linux-kernel - Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

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Date:   Wed, 19 Oct 2016 13:25:28 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Joseph Salisbury <joseph.salisbury@...onical.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Mike Galbraith <efault@....de>, omer.akram@...onical.com
Subject: Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

On 19 October 2016 at 11:46, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> On 18/10/16 12:56, Vincent Guittot wrote:
>> Le Tuesday 18 Oct 2016 à 12:34:12 (+0200), Peter Zijlstra a écrit :
>>> On Tue, Oct 18, 2016 at 11:45:48AM +0200, Vincent Guittot wrote:
>>>> On 18 October 2016 at 11:07, Peter Zijlstra <peterz@...radead.org> wrote:
>
> [...]
>
>>
>> The patch below fixes the issue on my platform:
>>
>> Dietmar, Omer can you confirm that this fix the problem of your platform too ?
>
> It fixes this broken BIOS issue on my T430 ( cpu_possible_mask >
> cpu_online_mask). I ran the original test with the cpu hogs (stress -c
> 4). Launch time of applications becomes normal again.
>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>

Thanks

>
> But this test only makes sure that we don't see any ghost contribution
> (from non-existing cpus) any more.
>
> We should study the tg->se[i]->avg.load_avg for the hierarchy of tg's
> (with the highest tg having a task enqueued) a little bit more, with and
> without your v5 'sched: reflect sched_entity move into task_group's load'.

Can you elaborate ?

Vincent

>
>> ---
>>  kernel/sched/fair.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 8b03fb5..89776ac 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -690,7 +690,14 @@ void init_entity_runnable_average(struct sched_entity *se)
>>        * will definitely be update (after enqueue).
>>        */
>>       sa->period_contrib = 1023;
>> -     sa->load_avg = scale_load_down(se->load.weight);
>> +     /*
>> +      * Tasks are intialized with full load to be seen as heavy task until
>> +      * they get a chance to stabilize to their real load level.
>> +      * group entity are intialized with null load to reflect the fact that
>> +      * nothing has been attached yet to the task group.
>> +      */
>> +     if (entity_is_task(se))
>> +             sa->load_avg = scale_load_down(se->load.weight);
>>       sa->load_sum = sa->load_avg * LOAD_AVG_MAX;
>>       /*
>>        * At this point, util_avg won't be used in select_task_rq_fair anyway