From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nicolas Pitre <nicolas.pitre@linaro.org>
Date: Thu, 24 Feb 2011 20:24:44 +0000
Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events
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To: Thomas Gleixner <tglx@linutronix.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, Peter Zijlstra <peterz@infradead.org>, 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, 24 Feb 2011, Thomas Gleixner wrote:

> On Thu, 24 Feb 2011, Alan Cox wrote:
> 
> > > Anybody who is interested in the latency of cpu hotplug is deluding
> > > himself, also cpu hotplug is _NOT_ a power management feature, so the
> > > rest of your justification just disappeared as well.
> > 
> > Actually CPU hotplug is a power management feature on some devices where
> > you need to shutdown one of the cores to enter low power modes.
> 
> Right.
> 
> > Remember we use it as part of the suspend paths and various processors
> > nowdays drop into a suspend to RAM type state on CPU idling.
> 
> The suspend path is using it, but the cpu idle one is hardly using the
> hotplug muck.

Most SMP ARM processors are going to use it soon.  Powering down idle 
cores provides substantial power saving.


Nicolas