linux-kernel - Re: [PATCH 2/6] sched/cpuset: Bring back cpuset

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Date:   Wed, 26 Apr 2023 13:57:31 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Qais Yousef <qyousef@...alina.io>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hao Luo <haoluo@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org,
        cgroups@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH 2/6] sched/cpuset: Bring back cpuset_mutex

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@...hat.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