linux-kernel - Re: [PATCH 07/10] nohz: Enforce timekeeping on CPU 0

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Message-ID: <20140719183132.GK3935@laptop>
Date:	Sat, 19 Jul 2014 20:31:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 07/10] nohz: Enforce timekeeping on CPU 0

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.
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