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

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Date:	Mon, 6 May 2013 20:06:33 -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 7:18 PM, Alex Shi <alex.shi@...el.com> wrote:
> 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.

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.

>>
>>> > 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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