linux-kernel - Re: [patch v3 6/8] sched: consider runnable load average in move

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Date:	Tue, 9 Apr 2013 13:56:42 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, Paul Turner <pjt@...gle.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	gregkh@...uxfoundation.org,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>, rafael.j.wysocki@...el.com,
	jkosina@...e.cz, Clark Williams <clark.williams@...il.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>, keescook@...omium.org,
	Mel Gorman <mgorman@...e.de>, riel@...hat.com
Subject: Re: [patch v3 6/8] sched: consider runnable load average in move_tasks

On 9 April 2013 12:38, Alex Shi <alex.shi@...el.com> wrote:
> On 04/09/2013 04:58 PM, Vincent Guittot wrote:
>>>> >> How do you ensure that runnable_avg_period and runnable_avg_sum are
>>>> >> coherent ? an update of the statistic can occur in the middle of your
>>>> >> sequence.
>>> >
>>> > Thanks for your question, Vincent!
>>> > the runnable_avg_period and runnable_avg_sum, only updated in
>>> > __update_entity_runnable_avg().
>>> > Yes, I didn't see some locks to ensure the coherent of them. but they
>>> > are updated closely, and it is not big deal even a little incorrect to
>>> > their value. These data are collected periodically, don't need very very
>>> > precise at every time.
>>> > Am I right? :)
>> The problem mainly appears during starting phase (the 1st 345ms) when
>> runnable_avg_period has not reached the max value yet so you can have
>> avg.runnable_avg_sum greater than avg.runnable_avg_period. In a worst
>> case, runnable_avg_sum could be twice runnable_avg_period
>
> Oh, That's a serious problem. Do you catch it in real word or in code?

I haven't trace that shows this issue but nothing prevent an update to
occur while you get values so you can have a mix of old and new
values.

> Could you explain more for details?

Both fields of a new task increase simultaneously but if you get the
old value for runnable_avg_period and the new one for
runnable_avg_sum, runnable_avg_sum will be greater than
runnable_avg_period during this starting phase.

The worst case appears 2ms after the creation of the task,
runnable_avg_period and runnable_avg_sum should go from 1024 to 2046.
So the  task_h_load_avg will be 199% of task_h_load If you have
runnable_avg_period with 1024 and runnable_avg_sum with 2046.

A simple solution is to check that runnable_avg_period is always
greater or equal to runnable_avg_sum like i have done in my packing
small tasks patches

Vincent
>
> --
> Thanks
>     Alex
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