Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754735AbZJUSev (ORCPT ); Wed, 21 Oct 2009 14:34:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753748AbZJUSeu (ORCPT ); Wed, 21 Oct 2009 14:34:50 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.123]:47308 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753532AbZJUSeu (ORCPT ); Wed, 21 Oct 2009 14:34:50 -0400 Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS From: Steven Rostedt Reply-To: rostedt@goodmis.org To: Nicholas Mc Guire Cc: David Daney , Wu Zhangjin , linux-kernel@vger.kernel.org, linux-mips@linux-mips.org, Thomas Gleixner , Ralf Baechle In-Reply-To: <20091021181705.GA5218@opentech.at> References: <53bdfdd95ec4fa00d4cc505bb5972cf21243a14d.1256135456.git.wuzhangjin@gmail.com> <1256141540.18347.3118.camel@gandalf.stny.rr.com> <4ADF38D5.9060100@caviumnetworks.com> <1256143568.18347.3169.camel@gandalf.stny.rr.com> <4ADF3FE0.5090104@caviumnetworks.com> <1256145813.18347.3210.camel@gandalf.stny.rr.com> <4ADF4982.9010306@caviumnetworks.com> <1256148562.18347.3264.camel@gandalf.stny.rr.com> <20091021181705.GA5218@opentech.at> Content-Type: text/plain Organization: Kihon Technologies Inc. Date: Wed, 21 Oct 2009 14:34:52 -0400 Message-Id: <1256150092.18347.3294.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.26.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1456 Lines: 35 On Wed, 2009-10-21 at 20:17 +0200, Nicholas Mc Guire wrote: > > > > > > We're not doing back traces. We need to modify the return of the > > function being called. Note, the above functions that end with ";" are > > leaf functions. Non leaf functions show "{" and end with "}". > > > > The trick here is to find a reliable way to modify the return address. > > > would it not more or less be the same thing if you used -finstrument-functions > then and provide a stub __cyg_profile_func_enter/exit initialized to an empty > function until you replace it during tracing. This does give you an overhead > when you are not tracing - but it would make the tracer implementation quite > generic. -finstrument-functions adds a substantial overhead when not tracing, and there's no easy way to remove it. The beauty with this approach is that -pg only adds a couple of instructions (one on x86). When tracing is disabled, that one line is converted to a nop. On x86, hackbench reports no difference between running with dynamic ftrace configured but disabled (includes function graph configured) and without it configured. This allows function tracing to be configured in on production environments. -- Steve -- 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/