From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751703AbYKMJYE (ORCPT ); Thu, 13 Nov 2008 04:24:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751631AbYKMJXs (ORCPT ); Thu, 13 Nov 2008 04:23:48 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:46884 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751516AbYKMJXr (ORCPT ); Thu, 13 Nov 2008 04:23:47 -0500 Date: Thu, 13 Nov 2008 10:23:40 +0100 From: Ingo Molnar To: =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker Cc: Steven Rostedt , Linux Kernel , Peter Zijlstra Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless Message-ID: <20081113092340.GJ25479@elte.hu> References: <491B4F0A.3080901@gmail.com> <20081112221552.GA6125@elte.hu> <20081113085551.GF25479@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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,DNS_FROM_SECURITYSAGE autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] 0.0 DNS_FROM_SECURITYSAGE RBL: Envelope sender in blackholes.securitysage.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Frédéric Weisbecker wrote: > 2008/11/13 Ingo Molnar : > > > > * Frédéric Weisbecker wrote: > > > >> BTW I'm wondering about consistency in time capturing. When I look > >> into kernel/sched_clock.c I see this in introduction: > >> > >> "The clock: sched_clock_cpu() is monotonic per cpu, and should be > >> somewhat consistent between cpus (never more than 2 jiffies > >> difference)." > >> > >> Two Jiffies, that could result in a lot of inconsistency in the way > >> of nanosec capturing. The current task can be preempted between the > >> call time and the return time and I'm doing a > >> cpu_clock(raw_smp_processor_id) on these two times. Should I keep > >> the same processor_id for these two captures? But what would happen > >> if this cpu is shut down between these two times? One other solution > >> would be to plan time capture in usec but I would mostly lose the > >> interest of function cost measuring.... > >> > >> What do you think? > > > > in practice the jitter is much lower - a couple of microseconds - up > > to a few dozen at most. > > > > But it's a possibility, and i think the best solution is something > > that Steve suggested yesterday: a /debug/tracing/trace_options flag > > that turns on global ordering for tracing timestamps. Something like: > > > > echo global_timestamps > /debug/tracing/trace_options > > > > tracers could also change the default of this flag. The function-cost > > tracer will probably want to default to globally synchronous > > timestamps, while the preempt and irqsoff tracers want to default to > > local timestamps only. > > > > Would something like this work for you? > > > > Ingo > > > > > But I guess this flag would apply on the timestamp inserted by the > ring-buffer. Unfortunately I can't use it since I have to capture > the clock for two times and not only during insertion in the > ring-buffer. i think the clock should be a property of the tracer, not of the ring buffer. Hence if a tracer has the option set, it will get coherent timestamps - including ringbuffer insertion timestamps. Ingo