From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 24 Feb 2011 20:49:49 +0000
Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events
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To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Vincent Guittot <vincent.guittot@linaro.org>, lkml <linux-kernel@vger.kernel.org>, linux-hotplug@vger.kernel.org, Frederic Weisbecker <fweisbec@gmail.com>, Steven Rostedt <rostedt@goodmis.org>, amit.kucheria@linaro.org, Rusty Russell <rusty@rustcorp.com.au>, Ingo Molnar <mingo@elte.hu>

On Thu, 2011-02-24 at 15:40 -0500, Nicolas Pitre wrote:
> On Thu, 24 Feb 2011, Peter Zijlstra wrote:
> 
> > On Thu, 2011-02-24 at 15:24 -0500, Nicolas Pitre wrote:
> > 
> > > Most SMP ARM processors are going to use it soon.  Powering down idle 
> > > cores provides substantial power saving.
> > 
> > And why can't regular idle paths be used? CPU hotplug is a massively
> > expensive operation.
> 
> The idle path assumes that the CPU state is preserved. 

Not much of it,

>  We're talking 
> about cores completely going down with power pulled beneath them and 
> eventually rebooted dynamically here.

I think you can get away with actually pulling the power and re-initing
the cpu to idle from the idle path and the rest of the kernel not
caring.