From 8a1c0f5c82843104d9bef2bc3726c89da70a9b91 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Sun, 27 Sep 2026 17:53:18 -0400 Subject: [PATCH] cgroup/cpuset: Run SCHED_DEADLINE shrink test on valid partition root only Commit f82f80426f7a ("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 a86ce68078b2 ("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: a86ce68078b2 ("cgroup/cpuset: Extract out CS_CPU_EXCLUSIVE & CS_SCHED_LOAD_BALANCE handling") Signed-off-by: Waiman Long Reviewed-by: Ridong Chen Reviewed-by: Guopeng Zhang Signed-off-by: Tejun Heo --- kernel/cgroup/cpuset.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 753aa65afcd7..104fe10c336d 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -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;