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cgroup/cpuset: Run SCHED_DEADLINE shrink test on valid partition root only

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Commit f82f80426f ("sched/deadline: Ensure that updates to exclusive
cpusets don't break AC") adds a check in validate_change() to make
sure that there is enough bandwidth for SCHED_DEADLINE tasks if we
shrink a v1 exclusive cpuset that has CS_CPU_EXCLUSIVE flag set.

With the introduction of cpuset partition in cgroup v2, we keep setting
the CS_CPU_EXCLUSIVE flag for a partition root so that the SCHED_DEADLINE
check will continue to work as intended. However it turns out that the
current code isn't perfect and there are cases where a cpuset isn't
a valid partition root, but the exclusive flag is still incorrectly
set. This can leads to SCHED_DEADLINE check being incorrectly triggered
when there are deadline tasks in the system. This can result in unexpected
-EBUSY failure when making changes to cpuset control files. Fix that
by checking for a valid partition root for v2 and is_cpu_exclusive()
for v1. It is far easier and less cumbersome than to make sure that
the exclusive flag is only set for valid partition roots.

Even though commit a86ce68078 ("cgroup/cpuset: Extract out
CS_CPU_EXCLUSIVE & CS_SCHED_LOAD_BALANCE handling") is marked as a
commit to be fixed, the problem may exist before that.

Fixes: a86ce68078 ("cgroup/cpuset: Extract out CS_CPU_EXCLUSIVE & CS_SCHED_LOAD_BALANCE handling")
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Waiman Long 2026-09-27 17:53:18 -04:00 committed by Tejun Heo
parent aa4decd2d9
commit 8a1c0f5c82
1 changed files with 4 additions and 3 deletions
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7
kernel/cgroup/cpuset.c
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@ -771,8 +771,8 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
* For v1, effective_cpus == cpus_allowed & user_xcpus() returns
* cpus_allowed.
*
* For v2, is_cpu_exclusive() & is_sched_load_balance() are true only
* for non-isolated partition root. At this point, the target
* For v2, is_partition_valid(cur) & is_sched_load_balance() are true
* only for non-isolated partition root. At this point, the target
* effective_cpus isn't computed yet. user_xcpus() is the best
* approximation.
*
@ -781,7 +781,8 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
* becomes an issue.
*/
ret = -EBUSY;
if (is_cpu_exclusive(cur) && is_sched_load_balance(cur) &&
if ((is_partition_valid(cur) || (!cpuset_v2() && is_cpu_exclusive(cur))) &&
is_sched_load_balance(cur) &&
!cpuset_cpumask_can_shrink(cur->effective_cpus, user_xcpus(trial)))
goto out;