From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752239AbYKRIsR (ORCPT ); Tue, 18 Nov 2008 03:48:17 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750779AbYKRIsB (ORCPT ); Tue, 18 Nov 2008 03:48:01 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:52348 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750756AbYKRIsB (ORCPT ); Tue, 18 Nov 2008 03:48:01 -0500 Date: Tue, 18 Nov 2008 09:47:55 +0100 From: Ingo Molnar To: Frederic Weisbecker Cc: Steven Rostedt , Linux Kernel Subject: Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun field Message-ID: <20081118084755.GK17838@elte.hu> References: <4920D571.4050007@gmail.com> <20081117084923.GD28786@elte.hu> <4921BA25.3090704@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4921BA25.3090704@gmail.com> 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 * Frederic Weisbecker wrote: > Ingo Molnar a écrit : > > * Frederic Weisbecker wrote: > > > >> Impact: help to find the better depth of trace > >> > >> We decided to arbitrary define the depth of function return trace as > >> "20". Perhaps this is not enough. To help finding an optimal depth, > >> we measure now the overrun: the number of functions that have been > >> missed for the current thread. By default this is not displayed, we > >> have to do set a particular flag on the return tracer: echo overrun > >>> /debug/tracing/trace_options And the overrun will be printed on > >> the right. > >> > >> As the trace shows below, the current 20 depth is not enough. > >> > >> update_wall_time+0x37f/0x8c0 -> update_xtime_cache (345 ns) (Overruns: 2838) > >> update_wall_time+0x384/0x8c0 -> clocksource_get_next (1141 ns) (Overruns: 2838) > >> do_timer+0x23/0x100 -> update_wall_time (3882 ns) (Overruns: 2838) > > > > hm, interesting. Have you tried to figure out what a practical depth > > limit would be? > > > > With lockdep we made the experience that function call stacks can be > > very deep - if we count IRQ contexts too it can be up to 100 in the > > extreme cases. (but at that stage kernel stack limits start hitting > > us) > > > > I'd say 50 would be needed. > > > > Ingo > > > Ok I will try with 50. If there are still a lot and often missing > traces with this depth, perhaps should we consider a hybrid solution > between ret stack and trampolines? We could use the normal ret stack > on struct info for most common cases and the trampoline when we are > exceeding the depth.... dunno, trampolines make me feel uneasy. Could you set it to some really large value (200) and add a "max depth seen" variable perhaps, and see the maximum depth? Ingo