linux-kernel - Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

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Message-ID: <20150327141922.GC5481@dhcp22.suse.cz>
Date:	Fri, 27 Mar 2015 15:19:22 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	akpm@...ux-foundation.org, hannes@...xchg.org, cl@...ux.com,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	vinmenon@...eaurora.org, shashim@...eaurora.org, mgorman@...e.de,
	dave@...olabs.net, koct9i@...il.com, linux-mm@...ck.org
Subject: Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

On Thu 26-03-15 11:09:01, Viresh Kumar wrote:
> A delayed work to schedule vmstat_shepherd() is queued at periodic intervals for
> internal working of vmstat core. This work and its timer end up waking an idle
> cpu sometimes, as this always stays on CPU0.
> 
> Because we re-queue the work from its handler, idle_cpu() returns false and so
> the timer (used by delayed work) never migrates to any other CPU.
> 
> This may not be the desired behavior always as waking up an idle CPU to queue
> work on few other CPUs isn't good from power-consumption point of view.

Wouldn't something like I was suggesting few months back
(http://article.gmane.org/gmane.linux.kernel.mm/127569) solve this
problem as well? Scheduler should be idle aware, no? I mean it shouldn't
wake up an idle CPU if the task might run on another one.
-- 
Michal Hocko
SUSE Labs
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