[PATCH for-7.4-fixes] cgroup/cpuset: Don't revive invalid partition without active CPU on hotplug

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

When a CPU hotplug leaves no CPU outside of the partitions for the top
cpuset, cpuset_handle_hotplug() clears subpartitions_cpus and gives all
the active CPUs back to the top cpuset. cpuset_hotplug_update_tasks()
then walks the hierarchy and invalidates the partitions, as
subpartitions_cpus is empty.

However, an invalid partition visited in the same walk may be revived
if its effective_xcpus is not empty. tasks_nocpu_error() doesn't catch
it when its CPUs are all offline but it has no task. Its CPUs are put
back into subpartitions_cpus, and the valid partitions visited after it
are no longer invalidated, as subpartitions_cpus is not empty any more.
The top cpuset is left with CPUs overlapping with these partitions.

It can be reproduced on a system with 8 CPUs:

  # cd /sys/fs/cgroup
  # echo +cpuset > cgroup.subtree_control
  # mkdir A1 B1
  # echo 1 > A1/cpuset.cpus
  # echo 1 > A1/cpuset.cpus.exclusive
  # echo 0,2-7 > B1/cpuset.cpus
  # echo root > B1/cpuset.cpus.partition
  # echo root > A1/cpuset.cpus.partition
  # cat A1/cpuset.cpus.partition
  root invalid (Parent unable to distribute cpu downstream)
  # echo 0 > /sys/devices/system/cpu/cpu1/online
  # cat A1/cpuset.cpus.partition B1/cpuset.cpus.partition
  root
  root
  # cat cpuset.cpus.effective
  0,2-7

A1 can't take CPU 1, the last CPU of the root cgroup which has tasks.
When CPU 1 is offlined, A1 is revived without any active CPU, and B1
stays valid while its CPUs are also in the effective CPUs of the root
cgroup, which triggers

  WARNING: kernel/cgroup/cpuset.c:919 at rebuild_sched_domains_locked

Only revive an invalid partition in hotplug if its effective_xcpus has
an active CPU, the same as what is required to create a partition with
tasks. With that, A1 and B1 are both invalidated in the above case.

Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20261114083124.1340140-1-ridong.chen%40linux.dev?part=11
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 kernel/cgroup/cpuset.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7e374210c123..bfc852090765 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4016,10 +4016,10 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 		partcmd = partcmd_invalidate;
 	/*
 	 * On the other hand, an invalid partition root may be transitioned
-	 * back to a regular one with a non-empty effective xcpus.
+	 * back to a regular one if its effective xcpus has an active CPU.
 	 */
 	else if (is_partition_valid(parent) && is_partition_invalid(cs) &&
-		 !cpumask_empty(cs->effective_xcpus))
+		 cpumask_intersects(cs->effective_xcpus, cpu_active_mask))
 		partcmd = partcmd_update;
 
 	if (partcmd >= 0) {

base-commit: e234ed1bb82c96a5643fd70784f16cf3d72eeed3
-- 
2.34.1