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

[Waiman Long <longman@redhat.com>]

On 11/14/26 7:05 AM, Ridong Chen wrote:
> 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
Reviewed-by: Waiman Long <longman@redhat.com>