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From:   Xuewen Yan <xuewen.yan94@gmail.com>
Date:   Thu, 27 Apr 2023 10:58:03 +0800
Message-ID: <CAB8ipk-ns=d+jNkKi1sjkSQmQidziCj34COkHZt6ZkRiG47HHA@mail.gmail.com>
Subject: Re: [PATCH 2/6] sched/cpuset: Bring back cpuset_mutex
To:     Waiman Long <longman@redhat.com>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        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,
        bristot@redhat.com, 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>,
        =?UTF-8?B?546L56eRIChLZSBXYW5nKQ==?= <Ke.Wang@unisoc.com>,
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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=E2=80=AFPM Waiman Long <longman@redhat.com> w=
rote:
>
> 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 implementin=
g
> >>> priority inheritance (which causes troubles with realtime workloads).
> >>>
> >>> Convert percpu_cpuset_rwsem back to regular cpuset_mutex. Also grab i=
t
> >>> 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 regula=
r
> >> rwsem as there are cases where a read lock is sufficient. This will al=
so
> >> 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.
>
> That is fine for me too.
>
> Cheers,
> Longman
>