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

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Message-Id: <20150326131822.fce6609efdd85b89ceb3f61c@linux-foundation.org>
Date:	Thu, 26 Mar 2015 13:18:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	hannes@...xchg.org, cl@...ux.com, linaro-kernel@...ts.linaro.org,
	linux-kernel@...r.kernel.org, vinmenon@...eaurora.org,
	shashim@...eaurora.org, mhocko@...e.cz, mgorman@...e.de,
	dave@...olabs.net, koct9i@...il.com, linux-mm@...ck.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work

On Thu, 26 Mar 2015 11:09:01 +0530 Viresh Kumar <viresh.kumar@...aro.org> 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.
> 
> In order to avoid waking up an idle core, we can replace schedule_delayed_work()
> with a normal work plus a separate timer. The timer handler will then queue the
> work after re-arming the timer. If the CPU was idle before the timer fired,
> idle_cpu() will mostly return true and the next timer shall be migrated to a
> non-idle CPU.
> 
> But the timer core has a limitation, when the timer is re-armed from its
> handler, timer core disables migration of that timer to other cores. Details of
> that limitation are present in kernel/time/timer.c:__mod_timer() routine.
> 
> Another simple yet effective solution can be to keep two timers with same
> handler and keep toggling between them, so that the above limitation doesn't
> hold true anymore.
> 
> This patch replaces schedule_delayed_work() with schedule_work() plus two
> timers. After this, it was seen that the timer and its do get migrated to other
> non-idle CPUs, when the local cpu is idle.

Shouldn't this be viewed as a shortcoming of the core timer code? 

vmstat_shepherd() is merely rescheduling itself with
schedule_delayed_work().  That's a dead bog simple operation and if
it's producing suboptimal behaviour then we shouldn't be fixing it with
elaborate workarounds in the caller?

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