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

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

On 11/10/2026 3:09 AM, Waiman Long wrote:
> 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

LGTM.

Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Thanks.

-- 
Best regards
Ridong