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

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Message-ID: <alpine.LFD.2.00.1102242159300.2701@localhost6.localdomain6>
Date:	Thu, 24 Feb 2011 22:03:17 +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 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.

As far as I can tell the isolation stuff covers lot's of the problems
including RCU, but there is still a way to go. And the good new is
that, when a cpu is idle it has not much state to preserve if its
properly isolated and the memory is not going away.

Thanks,

	tglx

 
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