin_unlock(&stats_flush_lock);
> +}
> +
> +static void flush_memcg_stats_dwork(struct work_struct *w)
> +{
> + mem_cgroup_flush_stats();
> + queue_delayed_work(system_unbound_wq, &stats_flush_dwork, 2UL*HZ);
> +}
> +
> +static void flush_memcg_stats_work(struct work_struct *w)
> +{
> + mem_cgroup_flush_stats();
> +}
> +
> static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
> {
> struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a7602f71ec04..1cc05ab8ca15 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2893,6 +2893,12 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
>
> again:
> + /*
> + * Flush the memory cgroup stats, so that we read accurate per-memcg
> + * lruvec stats for heuristics.
> + */
> + mem_cgroup_flush_stats();
> +
> memset(&sc->nr, 0, sizeof(sc->nr));
>
> nr_reclaimed = sc->nr_reclaimed;
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
Hi Marek
On Fri, Jul 16, 2021 at 8:03 AM Marek Szyprowski
<[email protected]> wrote:
>
> Hi,
>
> On 14.07.2021 03:39, Shakeel Butt wrote:
> > At the moment memcg stats are read in four contexts:
> >
> > 1. memcg stat user interfaces
> > 2. dirty throttling
> > 3. page fault
> > 4. memory reclaim
> >
> > Currently the kernel flushes the stats for first two cases. Flushing the
> > stats for remaining two casese may have performance impact. Always
> > flushing the memcg stats on the page fault code path may negatively
> > impacts the performance of the applications. In addition flushing in the
> > memory reclaim code path, though treated as slowpath, can become the
> > source of contention for the global lock taken for stat flushing because
> > when system or memcg is under memory pressure, many tasks may enter the
> > reclaim path.
> >
> > This patch uses following mechanisms to solve these challenges:
> >
> > 1. Periodically flush the stats from root memcg every 2 seconds. This
> > will time limit the out of sync stats.
> >
> > 2. Asynchronously flush the stats after fixed number of stat updates.
> > In the worst case the stat can be out of sync by O(nr_cpus * BATCH) for
> > 2 seconds.
> >
> > 3. For avoiding thundering herd to flush the stats particularly from the
> > memory reclaim context, introduce memcg local spinlock and let only one
> > flusher active at a time. This could have been done through
> > cgroup_rstat_lock lock but that lock is used by other subsystem and for
> > userspace reading memcg stats. So, it is better to keep flushers
> > introduced by this patch decoupled from cgroup_rstat_lock.
> >
> > Signed-off-by: Shakeel Butt <[email protected]>
>
> This patch landed in today's linux-next (next-20210716) as commit
> 42265e014ac7 ("memcg: infrastructure to flush memcg stats"). On my test
> system's I found that it triggers a kernel BUG on all ARM64 boards:
>
> BUG: sleeping function called from invalid context at
> kernel/cgroup/rstat.c:200
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 7, name:
> kworker/u8:0
> 3 locks held by kworker/u8:0/7:
> #0: ffff00004000c938 ((wq_completion)events_unbound){+.+.}-{0:0}, at:
> process_one_work+0x200/0x718
> #1: ffff80001334bdd0 ((stats_flush_dwork).work){+.+.}-{0:0}, at:
> process_one_work+0x200/0x718
> #2: ffff8000124f6d40 (stats_flush_lock){+.+.}-{2:2}, at:
> mem_cgroup_flush_stats+0x20/0x48
> CPU: 2 PID: 7 Comm: kworker/u8:0 Tainted: G W 5.14.0-rc1+ #3713
> Hardware name: Raspberry Pi 4 Model B (DT)
> Workqueue: events_unbound flush_memcg_stats_dwork
> Call trace:
> dump_backtrace+0x0/0x1d0
> show_stack+0x14/0x20
> dump_stack_lvl+0x88/0xb0
> dump_stack+0x14/0x2c
> ___might_sleep+0x1dc/0x200
> __might_sleep+0x4c/0x88
> cgroup_rstat_flush+0x2c/0x58
> mem_cgroup_flush_stats+0x34/0x48
> flush_memcg_stats_dwork+0xc/0x38
> process_one_work+0x2a8/0x718
> worker_thread+0x48/0x460
> kthread+0x12c/0x160
> ret_from_fork+0x10/0x18
>
> This can be also reproduced with QEmu. Please let me know if I can help
> fixing this issue.
