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

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Date:	Thu, 24 Feb 2011 21:47:23 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events

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.

To achieve the same result from idle, you need to exclude the core
from any unwanted wakeup. At the moment cpu unplug is the only way to
achieve that.

If you want to do the same from idle, then we need the isolation
features Frederic is working on for RT/HPC.

They allow us to isolate cores completely for totaly different
reasons, but it could be resused to provide full isolation of a core
in a very deep power state.

That would solve the problem w/o going through kstompmachine

Thanks,

	tglx
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