linux-kernel - Re: [PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity

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Message-ID: <YvwX24GXadKQNp6V@slm.duckdns.org>
Date:   Tue, 16 Aug 2022 12:19:07 -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 v5 3/3] cgroup/cpuset: Keep user set cpus affinity

Hello,

On Tue, Aug 16, 2022 at 06:11:03PM -0400, Waiman Long wrote:
> It is hard to synchronize different subsystems atomically without running
> into locking issue. Let me think about what can be done in this case.

I have a hard time seeing why this would be particularly difficult. cpuset
just needs to make the latest cpumask available to sched core in an easily
accessible form and whenever that changes, trigger a set_cpus_allowed call.
There's no need to entangle operations across the whole subsystems. All
that's needed to be communicated is the current cpumask.

> Is using a sequence number to check for race with retry good enough?

It seems unnecessarily fragile and complicated to me. If we're gonna change
it, let's change it right.

Thanks.

-- 
tejun