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Sender: Tejun Heo <htejun@gmail.com>
Date:   Thu, 28 Jul 2022 07:26:05 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Cc:     Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] cgroup: Skip subtree root in
 cgroup_update_dfl_csses()
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References: <20220728005815.1715522-1-longman@redhat.com>
 <20220728005815.1715522-2-longman@redhat.com>
 <20220728144426.GA26631@blackbody.suse.cz>
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On Thu, Jul 28, 2022 at 04:44:26PM +0200, Michal Koutn? wrote:
> On Wed, Jul 27, 2022 at 08:58:15PM -0400, Waiman Long <longman@redhat.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