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

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Date:	Tue, 09 Apr 2013 18:38:22 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
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@...il.com,
	"tony.luck@...el.com" <tony.luck@...el.com>, keescook@...omium.org,
	mgorman@...e.de, riel@...hat.com
Subject: Re: [patch v3 6/8] sched: consider runnable load average in move_tasks

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?
Could you explain more for details?

-- 
Thanks
    Alex
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