linux-kernel - Re: [PATCH v2 2/3] sched/fair: Skip detach and attach new group task

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Date:	Tue, 31 May 2016 14:39:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Yuyang Du <yuyang.du@...el.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2 2/3] sched/fair: Skip detach and attach new group task

On Tue, May 31, 2016 at 02:33:45PM +0200, Vincent Guittot wrote:

> The sched_avg of a task is not used anymore in copy_process with
> yuyang's patchset. To be fully correct, we still have a
> p->se.avg.last_update_time = 0 in cpu_cgroup_fork but this is just the
> side effect of factoring cpu_group_fork and cpu_group_move in
> sched_move_task and it will be overwritten by
> init_entity_runnable_average before being used.

Yeah, but why? Why does init_entity_runnable_average() live in
wake_up_new_task()? That doesn't seem to make sense.

Also note that vruntime_normalized() (ab)uses !se->sum_exec_runtime to
detect the new task state.

> Now, cpu_cgroup_fork() only sets task group and cfs_rq of the forked task
> 
> copy_process
>   sched_fork
>     ...
>     cgroup_post_fork
>       ss->fork() := cpu_cgroup_fork() which only set task group and
> cfs_rq of the sched_entity of the task
> 
> wake_up_new_task
>   init_entity_runnable_average
>   activate_task
>     enqueue_task
>       attach_task