linux-kernel - Re: [PATCH v7 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Date:   Fri, 27 Aug 2021 18:50:10 -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 v7 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

On 8/27/21 5:27 PM, Tejun Heo wrote:
> Hello,
>
> On Fri, Aug 27, 2021 at 05:19:31PM -0400, Waiman Long wrote:
>> Well, that is a valid point. The cpus may have been offlined when a
>> partition is being created. I can certainly relent on this check in forming
>> a partition. IOW, cpus_allowed can contain some or all offline cpus and a
>> valid (some are online) or invalid (all are offline) partition can be
>> formed. I can also allow an invalid child partition to be formed with an
>> invalid parent partition. However, the cpu exclusivity rules will still
>> apply.
>>
>> Other than that, do you envision any other circumstances where we should
>> allow an invalid partition to be formed?
> Now that most restrictions are removed from configuration side, just go all
> the way? Given that the user must check the status afterwards anyway, I
> don't see technical or even usability reasons for leaving some pieces
> behind. Going all the way would be easier to use too - bang in the target
> config and read the resulting state to reliably find out why a partition
> isn't valid, especially if we list *all* the reasons so that the user
> tell whether the configuration is as intended immediately.

The cpu exclusivity rule is due to the setting of CPU_EXCLUSIVE bit. 
This is a pre-existing condition unless you want to change how the 
cpuset.cpu_exclusive works.

So the new rules will be:

1) The "cpuset.cpus" is not empty and the list of CPUs are exclusive.
2) The parent cgroup is a partition root (can be an invalid one).
3) The "cpuset.cpus" is a subset of the parent's cpuset.cpus.allowed.
4) No child cgroup with cpuset enabled.

I think they are reasonable. What do you think?

Cheers,
Longman