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

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Message-ID: <20160531120352.GK3193@twins.programming.kicks-ass.net>
Date:	Tue, 31 May 2016 14:03:52 +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 01:55:49PM +0200, Vincent Guittot wrote:
> Hi Yuyang,
> 
> On 31 May 2016 at 00:32, Yuyang Du <yuyang.du@...el.com> wrote:
> > Vincent reported that the first task to a new task group's cfs_rq will
> > be attached in attach_task_cfs_rq() and once more when it is enqueued
> > (see https://lkml.org/lkml/2016/5/25/388).
> >
> > Actually, it is worse, attach_task_cfs_rq() is invoked for new task even
> > way before the new task is initiated in init_entity_runnable_average().
> >
> > Solve this by avoiding attach as well as detach new task in
> > task_move_group_fair(). To do it, we need to know whether the task
> > is forked or not, so we pass this info all the way from sched_move_task()
> > to attach_task_cfs_rq().
> 
> I have tested your patch and I can't the spurious detach
> /attach_task_cfs_rq anymore
> so you can add my tested-by.
> 

Could someone update the Changelog to better explain the whole
callchain.

Because a quick look seems to suggest something like:

	copy_process()
	  sched_fork()
	  ...
	  cgroup_post_fork()
	    ss->fork() := cpu_cgroup_fork()

Which seems to suggest init_entity_runnable_average() is placed wrong
and should live in sched_fork() ?

Or am I not getting it.. ?