From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765455AbZFOSnV (ORCPT ); Mon, 15 Jun 2009 14:43:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1765100AbZFOSmz (ORCPT ); Mon, 15 Jun 2009 14:42:55 -0400 Received: from tomts36.bellnexxia.net ([209.226.175.93]:54982 "EHLO tomts36-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1765242AbZFOSmy (ORCPT ); Mon, 15 Jun 2009 14:42:54 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AokFAMsvNkpMQWQl/2dsb2JhbACBT9VwhA0F Date: Mon, 15 Jun 2009 14:42:50 -0400 From: Mathieu Desnoyers To: Ingo Molnar Cc: Linus Torvalds , mingo@redhat.com, hpa@zytor.com, paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, penberg@cs.helsinki.fi, vegard.nossum@gmail.com, efault@gmx.de, jeremy@goop.org, npiggin@suse.de, tglx@linutronix.de, linux-tip-commits@vger.kernel.org Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods Message-ID: <20090615184250.GB6520@Krystal> References: <20090615171845.GA7664@elte.hu> <20090615180527.GB4201@Krystal> <20090615182348.GC11248@elte.hu> <20090615182828.GE11248@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20090615182828.GE11248@elte.hu> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 14:39:22 up 107 days, 15:05, 3 users, load average: 1.20, 1.27, 0.97 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Ingo Molnar (mingo@elte.hu) wrote: > > * Ingo Molnar wrote: > > > The GUP based method is pretty generic though - and can be used on > > other architectures as well. It's not as fast as direct access > > though. > > Another question is: your patch switches over all normal exceptions > from IRET to hand-unroll+RET. > Nope, it actually only switches the exceptions returning from an exception handler nested in NMI context to the hand-unroll+RET version. Given such exception nesting is expected to be very rare, it should not show any performance difference. I also organised the code to make sure I did not add any test to the fast paths in my original patch. > It would be really nice to benchmark it (via 'perf stat' for example > ;-) whether that's a slowdown or a speedup. > > If it's a slowdown then the decision is easy: we dont want this, we > want to push the overhead into the sampling code, away from common > codepaths. > I did not try to make the "hand unroll + ret" the default. I therefore don't know if it is faster or slower than iret. But I prefered to stay on the safe side and only modify the exceptions nested within NMI handlers. Mathieu > [ If on the other hand it's a speedup of a few cycles then we have > the problem of me suddenly liking this patch a whole lot more ;-) ] > > Ingo -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68