From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757747AbXGKAz2 (ORCPT ); Tue, 10 Jul 2007 20:55:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754535AbXGKAzU (ORCPT ); Tue, 10 Jul 2007 20:55:20 -0400 Received: from tomts10.bellnexxia.net ([209.226.175.54]:55500 "EHLO tomts10-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754477AbXGKAzS (ORCPT ); Tue, 10 Jul 2007 20:55:18 -0400 Date: Tue, 10 Jul 2007 20:55:14 -0400 From: Mathieu Desnoyers To: Matt Mackall Cc: Nick Piggin , Steven Rostedt , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [RFC] Thread Migration Preemption Message-ID: <20070711005514.GA29071@Krystal> References: <20070705215152.GA4865@Krystal> <468DDD3A.6010308@yahoo.com.au> <20070710233938.GT11166@waste.org> <46941E0F.3040209@yahoo.com.au> <20070711003611.GY11115@waste.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20070711003611.GY11115@waste.org> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 20:43:17 up 3 days, 14:48, 2 users, load average: 0.76, 0.74, 0.48 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org * Matt Mackall (mpm@selenic.com) wrote: > On Wed, Jul 11, 2007 at 10:02:23AM +1000, Nick Piggin wrote: > > >I like this patch a lot. Even if we don't add the underlying mechanism > > >right now, adding migration_disable as an alias for preempt_disable > > >will much better document quite a number of the users. > > > > I'd have no problem with that, and it might make it easier in future to > > justify a more complex scheme. > > What do you think, Mathieu? > That's an excellent idea. It should probably come with some documentation explaining what difference should be expected between preempt_disable and migrate_disable in the future so that people can choose the right alternative for their code. > Also, small nit: it ought to be migrate_disable to match the form of > preempt_disable. Yes, that's fine with me. :) I though a little more about this whole concept of migrate_disable, and I think it could be brought further. One of the main problems with per cpu variables is that this concept cannot be ported to this grey place called "user-space" because preemption cannot be disabled. The equivalent (kind of) is currently per thread variables, but it can consume a lot of space if many threads are running. However, it could be possible, if we implement a vsyscall based migration preemption counter accessible with read/write access from user-space, to tie a thread to a CPU during a user-space critical path. If we combine this with local atomic operations done in user-space, we could have highly scalable access to per cpu data structures reentrant with respect to signal handlers. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68