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 12:56:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.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 Mon, Jul 01, 2013 at 07:43:47PM +0000, Christoph Lameter wrote:
> 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?

Might be.  But what causes the breakpoints to be set on a system where
there is one CPU-bound nohz_full user-mode task with all other CPUs idle?
Or are you suggesting taking a breakpoint trap on each timekeeping access
to VDSO?

							Thanx, Paul

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