linux-kernel - Re: [PATCH v8 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Message-ID: <ec6e2b89-385a-fcc7-7cfa-7e9119fc34bc@redhat.com>
Date:   Wed, 1 Dec 2021 09:56:21 -0500
From:   Waiman Long <longman@...hat.com>
To:     Michal Koutný <mkoutny@...e.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 v8 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

On 12/1/21 09:13, Michal Koutný wrote:
> On Tue, Nov 30, 2021 at 10:56:34PM -0500, Waiman Long <longman@...hat.com> wrote:
>>>>       A valid parent partition may distribute out all its CPUs to
>>>>       its child partitions as long as it is not the root cgroup and
>>>>       there is no task associated with it.
>>> A valid parent partition which isn't root never has tasks in them to begin
>>> with.
>> I believe there is some corner cases where it is possible to put task in an
>> intermediate partition. That is why I put down this statement.
> Just mind the threads -- cpuset controller is threaded and having tasks
> in inner cgroup nodes is a real scenario. I wouldn't consider it a
> corner case.
>
> [ Actually, the paragraph could IMO be simplified:
Right, I shouldn't say corner cases. Having task in an intermediate 
partition is possible depending on event sequence. I am aware that there 
are code in the cpuset code to prevent that, but it didn't block all cases.
>> A valid parent partition may distribute out all its CPUs to
>>   its child partitions as long as there is no task associated with it.
> Assuming there's always at least one kernel thread in the root cgroup
> that can't be migrated anyway.]

I am aware of that. That is why I said root cgroup must have at least 
one cpu in its "cpuset.cpus.effective".

Cheers,
Longman