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

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Message-ID: <8b29f878-29bc-8efa-6d34-d5df3b68e020@canonical.com>
Date:   Wed, 19 Oct 2016 10:49:56 -0400
From:   Joseph Salisbury <joseph.salisbury@...onical.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 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!