linux-kernel - Re: [PATCH RFC nohz_full 0/7] v2 Provide infrastructure for full-system idle

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Date:	Mon, 1 Jul 2013 19:43:47 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH RFC nohz_full 0/7] v2 Provide infrastructure for full-system
 idle

On Fri, 28 Jun 2013, Paul E. McKenney wrote:

> Unfortunately, timekeeping CPU continues taking scheduling-clock
> interrupts even when all other CPUs are completely idle, which is
> not so good for energy efficiency and battery lifetime.  Clearly, it
> would be good to turn off the timekeeping CPU's scheduling-clock tick
> when all CPUs are completely idle.  This is conceptually simple, but
> we also need good performance and scalability on large systems, which
> rules out implementations based on frequently updated global counts of
> non-idle CPUs as well as implementations that frequently scan all CPUs.
> Nevertheless, we need a single global indicator in order to keep the
> overhead of checking acceptably low.

Can we turn off timekeeping when no cpu needs time in adaptive mode?
Setting breakpoints in the VDSO could force timekeeping on again whenever
something needs time. Would this not be simpler?
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