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Date: Wed, 19 Oct 2016 08:42:43 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Joonwoo Park <joonwoop@...eaurora.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
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 18 October 2016 at 23:58, Joonwoo Park <joonwoop@...eaurora.org> wrote:
>
>
> On 10/18/2016 04:56 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 ?
>
>
> I just noticed this thread after posting
> https://lkml.org/lkml/2016/10/18/719...
> Noticed this bug while a ago and had the patch above at least a week but
> unfortunately didn't have time to post...
> I think Omer had same problem I was trying to fix and I believe patch I post
> should address it.
>
> Vincent, your version fixes my test case as well.
Thanks for testing.
Can i consider this as a Tested-by ?
> This is sched_stat from the same test case I had in my changelog.
> Note dd-2030 which is in root cgroup had same runtime as dd-2033 which is in
> child cgroup.
>
> dd (2030, #threads: 1)
> -------------------------------------------------------------------
> se.exec_start : 275700.024137
> se.vruntime : 10589.114654
> se.sum_exec_runtime : 1576.837993
> se.nr_migrations : 0
> nr_switches : 159
> nr_voluntary_switches : 0
> nr_involuntary_switches : 159
> se.load.weight : 1048576
> se.avg.load_sum : 48840575
> se.avg.util_sum : 19741820
> se.avg.load_avg : 1022
> se.avg.util_avg : 413
> se.avg.last_update_time : 275700024137
> policy : 0
> prio : 120
> clock-delta : 34
> dd (2033, #threads: 1)
> -------------------------------------------------------------------
> se.exec_start : 275710.037178
> se.vruntime : 2383.802868
> se.sum_exec_runtime : 1576.547591
> se.nr_migrations : 0
> nr_switches : 162
> nr_voluntary_switches : 0
> nr_involuntary_switches : 162
> se.load.weight : 1048576
> se.avg.load_sum : 48316646
> se.avg.util_sum : 21235249
> se.avg.load_avg : 1011
> se.avg.util_avg : 444
> se.avg.last_update_time : 275710037178
> policy : 0
> prio : 120
> clock-delta : 36
>
> Thanks,
> Joonwoo
>
>
>>
>> ---
>> 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
>>
>>
>>
>>
>
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