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

Juri Lelli
2023-04-26T11:57:31Z

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.

Thanks!
Juri