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Date:   Wed, 26 Apr 2023 16:31:09 +0200
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Subject: Re: [PATCH 2/6] sched/cpuset: Bring back cpuset_mutex
To:     Juri Lelli <juri.lelli@redhat.com>,
        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>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hao Luo <haoluo@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it,
        claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it,
        mathieu.poirier@linaro.org, cgroups@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Wei Wang <wvw@google.com>, Rick Yiu <rickyiu@google.com>,
        Quentin Perret <qperret@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Sudeep Holla <sudeep.holla@arm.com>
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From:   Daniel Bristot de Oliveira <bristot@redhat.com>
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On 4/26/23 13:57, 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@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.

+1

-- Daniel