From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755481Ab1BXVDs (ORCPT ); Thu, 24 Feb 2011 16:03:48 -0500 Received: from www.tglx.de ([62.245.132.106]:55440 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753243Ab1BXVDp (ORCPT ); Thu, 24 Feb 2011 16:03:45 -0500 Date: Thu, 24 Feb 2011 22:03:17 +0100 (CET) From: Thomas Gleixner To: Peter Zijlstra cc: Nicolas Pitre , Alan Cox , Vincent Guittot , lkml , linux-hotplug@vger.kernel.org, Frederic Weisbecker , Steven Rostedt , amit.kucheria@linaro.org, Rusty Russell , Ingo Molnar , "Paul E. McKenney" Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events In-Reply-To: <1298581112.5226.838.camel@laptop> Message-ID: References: <1298573197.2428.457.camel@twins> <20110224201124.138311ba@lxorguk.ukuu.org.uk> <1298579452.5226.834.camel@laptop> <1298581112.5226.838.camel@laptop> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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