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

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Date:	Mon, 6 Jun 2016 20:32:49 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....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>,
	Yuyang Du <yuyang.du@...el.com>
Subject: Re: [PATCH v3] sched: fix first task of a task group is attached
 twice

On 01/06/16 16:54, Vincent Guittot wrote:
> On 1 June 2016 at 17:31, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> On 30/05/16 16:52, 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
>>
>> attached in .task_move_group ?
>>
>> I'm not sure if we can have a .switched_to() directly followed by a
>> .enqueue_task() into a cfs_rq with avg.last_update_time = 0.
> 
> I think it can happen as well if the task is not queued during the
> change of scheduling class because when we attach the task in
> switched_to, the task->se.avg.last_update_time will stay to 0. So when
> the task will be enqueued, it will be attached one more time

You're right. 

So I started msc_test as an rt task in task group tg_1 (css.id=2) (1) and when it slept 
I changed it to become a cfs task (2).


(1) # chrt -r 99 ./cg.sh /tg_1 ./msc_test

(2) # chrt -o -p 0 2093

...
---> busy
[   84.336570] dequeue_task_rt: cpu=2 comm=msc_test pid=2093 tg_css_id=2
[   84.342948] enqueue_task_rt: cpu=1 comm=msc_test pid=2093 tg_css_id=2
---> sleep
[   86.548655] dequeue_task_rt: cpu=1 comm=msc_test pid=2093 tg_css_id=2
[   91.008457] switched_from_rt: cpu=1 comm=msc_test pid=2093
[   91.013896] switched_to_fair: cpu=1 comm=msc_test pid=2093
[   91.019336] attach_entity_load_avg: cpu=1 comm=msc_test pid=2093 last_update_time=0 tg->css.id=2
[   91.028499] do_sched_setscheduler comm=msc_test pid=2093 policy=0 rv=0
[   91.548807] enqueue_task_fair: cpu=1 comm=msc_test pid=2093 tg_css_id=2
[   91.555363] attach_entity_load_avg: cpu=1 comm=msc_test pid=2093 last_update_time=91548795220 tg->css.id=2
---> busy
...