linux-kernel - Re: [PATCH 3/4] sched,cgroup: Fix cpu_cgroup

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Message-ID: <20160617140606.GR30154@twins.programming.kicks-ass.net>
Date:	Fri, 17 Jun 2016 16:06:06 +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>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH 3/4] sched,cgroup: Fix cpu_cgroup_fork()

On Fri, Jun 17, 2016 at 03:58:37PM +0200, Vincent Guittot wrote:
> On 17 June 2016 at 14:01, Peter Zijlstra <peterz@...radead.org> wrote:
> > From: Vincent Guittot <vincent.guittot@...aro.org>
> >
> > A new fair task is detached and attached from/to task_group with:
> >
> >   cgroup_post_fork()
> >     ss->fork(child) := cpu_cgroup_fork()
> >       sched_move_task()
> >         task_move_group_fair()
> >
> > Which is wrong, because at this point in fork() the task isn't fully
> > initialized and it cannot 'move' to another group, because its not
> > attached to any group as yet.
> >
> > In fact, cpu_cgroup_fork needs a small part of sched_move_task so we
> > can just call this small part directly instead sched_move_task. And
> > the task doesn't really migrate because it is not yet attached so we
> > need the sequence:
> >
> >   do_fork()
> >     sched_fork()
> >       __set_task_cpu()
> >
> >     cgroup_post_fork()
> >       set_task_rq() # set task group and runqueue
> >
> >     wake_up_new_task()
> >       select_task_rq() can select a new cpu
> >       __set_task_cpu
> >       post_init_entity_util_avg
> >         attach_task_cfs_rq()
> >       activate_task
> >         enqueue_task
> >
> > This patch makes that happen.
> >
> 
> With this patch and patch 1, the fork sequence looks correct in my test
> 
> > Maybe-Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> 
> You can remove the Maybe if you want

Thanks!