Re: [PATCH] sched: fix warning in sched_setaffinity

[Josh Don]

On Mon, Nov 11, 2024 at 10:27 AM Josh Don <joshdon@xxxxxxxxxx> wrote:
>
> Commit 8f9ea86fdf99b added some logic to sched_setaffinity that included
> a WARN when a per-task affinity assignment races with a cpuset update.
>
> Specifically, we can have a race where a cpuset update results in the
> task affinity no longer being a subset of the cpuset. That's fine; we
> have a fallback to instead use the cpuset mask. However, we have a WARN
> set up that will trigger if the cpuset mask has no overlap at all with
> the requested task affinity. This shouldn't be a warning condition; its
> trivial to create this condition.
>
> Reproduced the warning by the following setup:
>
> - $PID inside a cpuset cgroup
> - another thread repeatedly switching the cpuset cpus from 1-2 to just 1
> - another thread repeatedly setting the $PID affinity (via taskset) to 2
>
> Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
> Signed-off-by: Josh Don <joshdon@xxxxxxxxxx>
> Acked-by: Waiman Long <longman@xxxxxxxxxx>
> Tested-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx>
> Acked-and-tested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  kernel/sched/syscalls.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 4fae3cf25a3a..3a88f7c0cb69 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1321,7 +1321,7 @@ int __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
>                         bool empty = !cpumask_and(new_mask, new_mask,
>                                                   ctx->user_mask);
>
> -                       if (WARN_ON_ONCE(empty))
> +                       if (empty)
>                                 cpumask_copy(new_mask, cpus_allowed);
>                 }
>                 __set_cpus_allowed_ptr(p, ctx);
> --
> 2.46.0.469.g59c65b2a67-goog
>

Hey,

I wanted to bump this one last time. It's a pretty simple change that
already has ACK's from 3 other folks, but has seemed to fall through
the cracks [1].

Best,
Josh

[1] Timeline:
Original patch sent Aug 29 and quickly got ACK'd:
https://lkml.org/lkml/2024/8/29/1819
Pinged on Sep 30: https://lkml.org/lkml/2024/9/30/1528
Pinged on Oct 25: https://lkml.org/lkml/2024/10/25/1708
Resent as a new message on Nov 11: https://lkml.org/lkml/2024/11/11/1125