linux-kernel - Re: [PATCH RFC] sched/fair: let cpu's cfs

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Message-ID: <5706C34B.3020508@arm.com>
Date:	Thu, 7 Apr 2016 21:30:03 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	Morten Rasmussen <morten.rasmussen@....com>,
	Yuyang Du <yuyang.du@...el.com>, Leo Yan <leo.yan@...aro.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"eas-dev@...ts.linaro.org" <eas-dev@...ts.linaro.org>
Subject: Re: [PATCH RFC] sched/fair: let cpu's cfs_rq to reflect task
 migration

Hi  Vincent,

On 04/07/2016 02:04 PM, Vincent Guittot wrote:
> Hi Dietmar,
>
> On 6 April 2016 at 20:53, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> On 06/04/16 09:37, Morten Rasmussen wrote:
>>> On Tue, Apr 05, 2016 at 06:00:40PM +0100, Dietmar Eggemann wrote:

[...]

>> @@ -2910,8 +2920,13 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>>          if (!entity_is_task(se))
>>                  return;
>>
>> -       rq_of(cfs_rq)->cfs.avg.util_avg += se->avg.util_avg;
>> -       rq_of(cfs_rq)->cfs.avg.util_sum += se->avg.util_sum;
>> +       if (&rq_of(cfs_rq)->cfs == cfs_rq) {
>> +               rq_of(cfs_rq)->cfs.avg.util_avg += se->avg.util_avg;
>> +               rq_of(cfs_rq)->cfs.avg.util_sum += se->avg.util_sum;
>> +       } else {
>> +               rq_of(cfs_rq)->cfs.added_util_avg = se->avg.util_avg;
>> +               rq_of(cfs_rq)->cfs.added_util_sum = se->avg.util_sum;
>> +       }
>>   }
>
> Don't you also need similar thing for the detach ?

Maybe? I ran workloads in tg's and checked last_update_time of cfs_rq
and &rq_of(cfs_rq)->cfs and they always were in sync. That's obviously
only the call-stack 'task_move_group_fair() -> detach_task_cfs_rq() ->
detach_entity_load_avg()' and not the one starting from
switched_from_fair().

[...]

>> But attach_entity_load_avg() is not only called in enqueue_entity_load_avg() for migrated
>> tasks but also in attach_task_cfs_rq() which is called from switched_to_fair() and
>> task_move_group_fair() where we can't assume that after the enqueue_entity_load_avg() a
>> call to update_cfs_rq_load_avg() follows like in
>>
>> enqueue_task_fair():
>>
>>      for_each_sched_entity(se)
>>          enqueue_entity()
>>              enqueue_entity_load_avg()
>>                      update_cfs_rq_load_avg(now, cfs_rq)
>>                      if (migrated) attach_entity_load_avg()
>>
>>      for_each_sched_entity(se)
>>          update_load_avg()
>>              update_cfs_rq_load_avg(now, cfs_rq)
>>
>>
>> Not sure if we can just update the root cfs_rq to se->avg.last_update_time before we add
>> se->avg.util_[avg/sum] to rq_of(cfs_rq)->cfs.avg.util_[avg/sum] in attach_entity_load_avg()?
>>
>> cfs_rq throttling has to be considered as well ...
>
> IIUC this new proposal, the utilization of a task will be accounted on
> the utilization of the root cfs_rq thanks  to
> tsk->se->cfs_rq->tg->se[cpu]->... down to the root cfs_rq. Then, you
> directly add the utilization of the newly enqueued task in the root
> cfs_rq.

Not sure if you're referring to this, but in __update_load_avg() I
suppress the utilization update for se's w/ !entity_is_task(se) and
cfs_rq's w/ &rq_of(cfs_rq)->cfs != cfs_rq  so preventing the first case.

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