From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762891AbZFPWmH (ORCPT ); Tue, 16 Jun 2009 18:42:07 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762269AbZFPWlr (ORCPT ); Tue, 16 Jun 2009 18:41:47 -0400 Received: from terminus.zytor.com ([198.137.202.10]:56289 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761758AbZFPWlq (ORCPT ); Tue, 16 Jun 2009 18:41:46 -0400 Message-ID: <4A381E97.9010905@zytor.com> Date: Tue, 16 Jun 2009 15:37:11 -0700 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Mathieu Desnoyers CC: Ingo Molnar , Peter Zijlstra , Linus Torvalds , mingo@redhat.com, paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org, 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 References: <20090615223038.GA15903@Krystal> <4A36CCFC.8070908@zytor.com> <20090615224908.GA16661@Krystal> <4A36F520.6020604@zytor.com> <20090616030522.GA22162@Krystal> <20090616083343.GD16229@elte.hu> <20090616141956.GB6541@Krystal> <4A37B8CF.8090804@zytor.com> <20090616190634.GA16430@Krystal> <4A380000.5090206@zytor.com> <20090616211357.GB21305@Krystal> In-Reply-To: <20090616211357.GB21305@Krystal> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mathieu Desnoyers wrote: > > So, in summary : > > - near-zero measurable runtime cost. > - NMI-reentrancy constraint on a very small and well-defined trap > handler code path. > - simplifies life of tracer and profilers. (meaning : makes a lot of > _other_ kernel code much easier to write and understand) > - removes ad-hoc corner cases management from those users. > - provides early error detection because the nmi-reentrant code path is > shared by all users. > > So I'll use your own argument : making this trap handler code path > nmi-reentrant will simplify an already existing bunch of in-kernel users > (oprofile, perf counter tool, ftrace..). Moving the burden from > subsystems spread across the kernel tree to a single, well defined spot > looks like a constraint that will _diminish_ overall kernel development > cost. > No, this is utter bullshit. YOU ARE ADDING A CONSTRAINT TO ONE OF THE HOTTEST PATHS IN THE KERNEL. Constraining future optimizations. To support tools. That is what I'm objecting to. -hpa