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

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Message-ID: <20220630143211.GA22105@blackbody.suse.cz>
Date:   Thu, 30 Jun 2022 16:32:11 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Waiman Long <longman@...hat.com>,
        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 v11 7/8] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

On Tue, Jun 28, 2022 at 04:10:29AM +0900, Tejun Heo <tj@...nel.org> wrote:
> What I'm trying to say is that cpuset.cpus of child_1 and child_2 are
> owned by the parent,

Cf

On Mon, Jun 13, 2022 at 08:00:56AM -1000, Tejun Heo <tj@...nel.org> wrote:
> On Mon, Jun 13, 2022 at 07:55:49PM +0200, Michal Koutný wrote:
> > I don't think child_*/cpuset.cpus must be owned by root.
> 
> I meant the parent.

I'm slightly confused.

> so a feature which blocks siblings from intersecting each other
> doesn't make whole lot of sense because all those files are under the
> control of the parent who would have the power to enable or disable
> the restrition anyway.

file				owner
parent/				user (mkdir)
`- cpuset.cpus			root
`- cpuset.cpus.partition	root	(P)
`- child_1/			user
  ` cpuset.cpus			user	(*)
`- child_2/			user
  ` cpuset.cpus			user	(*)

The writes to child cpuset.cpus may/may not invalidate parent's (P)
partition validity (whether a cpu is left to it to host possible tasks).
child_1 vs child_2 overlap affects only whether the children cgroups are
a valid partition.

I think you mean: writes to children cpuset.cpus should be allowed,
possible exclusivity violation should be reported in
parent/cpuset.cpus.partition.

What I thought was OK: prevent (fail) writes to children cpuset.cpus
that'd violate the exclusivity (or would take the last cpu from parent
if it's necessary to host a task).
IMO, it's similar to failed writes to parent/cgroup.subtree_control in a
delegated subtree if the parent still has some tasks (that'd violate
internal node constraint).

What I think might still be OK: allow writes to children cpuset.cpus
that violate exclusivity and report that in children's
cpuset.cpus.partition. Writes that'd take last cpu from parent should
still fail (similar to the failing subtree_control writes above).

Hope that clarifies,
Michal