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Date:   Wed, 26 Apr 2023 13:57:31 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Qais Yousef <qyousef@layalina.io>, Tejun Heo <tj@kernel.org>,
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        Dietmar Eggemann <dietmar.eggemann@arm.com>,
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        Sudeep Holla <sudeep.holla@arm.com>
Subject: Re: [PATCH 2/6] sched/cpuset: Bring back cpuset_mutex
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 <fa585497-5c6d-f0ed-bdda-c71a81d315ad@redhat.com>
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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@redhat.com>
> 
> 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@linutronix.de/, so
I'd vote for a standard mutex unless we have a strong argument and/or
numbers.

Thanks!
Juri