linux-kernel - Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events

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Date:	Thu, 24 Feb 2011 15:24:44 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-hotplug@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, amit.kucheria@...aro.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events

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
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