[PATCH-next v2 1/2] cgroup/cpuset: Run SCHED_DEADLINE shrink test on valid partition root only

[Waiman Long]

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 <longman@xxxxxxxxxx>
---
 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;
 
-- 
2.55.0