linux-kernel - Re: [PATCH v9 2/7] cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective

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Message-ID: <20211215103629.GA25459@blackbody.suse.cz>
Date:   Wed, 15 Dec 2021 11:36:29 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, 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 v9 2/7] cgroup/cpuset: Allow no-task partition to have
 empty cpuset.cpus.effective

On Tue, Dec 14, 2021 at 10:24:22PM -0500, Waiman Long <longman@...hat.com> wrote:
> AFAICS, there are code in cpuset.c that disallows the an non-child node to
> hold tasks, but the check doesn't cover all the possible cases.
> I remembered that I was able to create such a scenario without using
> threaded domains.

On the default hierarchy (with controller(s) enabled)? That sounds like a bug.

> That is why I put in this conditional check. It has nothing to do with the
> use of threaded domains.

But threaded domains are important nevertheless.
I think that a structure like

	app-cgroup	cgroup.type=threaded domain	cpuset.partition=root
	`- rt		cgroup.type=threaded		cpuset.partition=isolated
	`- normal	cgroup.type=threaded

is a valid use case. Therefore I would not disallow partitioning inside
threaded subtrees (as suggested).


Michal