One of our machines keeled over trying to rebuild the scheduler domains.
Mainline produces the same splat:
BUG: unable to handle page fault for address: 0000607f820054db
CPU: 2 PID: 149 Comm: kworker/1:1 Not tainted 5.10.0-rc1-master+ #6
Workqueue: events cpuset_hotplug_workfn
RIP: build_sched_domains
Call Trace:
partition_sched_domains_locked
rebuild_sched_domains_locked
cpuset_hotplug_workfn
It happens with cgroup2 and exclusive cpusets only. This reproducer
triggers it on an 8-cpu vm and works most effectively with no
preexisting child cgroups:
cd $UNIFIED_ROOT
mkdir cg1
echo 4-7 > cg1/cpuset.cpus
echo root > cg1/cpuset.cpus.partition
# with smt/control reading 'on',
echo off > /sys/devices/system/cpu/smt/control
RIP maps to
sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
from sd_init(). sd_id is calculated earlier in the same function:
cpumask_and(sched_domain_span(sd), cpu_map, tl->mask(cpu));
sd_id = cpumask_first(sched_domain_span(sd));
tl->mask(cpu), which reads cpu_sibling_map on x86, returns an empty mask
and so cpumask_first() returns >= nr_cpu_ids, which leads to the bogus
value from per_cpu_ptr() above.
The problem is a race between cpuset_hotplug_workfn() and a later
offline of CPU N. cpuset_hotplug_workfn() updates the effective masks
when N is still online, the offline clears N from cpu_sibling_map, and
then the worker uses the stale effective masks that still have N to
generate the scheduling domains, leading the worker to read
N's empty cpu_sibling_map in sd_init().
rebuild_sched_domains_locked() prevented the race during the cgroup2
cpuset series up until the Fixes commit changed its check. Make the
check more robust so that it can detect an offline CPU in any exclusive
cpuset's effective mask, not just the top one.
Fixes: 0ccea8feb980 ("cpuset: Make generate_sched_domains() work with partition")
Signed-off-by: Daniel Jordan <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Prateek Sood <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
v2:
- Made the comment more descriptive (Peter)
kernel/cgroup/cpuset.c | 33 ++++++++++++++++++++++++++++-----
1 file changed, 28 insertions(+), 5 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 57b5b5d0a5fd..53c70c470a38 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -983,25 +983,48 @@ partition_and_rebuild_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
*/
static void rebuild_sched_domains_locked(void)
{
+ struct cgroup_subsys_state *pos_css;
struct sched_domain_attr *attr;
cpumask_var_t *doms;
+ struct cpuset *cs;
int ndoms;
lockdep_assert_cpus_held();
percpu_rwsem_assert_held(&cpuset_rwsem);
/*
- * We have raced with CPU hotplug. Don't do anything to avoid
+ * If we have raced with CPU hotplug, return early to avoid
* passing doms with offlined cpu to partition_sched_domains().
- * Anyways, hotplug work item will rebuild sched domains.
+ * Anyways, cpuset_hotplug_workfn() will rebuild sched domains.
+ *
+ * With no CPUs in any subpartitions, top_cpuset's effective CPUs
+ * should be the same as the active CPUs, so checking only top_cpuset
+ * is enough to detect racing CPU offlines.
*/
if (!top_cpuset.nr_subparts_cpus &&
!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
return;
- if (top_cpuset.nr_subparts_cpus &&
- !cpumask_subset(top_cpuset.effective_cpus, cpu_active_mask))
- return;
+ /*
+ * With subpartition CPUs, however, the effective CPUs of a partition
+ * root should be only a subset of the active CPUs. Since a CPU in any
+ * partition root could be offlined, all must be checked.
