linux-kernel - Re: [RFC PATCH 00/13] nohz: Use sysidle detection to let the timekeeper sleep

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Date:	Wed, 18 Dec 2013 18:43:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Alex Shi <alex.shi@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	John Stultz <john.stultz@...aro.org>,
	Kevin Hilman <khilman@...aro.org>
Subject: Re: [RFC PATCH 00/13] nohz: Use sysidle detection to let the
 timekeeper sleep

On Wed, Dec 18, 2013 at 10:04:43AM +0800, Alex Shi wrote:
> On 12/18/2013 06:51 AM, Frederic Weisbecker wrote:
> > So this is what this series brings, more details following:
> > 
> > * Some code, naming and whitespace cleanups
> > 
> > * Allow all CPUs outside the nohz_full range to handle the timekeeping
> >   duty, not just CPU 0. Balancing the timekeeping duty should improve
> >   powersavings.
> 
> If the system just has one nohz_full cpu running, it will need another
> cpu to do timerkeeper job. Then the system roughly needs 2 cpu living.
> From powersaving POV, that is not good compare to normal nohz idle.

Sure, but everything has a tradeoff :)

We could theoretically run with the timekeeper purely idle if the other
CPU in full dynticks mode runs in userspace for a long while and seldom
do syscalls and faults. Timekeeping could be updated on kernel/user
boundaries in this case without much impact on performances.

But then there is one strict condition for that: it can't read the timeofday
through the vdso but only through a syscall.

Then if we can meet that condition, then CPU 0 could as well be full dynticks.

Now that's real extreme HPC ;)
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