Re: [PATCH 2/6] sched/cpuset: Bring back cpuset_mutex

Xuewen Yan
2023-04-27T02:58:03Z

HI Juri,

Would this patch be merged tobe stable-rc? In kernel5.15, we also find
that the rwsem would be blocked for a long time, when we change the
task's cpuset cgroup.
And when we revert to the mutex, the delay would disappear.

BR
Thanks!

On Wed, Apr 26, 2023 at 10:50 PM Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>
> On 4/26/23 07:57, Juri Lelli wrote:
> > On 04/04/23 13:31, Waiman Long wrote:
> >> On 3/29/23 08:55, Juri Lelli wrote:
> >>> Turns out percpu_cpuset_rwsem - commit 1243dc518c9d ("cgroup/cpuset:
> >>> Convert cpuset_mutex to percpu_rwsem") - wasn't such a brilliant idea,
> >>> as it has been reported to cause slowdowns in workloads that need to
> >>> change cpuset configuration frequently and it is also not implementing
> >>> priority inheritance (which causes troubles with realtime workloads).
> >>>
> >>> Convert percpu_cpuset_rwsem back to regular cpuset_mutex. Also grab it
> >>> only for SCHED_DEADLINE tasks (other policies don't care about stable
> >>> cpusets anyway).
> >>>
> >>> Signed-off-by: Juri Lelli <juri.lelli-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> >> I am thinking that maybe we should switch the percpu rwsem to a regular
> >> rwsem as there are cases where a read lock is sufficient. This will also
> >> avoid the potential PREEMPT_RT problem with PI and reduce the time it needs
> >> to take a write lock.
> > I'm not a big fan of rwsems for reasons like
> > https://lore.kernel.org/lkml/20230321161140.HMcQEhHb-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org/, so
> > I'd vote for a standard mutex unless we have a strong argument and/or
> > numbers.
>
> That is fine for me too.
>
> Cheers,
> Longman
>