+ */
+ if (top_cpuset.nr_subparts_cpus) {
+ rcu_read_lock();
+ cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
+ if (!is_partition_root(cs)) {
+ pos_css = css_rightmost_descendant(pos_css);
+ continue;
+ }
+ if (!cpumask_subset(cs->effective_cpus,
+ cpu_active_mask)) {
+ rcu_read_unlock();
+ return;
+ }
+ }
+ rcu_read_unlock();
+ }
/* Generate domain masks and attrs */
ndoms = generate_sched_domains(&doms, &attr);
base-commit: 3d5e28bff7ad55aea081c1af516cc1c94a5eca7d
--
2.29.2
On Thu, Nov 12, 2020 at 12:17:11PM -0500, Daniel Jordan wrote:
> One of our machines keeled over trying to rebuild the scheduler domains.
> Mainline produces the same splat:
>
> BUG: unable to handle page fault for address: 0000607f820054db
> CPU: 2 PID: 149 Comm: kworker/1:1 Not tainted 5.10.0-rc1-master+ #6
> Workqueue: events cpuset_hotplug_workfn
> RIP: build_sched_domains
> Call Trace:
> partition_sched_domains_locked
> rebuild_sched_domains_locked
> cpuset_hotplug_workfn
>
> It happens with cgroup2 and exclusive cpusets only. This reproducer
> triggers it on an 8-cpu vm and works most effectively with no
> preexisting child cgroups:
>
> cd $UNIFIED_ROOT
> mkdir cg1
> echo 4-7 > cg1/cpuset.cpus
> echo root > cg1/cpuset.cpus.partition
>
> # with smt/control reading 'on',
> echo off > /sys/devices/system/cpu/smt/control
>
> RIP maps to
>
> sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
>
> from sd_init(). sd_id is calculated earlier in the same function:
>
> cpumask_and(sched_domain_span(sd), cpu_map, tl->mask(cpu));
> sd_id = cpumask_first(sched_domain_span(sd));
>
> tl->mask(cpu), which reads cpu_sibling_map on x86, returns an empty mask
> and so cpumask_first() returns >= nr_cpu_ids, which leads to the bogus
> value from per_cpu_ptr() above.
>
> The problem is a race between cpuset_hotplug_workfn() and a later
> offline of CPU N. cpuset_hotplug_workfn() updates the effective masks
> when N is still online, the offline clears N from cpu_sibling_map, and
> then the worker uses the stale effective masks that still have N to
> generate the scheduling domains, leading the worker to read
> N's empty cpu_sibling_map in sd_init().
>
> rebuild_sched_domains_locked() prevented the race during the cgroup2
> cpuset series up until the Fixes commit changed its check. Make the
> check more robust so that it can detect an offline CPU in any exclusive
> cpuset's effective mask, not just the top one.
>
> Fixes: 0ccea8feb980 ("cpuset: Make generate_sched_domains() work with partition")
> Signed-off-by: Daniel Jordan <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Li Zefan <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Prateek Sood <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Cc: Waiman Long <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
Works for me. TJ, do I take this or do you want it in the cgroup tree?
In that case:
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Hello,
On Fri, Nov 13, 2020 at 09:16:22AM +0100, Peter Zijlstra wrote:
> Works for me. TJ, do I take this or do you want it in the cgroup tree?
>
> In that case:
>
> Acked-by: Peter Zijlstra (Intel) <[email protected]>
Please take it through the sched tree, and FWIW
Acked-by: Tejun Heo <[email protected]>
Thank you.
--
tejun
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 406100f3da08066c00105165db8520bbc7694a36
Gitweb: https://git.kernel.org/tip/406100f3da08066c00105165db8520bbc7694a36
Author: Daniel Jordan <[email protected]>
AuthorDate: Thu, 12 Nov 2020 12:17:11 -05:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 19 Nov 2020 11:25:45 +01:00
cpuset: fix race between hotplug work and later CPU offline
One of our machines keeled over trying to rebuild the scheduler domains.
