From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754755AbZCELiw (ORCPT ); Thu, 5 Mar 2009 06:38:52 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752479AbZCELin (ORCPT ); Thu, 5 Mar 2009 06:38:43 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:53274 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752275AbZCELim (ORCPT ); Thu, 5 Mar 2009 06:38:42 -0500 Date: Thu, 5 Mar 2009 12:38:28 +0100 From: Ingo Molnar To: Frederic Weisbecker Cc: Steven Rostedt , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] tracing/function-graph-tracer: use the more lightweight local clock Message-ID: <20090305113828.GA9548@elte.hu> References: <49af243d.06e9300a.53ad.ffff840c@mx.google.com> <20090305110433.GH32407@elte.hu> <20090305112322.GH5359@nowhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090305112322.GH5359@nowhere> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Frederic Weisbecker wrote: > > There will be one problem though: function trace elapsed > > time measurements across idle. Those are not correctly > > measured by the local clock. > > Oh, why? If I'm not wrong it uses sched_clock() which uses tsc > on x86. And this register always goes forward at the same > rate. Unless some Cpu decrease their frequency while beeing > idle for some time and then become unreliable? TSC stops in idle on many CPUs. Ingo