linux-kernel - Re: [PATCH 1/2] cgroup/cpuset: Keep current cpus list if cpus affinity was explicitly set

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Message-ID: <20220728152355.GB25894@blackbody.suse.cz>
Date:   Thu, 28 Jul 2022 17:23:55 +0200
From:   Michal Koutný <mkoutny@...e.com>
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>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cgroup/cpuset: Keep current cpus list if cpus
 affinity was explicitly set

On Thu, Jul 28, 2022 at 10:59:01AM -0400, Waiman Long <longman@...hat.com> wrote:
> Cgroup v1 doesn't have this problem.

v1 analogy would be:

	echo 2-3 >$dst/cpuset.cpus
	# job runs in $dst
	# one task T in $dst sets affinity just to one cpu
	# I rethink my config, I want to allow $dst more space
	echo 2-5 >$dst/cpuset.cpus

Most tasks in $dst happily utilize the new cpus but it breaks affinity
for T -- this must have been broken since ever.

(Or I'd argue that per-thread affinities are just recommendations, if I
have a task for nohz CPU, I should enforce its placement with cpuset
from the beginning.)

Michal