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 14:46:24 -0400
From:   Waiman Long <llong@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Waiman Long <llong@...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 8/11/21 2:21 PM, Tejun Heo wrote:
> On Wed, Aug 11, 2021 at 02:18:17PM -0400, Waiman Long wrote:
>> I don't think that is true. A task can reside anywhere in the cgroup
>> hierarchy. I have encountered no problem moving tasks around.
> Oh, that shouldn't be happening with controllers enabled. Can you please
> share a repro?

I have done further testing. Enabling controllers won't prohibit moving 
a task into a parent cgroup as long as the child cgroups have no tasks. 
Once the child cgroup has task, moving another task to the parent is not 
allowed (-EBUSY). Similarly if a parent cgroup has tasks, you can't put 
new tasks into the child cgroup. I don't realize that we have such 
constraints as I usually do my testing with a cgroup hierarchy with no 
tasks initially. Anyway, a new lesson learned.

I will try to see how to address that in the patch, but the additional 
check added is still valid in some special case.

Cheers,
Longman