>
Thanks for the report. The issue can be fixed by changing
cgroup_rstat_flush() to cgroup_rstat_flush_irqsafe() in
mem_cgroup_flush_stats(). I will send out the updated patch in a
couple of hours after a bit more testing.
Hi,
On 16.07.2021 17:14, Shakeel Butt wrote:
> Hi Marek
>
> On Fri, Jul 16, 2021 at 8:03 AM Marek Szyprowski
> <[email protected]> wrote:
>> Hi,
>>
>> On 14.07.2021 03:39, Shakeel Butt wrote:
>>> At the moment memcg stats are read in four contexts:
>>>
>>> 1. memcg stat user interfaces
>>> 2. dirty throttling
>>> 3. page fault
>>> 4. memory reclaim
>>>
>>> Currently the kernel flushes the stats for first two cases. Flushing the
>>> stats for remaining two casese may have performance impact. Always
>>> flushing the memcg stats on the page fault code path may negatively
>>> impacts the performance of the applications. In addition flushing in the
>>> memory reclaim code path, though treated as slowpath, can become the
>>> source of contention for the global lock taken for stat flushing because
>>> when system or memcg is under memory pressure, many tasks may enter the
>>> reclaim path.
>>>
>>> This patch uses following mechanisms to solve these challenges:
>>>
>>> 1. Periodically flush the stats from root memcg every 2 seconds. This
>>> will time limit the out of sync stats.
>>>
>>> 2. Asynchronously flush the stats after fixed number of stat updates.
>>> In the worst case the stat can be out of sync by O(nr_cpus * BATCH) for
>>> 2 seconds.
>>>
>>> 3. For avoiding thundering herd to flush the stats particularly from the
>>> memory reclaim context, introduce memcg local spinlock and let only one
>>> flusher active at a time. This could have been done through
>>> cgroup_rstat_lock lock but that lock is used by other subsystem and for
>>> userspace reading memcg stats. So, it is better to keep flushers
>>> introduced by this patch decoupled from cgroup_rstat_lock.
>>>
>>> Signed-off-by: Shakeel Butt <[email protected]>
>> This patch landed in today's linux-next (next-20210716) as commit
>> 42265e014ac7 ("memcg: infrastructure to flush memcg stats"). On my test
>> system's I found that it triggers a kernel BUG on all ARM64 boards:
>>
>> BUG: sleeping function called from invalid context at
>> kernel/cgroup/rstat.c:200
>> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 7, name:
>> kworker/u8:0
>> 3 locks held by kworker/u8:0/7:
>> #0: ffff00004000c938 ((wq_completion)events_unbound){+.+.}-{0:0}, at:
>> process_one_work+0x200/0x718
>> #1: ffff80001334bdd0 ((stats_flush_dwork).work){+.+.}-{0:0}, at:
>> process_one_work+0x200/0x718
>> #2: ffff8000124f6d40 (stats_flush_lock){+.+.}-{2:2}, at:
>> mem_cgroup_flush_stats+0x20/0x48
>> CPU: 2 PID: 7 Comm: kworker/u8:0 Tainted: G W 5.14.0-rc1+ #3713
>> Hardware name: Raspberry Pi 4 Model B (DT)
>> Workqueue: events_unbound flush_memcg_stats_dwork
>> Call trace:
>> dump_backtrace+0x0/0x1d0
>> show_stack+0x14/0x20
>> dump_stack_lvl+0x88/0xb0
>> dump_stack+0x14/0x2c
>> ___might_sleep+0x1dc/0x200
>> __might_sleep+0x4c/0x88
>> cgroup_rstat_flush+0x2c/0x58
>> mem_cgroup_flush_stats+0x34/0x48
>> flush_memcg_stats_dwork+0xc/0x38
>> process_one_work+0x2a8/0x718
>> worker_thread+0x48/0x460
>> kthread+0x12c/0x160
>> ret_from_fork+0x10/0x18
>>
>> This can be also reproduced with QEmu. Please let me know if I can help
>> fixing this issue.
>>
> Thanks for the report. The issue can be fixed by changing
> cgroup_rstat_flush() to cgroup_rstat_flush_irqsafe() in
> mem_cgroup_flush_stats(). I will send out the updated patch in a
> couple of hours after a bit more testing.
Right, this fixes the issue on my test systems. Feel free to add:
Reported-by: Marek Szyprowski <[email protected]>
Tested-by: Marek Szyprowski <[email protected]>
to the fixup patch if the target kernel tree won't be rebased and the
original patch (42265e014ac7) stays.
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland