Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754625AbZKSSs3 (ORCPT ); Thu, 19 Nov 2009 13:48:29 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754565AbZKSSs2 (ORCPT ); Thu, 19 Nov 2009 13:48:28 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:43278 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753702AbZKSSs1 (ORCPT ); Thu, 19 Nov 2009 13:48:27 -0500 Date: Thu, 19 Nov 2009 19:47:16 +0100 From: Ingo Molnar To: Linus Torvalds Cc: Richard Guenther , rostedt@goodmis.org, Thomas Gleixner , "H. Peter Anvin" , LKML , Andrew Morton , Heiko Carstens , feng.tang@intel.com, Fr??d??ric Weisbecker , Peter Zijlstra , jakub@redhat.com, gcc@gcc.gnu.org Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions Message-ID: <20091119184716.GA25458@elte.hu> References: <20091119072040.GA23579@elte.hu> <1258653562.22249.682.camel@gandalf.stny.rr.com> <84fc9c000911191003t244eb864o3d5b355ab5485f@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: 0.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=0.0 required=5.9 tests=none autolearn=no SpamAssassin version=3.2.5 _SUMMARY_ Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1144 Lines: 25 * Linus Torvalds wrote: > Admittedly, anybody who compiles with -pg probably doesn't care deeply > about smaller and more efficient code, since the mcount call overhead > tends to make the thing moot anyway, but it really looks like a > win-win situation to just fix the mcount call sequence regardless. Just a sidenote: due to dyn-ftrace, which patches out all mcounts during bootup to be NOPs (and opt-in patches them in again if someone runs the function tracer), the cost is not as large as one would have it with say -pg based user-space profiling. It's not completely zero-cost as the pure NOPs balloon the i$ footprint a bit and GCC generates different code too in some cases. But it's certainly good enough that it's generally pretty hard to prove overhead via micro or macro benchmarks that the patched out mcounts call sites are there. Ingo -- 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/