linux-kernel - Re: [PATCH v4 3/3] cgroup/cpuset: Keep user set cpus affinity

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Message-ID: <YvvZc7KgZT/bRoKl@slm.duckdns.org>
Date:   Tue, 16 Aug 2022 07:52:51 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Will Deacon <will@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4 3/3] cgroup/cpuset: Keep user set cpus affinity

Hello,

On Tue, Aug 16, 2022 at 01:38:17PM -0400, Waiman Long wrote:
> Yes, a race like this is possible. To completely eliminate the race may
> require taking task_rq_lock() and then calling
> __set_cpus_allowed_ptr_locked() which is internal to kernel/sched/core.c.
> 
> Alternatively, we can check user_cpus_ptr again after the scond
> set_cpus_allowed_ptr() and retry it with the other path if set. That will
> probably address your concern. Please let me know if you are OK with that.

I think this would look better if structured the other way around - make the
scheduler side call out to cpuset to query the current restrictions and
apply it atomically.

Thanks.

-- 
tejun