Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757523AbXHUXue (ORCPT ); Tue, 21 Aug 2007 19:50:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754732AbXHUXuZ (ORCPT ); Tue, 21 Aug 2007 19:50:25 -0400 Received: from tomts22-srv.bellnexxia.net ([209.226.175.184]:45018 "EHLO tomts22-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752928AbXHUXuY (ORCPT ); Tue, 21 Aug 2007 19:50:24 -0400 Date: Tue, 21 Aug 2007 19:50:23 -0400 From: Mathieu Desnoyers To: Christoph Lameter Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, mingo@redhat.com Subject: Re: [PATCH] SLUB use cmpxchg_local Message-ID: <20070821235023.GF29691@Krystal> References: <20070820204126.GA22507@Krystal> <20070820212922.GA27011@Krystal> <20070820215413.GA28452@Krystal> <20070821173849.GA8360@Krystal> <20070821232320.GC29691@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20070821232320.GC29691@Krystal> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 19:47:32 up 23 days, 6 min, 3 users, load average: 0.36, 0.34, 0.34 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1686 Lines: 36 * Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote: > * Christoph Lameter (clameter@sgi.com) wrote: > > On Tue, 21 Aug 2007, Mathieu Desnoyers wrote: > > > > > - Changed smp_rmb() for barrier(). We are not interested in read order > > > across cpus, what we want is to be ordered wrt local interrupts only. > > > barrier() is much cheaper than a rmb(). > > > > But this means a preempt disable is required. RT users do not want that. > > Without preemption the processor can be moved after c has been determined. > > That is why the smp_rmb() is there. > > preemption is required if we want to use cmpxchg_local anyway. > > We may have to find a way to use preemption while being able to give an > upper bound on the preempt disabled execution time. I think I got a way > to do this yesterday.. I'll dig in my patches. > Yeah, I remember having done so : moving the preempt disable nearer to the cmpxchg, checking if the cpuid has changed between the raw_smp_processor_id() read and the preempt_disable done later, redo if it is different. It makes the slow path faster, but makes the fast path more complex, therefore I finally dropped the patch. And we talk about ~10 cycles for the slow path here, I doubt it's worth the complexity added to the fast path. -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - 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/