linux-kernel - Re: [PATCH v11 7/8] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Date:   Sun, 12 Jun 2022 23:02:38 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
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 v11 7/8] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

On 6/12/22 13:49, Tejun Heo wrote:
> Hello,
>
> On Tue, May 10, 2022 at 11:34:12AM -0400, Waiman Long wrote:
>> +	External events like hotplug or changes to "cpuset.cpus" can
>> +	cause a valid partition root to become invalid and vice versa.
>> +	Note that a task cannot be moved to a cgroup with empty
>> +	"cpuset.cpus.effective".
>> +
>> +	For a valid partition root or an invalid partition root with
>> +	the exclusivity rule enabled, changes made to "cpuset.cpus"
>> +	that violate the exclusivity rule will not be allowed.
> My memory is failing but this is the same thing that we were discussing
> before, right? The point was that the different behaviors re. system events
> and config actions seemed unncessary and IIRC Michal was of the same opinion
> (please correct me if I'm misremembering).

That is the behavior enforced by setting the CPU_EXCLUSIVE bit in cgroup 
v1. I haven't explicitly change it to make it different in cgroup v2. 
The major reason is that I don't want change to one cpuset to affect a 
sibling partition as it may make the code more complicate to validate if 
a partition is valid.

>
>> +	A valid non-root parent partition may distribute out all its CPUs
>> +	to its child partitions when there is no task associated with it.
> I'm probably forgetting something. Was this necessary because of threaded
> cgroup support because otherwise the above condition is superflous?

The top cpuset cannot have empty cpus.effective whereas the non-root 
partition roots can. Maybe I should reword it to make it more clear.

Thanks,
Longman