From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756605AbZAWLCM (ORCPT ); Fri, 23 Jan 2009 06:02:12 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761876AbZAWLAs (ORCPT ); Fri, 23 Jan 2009 06:00:48 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:34876 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761798AbZAWLAr (ORCPT ); Fri, 23 Jan 2009 06:00:47 -0500 Date: Fri, 23 Jan 2009 12:00:37 +0100 From: Ingo Molnar To: =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker Cc: Steven Rostedt , Linux Kernel Mailing List Subject: Re: [PATCH 1/2 v2] tracing/function-graph-tracer: various fixes and features Message-ID: <20090123110037.GI15188@elte.hu> References: <497917b5.09cc660a.5f4c.ffffc568@mx.google.com> <20090123101949.GD15188@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 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 * Frédéric Weisbecker wrote: > > Still needs a solution - if we do cross-CPU traces we want to have a > > global trace clock with 'seemless' transition between CPUs. > > So it doesn't only need a monotonic clock. It needs a global consistent > clock like ktime for example? Unfortunately this one uses seq_locks and > would add some drawbacks like verifying if the traced function doesn't > hold the write seq_lock and it will bring some more ftrace recursion... using ktime_get() is indeed out of question - GTOD callpaths are too complex (and also too slow). I'd not change anything in the current logic, but i was thinking of a new trace_option, which can be set optionally. If that trace option is set then this bit of ring_buffer_time_stamp(): time = sched_clock() << DEBUG_SHIFT; gets turned into: time = cpu_clock(cpu) << DEBUG_SHIFT; This way we default to sched_clock(), but also gain some 'global' properties if the trace_option is set. Furthermore, another trace_option could introduce a third 'strongly ordered' trace-clock variant, which would use cmpxchg and per cpu timestamps, something like this: atomic64_t curr_time; DEFINE_PER_CPU(u64, prev_cpu_time); ... retry: prev_cpu_time = per_cpu(prev_cpu_time, cpu); cpu_time = sched_clock(); old_time = atomic64_read(&curr_time); delta = cpu_time - prev_cpu_time; if (unlikely((s64)delta <= 0)) delta = 1; new_time = old_time + delta; if (atomic64_cmpxchg(&curr_time, old_time, new_time) != new_time) goto repeat; time = new_time << DEBUG_SHIFT; This would be a monotonic, global clock wrapped around sched_clock(). It uses a cmpxchg to achieve it, but we have to use global ordering anyway. It would still be _much_ faster than any GTOD clocksource we have. Hm? Ingo