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Date: Thu, 24 Feb 2011 13:11:17 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Vincent Guittot <vincent.guittot@linaro.org>,
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        Steven Rostedt <rostedt@goodmis.org>, amit.kucheria@linaro.org,
        Rusty Russell <rusty@rustcorp.com.au>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events
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On Thu, Feb 24, 2011 at 09:58:32PM +0100, Peter Zijlstra wrote:
> On Thu, 2011-02-24 at 21:47 +0100, Thomas Gleixner 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.
> > 
> > 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.
> 
> Right, everything is a nail because all we have is a hammer like.
> 
> > 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.
> 
> Exactly.
> 
> > That would solve the problem w/o going through kstompmachine
> 
> Right, kstopmachine is a large part of the problem, but cpu hotplug
> really does an insane amount of work if all you want is to idle the
> core.

You do indeed need to know that the CPU will be powered off for quite
some time to be worth the extra work.  And a number of embedded devices
do have this level of foreknowledge -- for example, they might use the
second CPU only when the user is doing some computationally intensive
task, so that the device would normally be using only a single CPU.

							Thanx, Paul