linux-kernel - Re: [PATCH 2/2] cgroup: Skip subtree root in cgroup_update_dfl

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Message-ID: <YuLGrXyXj6a2yigJ@slm.duckdns.org>
Date:   Thu, 28 Jul 2022 07:26:05 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cgroup: Skip subtree root in
 cgroup_update_dfl_csses()

On Thu, Jul 28, 2022 at 04:44:26PM +0200, Michal Koutný wrote:
> On Wed, Jul 27, 2022 at 08:58:15PM -0400, Waiman Long <longman@...hat.com> wrote:
> > The cgroup_update_dfl_csses() function updates css associations when a
> > cgroup's subtree_control file is modified. Any changes made to a cgroup's
> > subtree_control file, however, will only affect its descendants but not
> > the cgroup itself. 
> 
> I find this correct.
> 
> > So there is no point in migrating csses associated with that cgroup.
> > We can skip them instead.
> 
> Alone it's not such a big win but it componds with the recent Tejun's
> threadgroup_rwsem elision.

The chance is that if you're writing to a cgroup's subtree_control, that
cgroup is gonna be empty. The only case I can think of that this would make
a difference is w/ threaded cgroups, but it does make sense.

Thanks.

-- 
tejun