Re: [PATCH v2] sched/core: Mask out offline CPUs when user_cpus_ptr is used

[Waiman Long <llong@redhat.com>]

On 7/18/25 12:41 PM, Waiman Long wrote:
> Chen Ridong reported that cpuset could report a kernel warning for a task
> due to set_cpus_allowed_ptr() returning failure in the corner case that:
>
> 1) the task used sched_setaffinity(2) to set its CPU affinity mask to
>     be the same as the cpuset.cpus of its cpuset,
> 2) all the CPUs assigned to that cpuset were taken offline, and
> 3) cpuset v1 is in use and the task had to be migrated to the top cpuset.
>
> Due to the fact that CPU affinity of the tasks in the top cpuset are
> not updated when a CPU hotplug online/offline event happens, offline
> CPUs are included in CPU affinity of those tasks. It is possible
> that further masking with user_cpus_ptr set by sched_setaffinity(2)
> in __set_cpus_allowed_ptr() will leave only offline CPUs in the new
> mask causing the subsequent call to __set_cpus_allowed_ptr_locked()
> to return failure with an empty CPU affinity.
>
> Fix this failure by skipping user_cpus_ptr masking if there is no online
> CPU left.
>
> Reported-by: Chen Ridong <chenridong@huaweicloud.com>
> Closes: https://lore.kernel.org/lkml/20250714032311.3570157-1-chenridong@huaweicloud.com/
> Fixes: da019032819a ("sched: Enforce user requested affinity")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/sched/core.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)

Sorry, I forgot to change the patch title. Will send out a v3.

Cheers,
Longman

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 81c6df746df1..208f8af73134 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3170,12 +3170,13 @@ int __set_cpus_allowed_ptr(struct task_struct *p, struct affinity_context *ctx)
>   
>   	rq = task_rq_lock(p, &rf);
>   	/*
> -	 * Masking should be skipped if SCA_USER or any of the SCA_MIGRATE_*
> -	 * flags are set.
> +	 * Masking should be skipped if SCA_USER, any of the SCA_MIGRATE_*
> +	 * flags are set or no online CPU left.
>   	 */
>   	if (p->user_cpus_ptr &&
>   	    !(ctx->flags & (SCA_USER | SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) &&
> -	    cpumask_and(rq->scratch_mask, ctx->new_mask, p->user_cpus_ptr))
> +	    cpumask_and(rq->scratch_mask, ctx->new_mask, p->user_cpus_ptr) &&
> +	    cpumask_intersects(rq->scratch_mask, cpu_active_mask))
>   		ctx->new_mask = rq->scratch_mask;
>   
>   	return __set_cpus_allowed_ptr_locked(p, ctx, rq, &rf);