linux-kernel - Re: [patch v4 07/18] sched: set initial load avg of new forked task

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Date:	Wed, 20 Feb 2013 00:34:44 +1300
From:	Paul Turner <pjt@...gle.com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	torvalds@...ux-foundation.org, mingo@...hat.com,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, namhyung@...nel.org,
	efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v4 07/18] sched: set initial load avg of new forked task

On Fri, Feb 15, 2013 at 2:07 AM, Alex Shi <alex.shi@...el.com> wrote:
>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 1dff78a..9d1c193 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1557,8 +1557,8 @@ static void __sched_fork(struct task_struct *p)
>>   * load-balance).
>>   */
>>  #if defined(CONFIG_SMP) && defined(CONFIG_FAIR_GROUP_SCHED)
>> -       p->se.avg.runnable_avg_period = 0;
>> -       p->se.avg.runnable_avg_sum = 0;
>> +       p->se.avg.runnable_avg_period = 1024;
>> +       p->se.avg.runnable_avg_sum = 1024;
>
> It can't work.
> avg.decay_count needs to be set to 0 before enqueue_entity_load_avg(), then
> update_entity_load_avg() can't be called, so, runnable_avg_period/sum
> are unusable.

Well we _could_ also use a negative decay_count here and treat it like
a migration; but the larger problem is the visibility of p->on_rq;
which is gates whether we account the time as runnable and occurs
after activate_task() so that's out.

>
> Even we has chance to call __update_entity_runnable_avg(),
> avg.last_runnable_update needs be set before that, usually, it needs to
> be set as 'now', that cause __update_entity_runnable_avg() function
> return 0, then update_entity_load_avg() still can not reach to
> __update_entity_load_avg_contrib().
>
> If we embed a simple new task load initialization to many functions,
> that is too hard for future reader.

This is my concern about making this a special case with the
introduction ENQUEUE_NEWTASK flag; enqueue jumps through enough hoops
as it is.

I still don't see why we can't resolve this at init time in
__sched_fork(); your patch above just moves an explicit initialization
of load_avg_contrib into the enqueue path.  Adding a call to
__update_task_entity_contrib() to the previous alternate suggestion
would similarly seem to resolve this?

>
>>  #endif
>>  #ifdef CONFIG_SCHEDSTATS
>>         memset(&p->se.statistics, 0, sizeof(p->se.statistics));
>>
>>
>>
>>>
>>>
>>> --
>>> Thanks
>>>     Alex
>
>
> --
> Thanks
>     Alex
--
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