[PATCH v2 3/3] cgroup/cpuset: Release isolated CPUs when the parent is invalid

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

An isolated partition can be invalidated after its parent has already
become invalid. update_parent_effective_cpumask() then skips returning
CPUs to the parent, but also skips removing them from isolated_cpus.
The invalid child continues to isolate CPUs that no longer belong to
any valid isolated partition.

For example, on a 32-CPU system with all CPUs online and without
isolcpus= or nohz_full= boot parameters:

  cd /sys/fs/cgroup
  echo +cpuset > cgroup.subtree_control
  mkdir A
  echo 0,2-31 > A/cpuset.cpus
  echo root > A/cpuset.cpus.partition
  echo +cpuset > A/cgroup.subtree_control
  mkdir A/B
  echo 2-3 > A/B/cpuset.cpus
  echo isolated > A/B/cpuset.cpus.partition
  cat cpuset.cpus.isolated
  2-3
  echo 0 > /sys/devices/system/cpu/cpu1/online
  cat A/B/cpuset.cpus.partition
  isolated invalid
  cat cpuset.cpus.isolated
  2-3

Release the invalidated child's isolated CPUs when its parent is no
longer a valid partition. Use isolated_cpus_release() to preserve CPUs
still covered by a valid isolated ancestor and by boot isolation.

After this fix, cpuset.cpus.isolated is empty for this reproducer.

Fixes: 11e5f407b64a ("cgroup/cpuset: Keep track of CPUs in isolated partitions")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/cgroup/cpuset.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index fc5454c2740f..f3b808cea51c 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1976,10 +1976,15 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 		/*
 		 * Make the current partition invalid.
 		 */
-		if (is_partition_valid(parent))
+		if (is_partition_valid(parent)) {
 			adding = cpumask_and(tmp->addmask,
 					     cs->effective_xcpus,
 					     parent->effective_xcpus);
+		} else if (old_prs == PRS_ISOLATED) {
+			/* No valid parent will release these isolated CPUs. */
+			cpumask_copy(tmp->addmask, cs->effective_xcpus);
+			isolated_cpus_release(cs, tmp->addmask);
+		}
 		if (old_prs > 0)
 			new_prs = -old_prs;
 
-- 
2.43.0