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

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Date:   Mon, 20 Mar 2023 20:40:51 +0800
From:   chi wu <wuchi.zero@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

Peter Zijlstra <peterz@...radead.org> 于2023年3月20日周一 16:28写道：
>
> 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?

cpu:bogomips=4600.00
kernel:6.3.0-rc3
cpu cgroup: 6:cpu,cpuacct:/user.slice

run cmds:

#!/bin/bash
for i in {0..10000}; do
sleep 0 &
done
wait

bpftrace -e 'k:sched_move_task { @ts[tid] = nsecs; }
kr:sched_move_task /@ts[tid]/ { @ns += nsecs - @ts[tid];
delete(@ts[tid]); }'

cost time(ns):
 without patch: 43528033
 with      patch: 18934496
                  diff: -24593537  -56.5%

About the change, move sched_task_group_changed before
update_rq_clock as following and add the performance results in
the Changelog?

@@ -10369,6 +10390,9 @@ void sched_move_task(struct task_struct *tsk)
        rq = task_rq_lock(tsk, &rf);

+       if (!sched_task_group_changed(tsk))
+               goto unlock;
+
        update_rq_clock(rq);