linux-kernel - Re: [PATCH v3] sched: fix first task of a task group is attached twice

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Date:	Tue, 31 May 2016 08:44:00 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v3] sched: fix first task of a task group is attached
 twice

On Tue, May 31, 2016 at 09:28:30AM +0200, Vincent Guittot wrote:
> Hi Yuyang,
> 
> On 30 May 2016 at 21:48, Yuyang Du <yuyang.du@...el.com> wrote:
> > On Mon, May 30, 2016 at 05:52:20PM +0200, Vincent Guittot wrote:
> >> The cfs_rq->avg.last_update_time is initialize to 0 with the main effect
> >> that the 1st sched_entity that will be attached, will keep its
> >> last_update_time set to 0 and will attached once again during the
> >> enqueue.
> >> Initialize cfs_rq->avg.last_update_time to 1 instead.
> >>
> >> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> >> ---
> >>
> >> v3:
> >> - add initialization of load_last_update_time_copy for not 64bits system
> >> - move init into init_cfs_rq
> >>
> >> v2:
> >> - rq_clock_task(rq_of(cfs_rq)) can't be used because lock is not held
> >>
> >>  kernel/sched/fair.c | 10 ++++++++++
> >>  1 file changed, 10 insertions(+)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 218f8e8..86be9c1 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -8459,6 +8459,16 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
> >>       cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> >>  #endif
> >>  #ifdef CONFIG_SMP
> >> +     /*
> >> +      * Set last_update_time to something different from 0 to make
> >> +      * sure the 1st sched_entity will not be attached twice:once
> >> +      * when attaching the task to the group and one more time when
> >> +      * enqueueing the task.
> >> +      */
> >
> > The first time: "once when attaching the task to the group".
> >
> > That attaching is purely wrong, but will not have any effect (at least
> > load/util wise), because the task will later be inited in
> > init_entity_runnable_average().
> 
> This patch is not related to the init of a task but related to the
> init of the cfs_rq and to what happen with the 1st task that is
> enqueued on it.
> 
> Lets take a task A that has already been scheduled on other cfs_rq so
> its se->avg.last_update_time is different from 0.

I understand it, finally, :)
 
> Create a new task group TGB
> At creation, the cfs_rq->avg.last_update_time of this TGB is set to 0.
> 
> Now move task A on TGB.
> A is attached to TGB so  se->avg.last_update_time =
> cfs_rq->avg.last_update_time which is 0
> A is then enqueued on th cfs_rq and because se->avg.last_update_time
> == 0, A will be attached one more time on the cfs_rq
> 
> This patch set cfs_rq->avg.last_update_time to 1 at creation so the
> 1st time that A is attached to TGB,  se->avg.last_update_time =
> cfs_rq->avg.last_update_time = 1 and A will not bve attached one more
> time during the enqueue.