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

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Date:   Wed, 19 Oct 2016 16:53:01 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Joseph Salisbury <joseph.salisbury@...onical.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....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 16:49, Joseph Salisbury
<joseph.salisbury@...onical.com> wrote:
> On 10/18/2016 07:56 AM, 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:
>>>>> So aside from funny BIOSes, this should also show up when creating
>>>>> cgroups when you have offlined a few CPUs, which is far more common I'd
>>>>> think.
>>>> The problem is also that the load of the tg->se[cpu] that represents
>>>> the tg->cfs_rq[cpu] is initialized to 1024 in:
>>>> alloc_fair_sched_group
>>>>      for_each_possible_cpu(i) {
>>>>          init_entity_runnable_average(se);
>>>>             sa->load_avg = scale_load_down(se->load.weight);
>>>>
>>>> Initializing  sa->load_avg to 1024 for a newly created task makes
>>>> sense as we don't know yet what will be its real load but i'm not sure
>>>> that we have to do the same for se that represents a task group. This
>>>> load should be initialized to 0 and it will increase when task will be
>>>> moved/attached into task group
>>> Yes, I think that makes sense, not sure how horrible that is with the
>> That should not be that bad because this initial value is only useful for
>> the few dozens of ms that follow the creation of the task group
>>
>>> current state of things, but after your propagate patch, that
>>> reinstates the interactivity hack that should work for sure.
>> The patch below fixes the issue on my platform:
>>
>> Dietmar, Omer can you confirm that this fix the problem of your platform too ?
>>
>> ---
>>  kernel/sched/fair.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)Vinc
>>
>> 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
>>
>>
>>
>>
> Omer also reports that this patch fixes the bug for him as well.  Thanks
> for the great work, Vincent!

Thanks

>