[PATCH-next v2 3/5] cgroup/cpuset: Fix incorrect partition invalidation logic in cpuset_hotplug_update_tasks()

[Waiman Long <longman@redhat.com>]

When most of the active CPUs of a system are allocated to
sub-partitions below the top_cpuset partition, it is possible that a
CPU offline operation may remove the last active CPU in the top_cpuset
partition. When that really happens, cpuset_handle_hotplug() will clear
the subpartitions_cpus to make sure active CPUs are present in the top
partition and let the other sub-partitions get invalidated and then
competes for their exclusive CPUs again.

In cpuset_hotplug_update_tasks(), the current code use
"cpumask_empty(subpartitions_cpus)" to determine if that partition
should be invalidated. That is fine if there is only one sub-partition.
As reported by sashiko that when there are more than one sub-partitions
and the first sub-partition will get invalidated and then it will
try to acquire the exclusive CPUs and become valid again. When this
happens, the subpartitions_cpus will no longer be empty. The remaining
sub-partitions will assume their exclusive CPUs are safe and proceed
as normal. As a result, active CPUs owned by those sub-partitions
remain in both top_cpuset.effective_cpus and the sub-partitions'
effective_cpus while missing from subpartitions_cpus, breaking partition
CPU exclusivity and triggering WARN_ON_ONCE() in generate_sched_domains()
and remote_partition_disable().

Fix that by checking if the sub-partitions' effective_xcpus are subset
of subpartitions_cpus to determine if they should be invalidated. Also
remove the obsolete WARN_ON_ONCE() call in remote_partition_disable().

Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/bug/linux-bf69cd3c-833a-4599-a1f9-8dd9e044cd7c
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 3955d2d574cf..e9b84671af21 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1766,15 +1766,6 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
 {
 	WARN_ON_ONCE(!is_remote_partition(cs));
-	/*
-	 * When a CPU is offlined, top_cpuset may end up with no available CPUs,
-	 * which should clear subpartitions_cpus. We should not emit a warning for this
-	 * scenario: the hierarchy is updated from top to bottom, so subpartitions_cpus
-	 * may already be cleared when disabling the partition.
-	 */
-	WARN_ON_ONCE(!cpumask_subset(cs->effective_xcpus, subpartitions_cpus) &&
-		     !cpumask_empty(subpartitions_cpus));
-
 	spin_lock_irq(&callback_lock);
 	cs->remote_partition = false;
 	partition_xcpus_del(cs->partition_root_state, NULL, cs->effective_xcpus);
@@ -4020,9 +4011,9 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	 * or update_parent_effective_cpumask() is called as cs->effective_cpus
 	 * may have been updated.
 	 */
-	if (is_remote_partition(cs) && (cpumask_empty(subpartitions_cpus) ||
-				       (cpumask_empty(&new_cpus) &&
-					partition_is_populated(cs, NULL)))) {
+	if (is_remote_partition(cs) &&
+	    (!cpumask_subset(cs->effective_xcpus, subpartitions_cpus) ||
+	     (cpumask_empty(&new_cpus) && partition_is_populated(cs, NULL)))) {
 		WRITE_ONCE(cs->prs_err, PERR_HOTPLUG);
 		remote_partition_disable(cs, tmp);
 		compute_effective_cpumask(&new_cpus, cs, parent);
@@ -4035,12 +4026,12 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	 * 1) empty effective cpus but not valid empty partition.
 	 * 2) parent is invalid or doesn't grant any cpus to child
 	 *    partitions.
-	 * 3) subpartitions_cpus is empty.
+	 * 3) effective_xcpus isn't a subset of subpartitions_cpus.
 	 */
 	if (is_local_partition(cs) &&
 	    (!is_partition_valid(parent) ||
 	     tasks_nocpu_error(parent, cs, &new_cpus) ||
-	     cpumask_empty(subpartitions_cpus)))
+	     !cpumask_subset(cs->effective_xcpus, subpartitions_cpus)))
 		partcmd = partcmd_invalidate;
 	/*
 	 * On the other hand, an invalid partition root may be transitioned
-- 
2.56.0