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

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Date:	Wed, 13 Feb 2013 07:41:32 -0800
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 Wed, Feb 13, 2013 at 7:14 AM, Alex Shi <alex.shi@...el.com> wrote:
> On 02/12/2013 06:26 PM, Peter Zijlstra wrote:
>> On Thu, 2013-01-24 at 11:06 +0800, Alex Shi wrote:
>>> +       /*
>>> +        * set the initial load avg of new task same as its load
>>> +        * in order to avoid brust fork make few cpu too heavier
>>> +        */
>>> +       if (flags & ENQUEUE_NEWTASK)
>>> +               se->avg.load_avg_contrib = se->load.weight;
>>
>> I seem to have vague recollections of a discussion with pjt where we
>> talk about the initial behaviour of tasks; from this haze I had the
>> impression that new tasks should behave like full weight..
>>
>
> Here just make the new task has full weight..
>
>> PJT is something more fundamental screwy?
>>

So tasks get the quotient of their runnability over the period.  Given
the period initially is equivalent to runnability it's definitely the
*intent* to start at full-weight and ramp-down.

Thinking on it, perhaps this is running a-foul of amortization -- in
that we only recompute this quotient on each 1024ns boundary; perhaps
in the fork-bomb case we're too slow to accumulate these.

Alex, does something like the following help?  This would force an
initial __update_entity_load_avg_contrib() update the first time we
see the task.

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



>
>
> --
> Thanks
>     Alex
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