From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753764AbYKMMyk (ORCPT ); Thu, 13 Nov 2008 07:54:40 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752070AbYKMMyc (ORCPT ); Thu, 13 Nov 2008 07:54:32 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:33542 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752000AbYKMMyb (ORCPT ); Thu, 13 Nov 2008 07:54:31 -0500 Date: Thu, 13 Nov 2008 13:54:19 +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: <20081113125419.GA32574@elte.hu> References: <491B4F0A.3080901@gmail.com> <20081112221552.GA6125@elte.hu> <20081113085551.GF25479@elte.hu> <20081113092340.GJ25479@elte.hu> <20081113094027.GK25479@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: > Ok, so correct me if I'm wrong. Global timestamp would be captured > by using sched_clock(). That's what is done currently in > ring_buffer_time_stamp() And the global timestamp would be > combination of a last global timestamp and a relative position from > now to this last at each insertion in the ring-buffer (or tracing > time capture). Am I right? I don't really understand why you want to > update with a cmpxchg loop... the cmpxchg loop would be needed to ensure timestamp monotonicity: every new "global time" is cmpxchg-ed with the "previous global time" (and is first monotonicity checked). "prev_global_time" also acts as a global serializer: it ensures that events are timestamped in a monotonic and ordered way. i.e. something like this (pseudocode, without the cmpxchg): u64 prev_global_time; DEFINE_PER_CPU(prev_local_time); u64 global_time() { u64 now, delta, now_global; prev_global = prev_global_time; now = sched_clock(); delta = now - per_cpu(prev_local_time, this_cpu); per_cpu(prev_local_time, this_cpu) = now; now_global = prev_global + delta; prev_global = now_global; return now_global; } note how we build "global time" out of "local time". The cmpxchg would be used to put the above one into a loop, and instead of updating the global time in a racy way: prev_global = now_global; We'd update it via the cmpxchg: atomic64_t prev_global_time; ... while (atomic64_cmpxchg(&prev_global_time, prev_global, now_global) != prev_global) { [...] } To make sure the global time goes monotonic. (this way we also avoid a spinlock - locks are fragile for instrumentation) Ingo