linux-kernel - Re: [PATCH v11 3/8] cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective

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Date:   Mon, 13 Jun 2022 07:23:03 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v11 3/8] cgroup/cpuset: Allow no-task partition to have
 empty cpuset.cpus.effective

Hello,

On Mon, Jun 13, 2022 at 12:47:37PM -0400, Waiman Long wrote:
> On 6/13/22 10:02, Michal Koutný wrote:
> > On Sun, Jun 12, 2022 at 04:55:13PM -1000, Tejun Heo <tj@...nel.org> wrote:
> > > But how would that happen? A lot of other things would break too if that
> > > were to happen.
> > cpuset is a threaded controller where the internal-node-constraint does
> > not hold. So the additional condition for cpuset migrations is IMO
> > warranted (and needed if there's no "fall up").
> 
> Yes, you are right. cpuset is threaded and so it may have tasks even if it
> is not the leaf node.

And we had this same exchange the last time. Can you please add a comment?
We might also already have had this exchange before too but is it necessary
to allow threaded cgroups to be isolated roots? The interaction between
being threaded and isolated is cleaner at that layer as it's interactions
between two explicit mode changes.

Thanks.

-- 
tejun