linux-kernel - Re: [PATCH 1/3 cgroup/for-5.20] cgroup: Elide write-locking threadgroup_rwsem when updating csses on an empty subtree

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Date:   Tue, 26 Jul 2022 16:31:56 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Christian Brauner <brauner@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 1/3 cgroup/for-5.20] cgroup: Elide write-locking
 threadgroup_rwsem when updating csses on an empty subtree

Hello.

On Thu, Jul 14, 2022 at 06:38:15PM -1000, Tejun Heo <tj@...nel.org> wrote:
> However, if the subtree doesn't contain any tasks, there aren't gonna
> be any cgroup migrations.

Nice catch.

> This condition can be trivially detected by testing whether
> mgctx.preloaded_src_csets is empty. Elide write-locking
> threadgroup_rwsem if the subtree is empty.

This check is perhaps even more robust than, e.g. cgroup_is_populated()
due to possible zombie cases.

>  kernel/cgroup/cgroup.c |   16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

Reviewed-by: Michal Koutný <mkoutny@...e.com>

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