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

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Date:	Tue, 07 May 2013 10:18:08 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Paul Turner <pjt@...gle.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 05/06/2013 06:17 PM, Paul Turner wrote:
>>> >> Rather than exposing the representation of load_avg_contrib to
>>> >> __sched_fork it might also be better to call:
>>> >>   __update_task_entity_contrib(&p->se)
>>> >> After the initialization above; this would also avoid potential bugs
>>> >> like the missing scale_load() above.
>> >
>> > Above simple change can not work.
> Could you provide additional detail here?  Note that the sum change I
> was suggesting above was:
> 
> __sched_fork():
> +       p->se.avg.decay_count = 0;
> +       p->se.avg.runnable_avg_period = 1024;
> +       p->se.avg.runnable_avg_sum = 1024;
> +       __update_task_entity_contrib(&p->se);
> 
> [ Also: move __sched_fork() beyond p->sched_reset_on_fork in sched_fork(). ]

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.
> 
>> > We had talked this solution months ago. And get agreement on this patch.
>> > https://lkml.org/lkml/2013/2/20/48  :)
> Yes, I made the same suggestion in the last round, see:
> https://lkml.org/lkml/2013/2/19/176
> 
> Your reply there seems like an ack of my suggestion, the only
> difference I'm seeing is that using __update_task_entity_contrib() as
> originally suggested is safer since it keeps the representation of
> load_avg_contrib opaque.

Yes, using __update_task_entity_contrib make load_avg_contrib opaque.
but just initial value 1024 is a bit arbitrary.
> 


-- 
Thanks
    Alex
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