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

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Date:	Sat, 19 Jul 2014 13:31:25 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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, 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.

Nicolas
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