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 13:49:30 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Alex Shi <alex.shi@...aro.org>,
	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>,
	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 01:29:53PM -0800, Andy Lutomirski wrote:
> On 12/18/2013 09:43 AM, Frederic Weisbecker wrote:
> > 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.
> 
> Where's your ambition? :)
> 
> If the vdso timing functions could see that it's been too long since a
> real timekeeping update, they could fall back to a syscall.  Otherwise,
> they could using rdtsc or whatever is in use.

One objection to that approach in the past has been that it injects
avoidable latency into the worker CPUs.  I suppose that you could argue
that the cache misses due to a timekeeping-CPU update are not free, but
then again, the syscall is likely to also incur a few cache misses as
well.

I bet that the timekeeping-CPU approach wins, but it would be cool to
see you prove me wrong.

							Thanx, Paul

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