From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933679Ab3BTKJJ (ORCPT ); Wed, 20 Feb 2013 05:09:09 -0500 Received: from www.linutronix.de ([62.245.132.108]:38293 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757746Ab3BTKJG (ORCPT ); Wed, 20 Feb 2013 05:09:06 -0500 Date: Wed, 20 Feb 2013 11:09:03 +0100 (CET) From: Thomas Gleixner To: Andy Lutomirski cc: Daniel Lezcano , John Stultz , Frederic Weisbecker , Linux Kernel Mailing List , "linaro-dev >> Lists Linaro-dev" Subject: Re: [resend] Timer broadcast question In-Reply-To: <512400B2.9010801@mit.edu> Message-ID: References: <5123BE35.8070902@linaro.org> <5123C299.3080005@linaro.org> <512400B2.9010801@mit.edu> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 19 Feb 2013, Andy Lutomirski wrote: > On 02/19/2013 10:21 AM, Daniel Lezcano wrote: > > On 02/19/2013 07:10 PM, Thomas Gleixner wrote: > >> On Tue, 19 Feb 2013, Daniel Lezcano wrote: > >>> I am working on identifying the different wakeup sources from the > >>> interrupts and I have a question regarding the timer broadcast. > >>> > >>> The broadcast timer is setup to the next event and that will wake up any > >>> idle cpu belonging to the "broadcast cpumask", right ? > >>> > >>> The cpu which has been woken up will look for each cpu the next-event > >>> and send an IPI to wake it up. > >>> > >>> Although, it is possible the sender of this IPI may not be concerned by > >>> the timer expiration and has been woken up just for sending the IPI, right ? > >> > >> Correct. > >> > >>> If this is correct, is it possible to setup the timer irq affinity to a > >>> cpu which will be concerned by the timer expiration ? so we prevent an > >>> unnecessary wake up for a cpu. > >> > >> It is possible, but we never implemented it. > >> > >> If we go there, we want to make that conditional on a property flag, > >> because some interrupt controllers especially on x86 only allow to > >> move the affinity from interrupt context, which is pointless. > > > > Thanks Thomas for your quick answer. I will write a RFC patchset. > > I'm curious what the use case is. I played with this code awhile ago, > and AFAICT it's not used on sensible (i.e. modern) systems. Is there > anything other than old x86 machines that needs it? If the local apic timer is not affected by C-States, it's irrelevant, but there are enough machines out there which do not have that. The point is that we want a flag on the broadcast device which tells us whether we should use dynamic affinity settings or not. On x86 we would not set that flag ever. Thanks, tglx