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Date:	Sat, 19 Jul 2014 14:46:56 -0400 (EDT)
From:	Nicolas Pitre <>
To:	Peter Zijlstra <>
cc:	Frederic Weisbecker <>,
	LKML <>,
	Ingo Molnar <>,
	"Paul E. McKenney" <>,
	Steven Rostedt <>,
	Thomas Gleixner <>,
	Viresh Kumar <>
Subject: Re: [PATCH 07/10] nohz: Enforce timekeeping on CPU 0

On Sat, 19 Jul 2014, Peter Zijlstra wrote:

> On Sat, Jul 19, 2014 at 01:31:25PM -0400, Nicolas Pitre wrote:
> > On Sat, 19 Jul 2014, Frederic Weisbecker wrote:
> > 
> > > The timekeeper gets initialized to the value of the CPU where the
> > > first clockevent device is setup. This works well because the timekeeper
> > > can be any online CPU in most configs.
> > > 
> > > Full dynticks has its own requirement though and needs the timekeeper
> > > to always be 0. And this requirement seem to accomodate pretty well with
> > > the above described boot timekeeper setting because the first clockevent
> > > device happens to be initialized, most of the time, on the boot CPU
> > > (which should be CPU 0).
> > 
> > This might have been discussed before... but this isn't ARM big.LITTLE 
> > friendly at all.
> > 
> > Could we accommodate for any arbitrary CPU instead of making CPU 0 
> > "special" other than its role as the boot CPU please?  It doesn't have 
> > to be completely dynamic, but CPU 0 might be a really bad choice for 
> > ongoing periodic duties in some configurations.  For example, we might 
> > highly prefer to do this on CPU 4 for power efficiency reasons once it 
> > is online and keep CPU 0 in a deep C-state as much as possible.
> This is because CPU0 can be a big core, right? IIRC this is done because
> a big core as boot cpu, boots faster and some people think boot time is
> relevant.

People think all sorts of things.  And it becomes very irritating when 
thoughtful assumptions get burned into ROM for example.  We should be 
able to do better in the kernel.

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