linux-kernel - Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task

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Date:	Thu, 9 May 2013 02:34:34 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new
 forked task

On Mon, May 6, 2013 at 8:24 PM, Alex Shi <alex.shi@...el.com> wrote:
> On 05/07/2013 11:06 AM, Paul Turner wrote:
>>> > Thanks Paul!
>>> > It seems work with this change if new __sched_fork move after the
>>> > p->sched_reset_on_fork setting.
>>> >
>>> > But why we initial avg sum to 1024? new task may goes to sleep, the
>>> > initial 1024 give a unreasonable initial value.
>>> >
>>> > guess let the task accumulate itself avg sum and period is more natural.
>> 1024 is a full single unit period representing ~1ms of time.
>>
>> The reason to store a small initial "observation" here is so that as
>> when we reach our next period edge our load converges (presumably
>> down) towards its true target more smoothly; as well as providing a
>> task additional protection from being considered "small" through
>> start-up.
>>
>
> It will give new forked task 1 ms extra running time. That will bring
> incorrect info if the new forked goes to sleep a while.

This is intentional.

Either:
The sleep was representative, we still converge reasonably quickly to zero.
The sleep was not representative and we have not under-represented the task.

Providing it initial time is entirely about improving numerical stability.

> But this info should benefit to some benchmarks like aim7,
> pthread_cond_broadcast. So I am convinced. :)
>
> What's your opinion of this, Peter?
>
> --
> Thanks
>     Alex
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