From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Date: Thu, 24 Feb 2011 21:11:17 +0000 Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events Message-Id: <20110224211117.GQ2264@linux.vnet.ibm.com> List-Id: References: <1298573197.2428.457.camel@twins> <20110224201124.138311ba@lxorguk.ukuu.org.uk> <1298579452.5226.834.camel@laptop> <1298581112.5226.838.camel@laptop> In-Reply-To: <1298581112.5226.838.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Zijlstra Cc: Thomas Gleixner , Nicolas Pitre , Alan Cox , Vincent Guittot , lkml , linux-hotplug@vger.kernel.org, Frederic Weisbecker , Steven Rostedt , amit.kucheria@linaro.org, Rusty Russell , Ingo Molnar 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