From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935711AbZFOVeg (ORCPT ); Mon, 15 Jun 2009 17:34:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S934528AbZFOVea (ORCPT ); Mon, 15 Jun 2009 17:34:30 -0400 Received: from tomts43-srv.bellnexxia.net ([209.226.175.110]:63320 "EHLO tomts43-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934479AbZFOVe3 (ORCPT ); Mon, 15 Jun 2009 17:34:29 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AokFAL9ZNkpMQWQl/2dsb2JhbACBT9NRhA0F Date: Mon, 15 Jun 2009 17:34:29 -0400 From: Mathieu Desnoyers To: Ingo Molnar Cc: "H. Peter Anvin" , 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 Message-ID: <20090615213429.GD12919@Krystal> References: <20090615184858.GD6520@Krystal> <1245091917.6741.185.camel@laptop> <20090615185907.GF6520@Krystal> <1245092561.6741.205.camel@laptop> <4A369CD8.3090505@zytor.com> <20090615192720.GA9056@Krystal> <4A36A1C7.6080005@zytor.com> <20090615210119.GD24554@elte.hu> <20090615211207.GB12919@Krystal> <20090615211605.GC27100@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20090615211605.GC27100@elte.hu> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 17:27:09 up 107 days, 17:53, 3 users, load average: 0.30, 0.26, 0.33 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: > > * Mathieu Desnoyers wrote: > > > Just for the sake of making NMI handlers less tricky, supporting > > page faults caused by faulting kernel instructions (rather than > > only supporting explicit faulting from get_user_pages_inatomic) > > would be rather nice design-wise if it only costs 2-3 cycles. > > > > And I would not want to touch the page fault handler itself to > > write the saved cr2 value before the handler exits, because this > > would add a branch on a very hot path. > > _That_ path is not hot at all - it's the 'we are in atomic section > and faulted' rare path (laced with an exception table search - which > is extremely slow compared to other bits of the pagefault path). > > But ... it's not an issue: a check can be made in the NMI code too, > as we always know about pagefaults there, by virtue of getting > -EFAULT back from the attempted-user-copy. As the maintainer of the out-of-tree LTTng tracer, which hooks in the page fault handler with tracepoints, and which can build almost entirely as modules, I am very tempted to argue that having the nmi-code entirely robust wrt in-kernel page faults would be a very-nice-to-have feature. Requiring that code to be either built-in or to call vmalloc_sync_all() after any vmalloc or after module load is just painful and error-prone. Plus, tracing is a feature that some users will only use in specific occasions. I don't see why it should be built-into the kernel at all. That's just a waste of memory in many cases. (I am not talking about users who want to do continuous system tracing here, which is a totally different scenario). Mathieu > > Ingo -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68