linux-kernel - Re: [PATCH] sched/core: Reduce cost of sched_move

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Message-ID: <20230320082811.GD2194297@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Mar 2023 09:28:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     wuchi <wuchi.zero@...il.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: Reduce cost of sched_move_task when config
 autogroup

On Sun, Mar 19, 2023 at 03:56:43PM +0800, wuchi wrote:
> Some sched_move_task calls of autogroup is useless when the
> task_struct->sched_task_group isn't changed because of task_group
> of cpu_cgroup overlay task_group of autogroup. The overlay key codes
> are as follows:
> 
> sched_cgroup_fork->autogroup_task_group->task_wants_autogroup
> sched_change_group->autogroup_task_group->autogroup_task_group
> 
> sched_move_task eg:
> task A belongs to cpu_cgroup0 and autogroup0, it will always to
> cpu_cgroup0 when doing exit. So there is no need to do {de|en}queue.
> The call graph is as follow.
> 
> do_exit
>   sched_autogroup_exit_task
>     sched_move_task
>       dequeue_task
>         sched_change_group
> 	  A.sched_task_group = sched_get_task_group
>       enqueue_task
> 
> So do some check before dequeue task in sched_move_task.

No immediate objection; but the $subject seems to suggest you did this
for performance reasons, yet there are no performance results in this
Changelog. How much does this save under what load?