Mainline produces the same splat:
BUG: unable to handle page fault for address: 0000607f820054db
CPU: 2 PID: 149 Comm: kworker/1:1 Not tainted 5.10.0-rc1-master+ #6
Workqueue: events cpuset_hotplug_workfn
RIP: build_sched_domains
Call Trace:
partition_sched_domains_locked
rebuild_sched_domains_locked
cpuset_hotplug_workfn
It happens with cgroup2 and exclusive cpusets only. This reproducer
triggers it on an 8-cpu vm and works most effectively with no
preexisting child cgroups:
cd $UNIFIED_ROOT
mkdir cg1
echo 4-7 > cg1/cpuset.cpus
echo root > cg1/cpuset.cpus.partition
# with smt/control reading 'on',
echo off > /sys/devices/system/cpu/smt/control
RIP maps to
sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
from sd_init(). sd_id is calculated earlier in the same function:
cpumask_and(sched_domain_span(sd), cpu_map, tl->mask(cpu));
sd_id = cpumask_first(sched_domain_span(sd));
tl->mask(cpu), which reads cpu_sibling_map on x86, returns an empty mask
and so cpumask_first() returns >= nr_cpu_ids, which leads to the bogus
value from per_cpu_ptr() above.
The problem is a race between cpuset_hotplug_workfn() and a later
offline of CPU N. cpuset_hotplug_workfn() updates the effective masks
when N is still online, the offline clears N from cpu_sibling_map, and
then the worker uses the stale effective masks that still have N to
generate the scheduling domains, leading the worker to read
N's empty cpu_sibling_map in sd_init().
rebuild_sched_domains_locked() prevented the race during the cgroup2
cpuset series up until the Fixes commit changed its check. Make the
check more robust so that it can detect an offline CPU in any exclusive
cpuset's effective mask, not just the top one.
Fixes: 0ccea8feb980 ("cpuset: Make generate_sched_domains() work with partition")
Signed-off-by: Daniel Jordan <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
---
kernel/cgroup/cpuset.c | 33 ++++++++++++++++++++++++++++-----
1 file changed, 28 insertions(+), 5 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 57b5b5d..53c70c4 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -983,25 +983,48 @@ partition_and_rebuild_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
*/
static void rebuild_sched_domains_locked(void)
{
+ struct cgroup_subsys_state *pos_css;
struct sched_domain_attr *attr;
cpumask_var_t *doms;
+ struct cpuset *cs;
int ndoms;
lockdep_assert_cpus_held();
percpu_rwsem_assert_held(&cpuset_rwsem);
/*
- * We have raced with CPU hotplug. Don't do anything to avoid
+ * If we have raced with CPU hotplug, return early to avoid
* passing doms with offlined cpu to partition_sched_domains().
- * Anyways, hotplug work item will rebuild sched domains.
+ * Anyways, cpuset_hotplug_workfn() will rebuild sched domains.
+ *
+ * With no CPUs in any subpartitions, top_cpuset's effective CPUs
+ * should be the same as the active CPUs, so checking only top_cpuset
+ * is enough to detect racing CPU offlines.
*/
if (!top_cpuset.nr_subparts_cpus &&
!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
return;
- if (top_cpuset.nr_subparts_cpus &&
- !cpumask_subset(top_cpuset.effective_cpus, cpu_active_mask))
- return;
+ /*
+ * With subpartition CPUs, however, the effective CPUs of a partition
+ * root should be only a subset of the active CPUs. Since a CPU in any
+ * partition root could be offlined, all must be checked.
+ */
+ if (top_cpuset.nr_subparts_cpus) {
+ rcu_read_lock();
+ cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
+ if (!is_partition_root(cs)) {
+ pos_css = css_rightmost_descendant(pos_css);
+ continue;
+ }
+ if (!cpumask_subset(cs->effective_cpus,
+ cpu_active_mask)) {
+ rcu_read_unlock();
+ return;
+ }
+ }
+ rcu_read_unlock();
+ }
/* Generate domain masks and attrs */
ndoms = generate_sched_domains(&doms, &attr);