linux-kernel - Re: [PATCH v4 4/6] cgroup/cpuset: Allow non-top parent partition root to distribute out all CPUs

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Date:   Wed, 11 Aug 2021 08:13:33 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     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>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v4 4/6] cgroup/cpuset: Allow non-top parent partition
 root to distribute out all CPUs

On Tue, Aug 10, 2021 at 11:06:05PM -0400, Waiman Long wrote:
> Currently, a parent partition root cannot distribute all its CPUs to
> child partition roots with no CPUs left. However in some use cases,
> a management application may want to create a parent partition root as
> a management unit with no task associated with it and has all its CPUs
> distributed to various child partition roots dynamically according to
> their needs. Leaving a cpu in the parent partition root in such a case is
> now a waste.
> 
> To accommodate such use cases, a parent partition root can now have
> all its CPUs distributed to its child partition roots as long as:
>  1) it is not the top cpuset; and

>  2) there is no task directly associated with the parent.
> 
> Once an empty parent partition root is formed, no new task can be moved
> into it.

The above are already enforced by cgroup2 core, right? No intermediate
cgroup with controllers enabled can have processes. From controllers' POV,
only leaves can have processes.

Thanks.

-- 
tejun