Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751296AbWIOR7U (ORCPT ); Fri, 15 Sep 2006 13:59:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751290AbWIOR7U (ORCPT ); Fri, 15 Sep 2006 13:59:20 -0400 Received: from smtp.osdl.org ([65.172.181.4]:8648 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1751296AbWIOR7S (ORCPT ); Fri, 15 Sep 2006 13:59:18 -0400 Date: Fri, 15 Sep 2006 10:57:17 -0700 From: Andrew Morton To: fche@redhat.com (Frank Ch. Eigler) Cc: Alan Cox , karim@opersys.com, Roman Zippel , Tim Bird , Ingo Molnar , Mathieu Desnoyers , linux-kernel@vger.kernel.org, Christoph Hellwig , Ingo Molnar , Greg Kroah-Hartman , Thomas Gleixner , Tom Zanussi , ltt-dev@shafik.org, Michel Dagenais Subject: Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108 Message-Id: <20060915105717.b8e35cd6.akpm@osdl.org> In-Reply-To: References: <20060914033826.GA2194@Krystal> <20060914112718.GA7065@elte.hu> <20060914135548.GA24393@elte.hu> <20060914171320.GB1105@elte.hu> <20060914181557.GA22469@elte.hu> <4509B03A.3070504@am.sony.com> <1158320406.29932.16.camel@localhost.localdomain> <1158323938.29932.23.camel@localhost.localdomain> <1158327696.29932.29.camel@localhost.localdomain> <1158331277.29932.66.camel@localhost.localdomain> <450ABA2A.9060406@opersys.com> <1158332324.29932.82.camel@localhost.localdomain> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.6; i686-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2072 Lines: 60 On 15 Sep 2006 13:08:29 -0400 fche@redhat.com (Frank Ch. Eigler) wrote: > Alan Cox writes: > > > [...] > > > > > > prepare_arch_switch(rq, next); > > > + TRACE_SCHEDCHANGE(prev, next); > > > prev = context_switch(rq, prev, next); > > > barrier(); > > > > The gdb debug data lets you find each line and also the variable > > assignments (except when highly optimised in some cases). [...] > > Unfortunately, variables and even control flow are quite regularly > made non-probe-capable by modern gcc. Statement boundaries and > variables are not preserved. There is an arms race within gcc to both > improve code optimization and its own "reverse-engineering" debugging > data generation, and the former is always ahead. > > The end result is that there are many spots that we'd like to probe in > systemtap, but can't place exactly or extract all the data we'd like. > Really. Useful info, thanks. > There are also spots that for other reasons cannot tolerate a fully > dynamic kprobes-style probe: > > - where 1000-cycle int3-dispatching overheads too high Is that still true of the recent kprobes "boosting" changes? > - in low-level code such as fault handling or locking, that, if probed > dynamically, could entail infinite regress > - debugging information may not be available > > This is the reason why I'm in favour of some lightweight event-marking > facility: a way of catching those points where dynamic probing is not > sufficiently fast or dependable. OK. > > [...] > > All we appear to lack is systemtap ability to parse debug data so it can > > be told "trace on line 9 of sched.c and record rq and next" > > Actually: > > #! stap > probe kernel.function("*@kernel/sched.c:9") { printf("%p %p", $rq, $next) } > Really. That's impressive progress. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/