From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753592Ab1JJLCN (ORCPT ); Mon, 10 Oct 2011 07:02:13 -0400 Received: from am1ehsobe005.messaging.microsoft.com ([213.199.154.208]:50645 "EHLO AM1EHSOBE005.bigfish.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753474Ab1JJLCM (ORCPT ); Mon, 10 Oct 2011 07:02:12 -0400 X-SpamScore: -14 X-BigFish: VPS-14(zzbb2dK9371K1432N98dKzz1202hzzz32i668h839h944h61h) X-Spam-TCS-SCL: 0:0 X-Forefront-Antispam-Report: CIP:163.181.249.109;KIP:(null);UIP:(null);IPVD:NLI;H:ausb3twp02.amd.com;RD:none;EFVD:NLI X-FB-SS: 0, X-WSS-ID: 0LSUJXH-02-382-02 X-M-MSG: From: Stephan Diestelhorst To: CC: Jeremy Fitzhardinge , Jeremy Fitzhardinge , Nick Piggin , KVM , Peter Zijlstra , the arch/x86 maintainers , Linux Kernel Mailing List , Marcelo Tosatti , Andi Kleen , Avi Kivity , Jan Beulich , "H. Peter Anvin" , Ingo Molnar , Linus Torvalds Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks Date: Mon, 10 Oct 2011 13:00:50 +0200 Message-ID: <2523929.AGG4U997NO@d-allen> Organization: AMD OSRC User-Agent: KMail/4.7.1 (Linux/3.0.4-030004-generic; KDE/4.7.1; x86_64; ; ) In-Reply-To: <4E8DE7F1.3050108@goop.org> References: <2707952.s3VYcmPHUN@chlor> <4E8DE7F1.3050108@goop.org> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-OriginatorOrg: amd.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thursday 06 October 2011, 13:40:01 Jeremy Fitzhardinge wrote: > On 10/06/2011 07:04 AM, Stephan Diestelhorst wrote: > > On Wednesday 28 September 2011, 14:49:56 Linus Torvalds wrote: > >> Which certainly should *work*, but from a conceptual standpoint, isn't > >> it just *much* nicer to say "we actually know *exactly* what the upper > >> bits were". > > Well, we really do NOT want atomicity here. What we really rather want > > is sequentiality: free the lock, make the update visible, and THEN > > check if someone has gone sleeping on it. > > > > Atomicity only conveniently enforces that the three do not happen in a > > different order (with the store becoming visible after the checking > > load). > > > > This does not have to be atomic, since spurious wakeups are not a > > problem, in particular not with the FIFO-ness of ticket locks. > > > > For that the fence, additional atomic etc. would be IMHO much cleaner > > than the crazy overflow logic. > > All things being equal I'd prefer lock-xadd just because its easier to > analyze the concurrency for, crazy overflow tests or no. But if > add+mfence turned out to be a performance win, then that would obviously > tip the scales. > > However, it looks like locked xadd is also has better performance: on > my Sandybridge laptop (2 cores, 4 threads), the add+mfence is 20% slower > than locked xadd, so that pretty much settles it unless you think > there'd be a dramatic difference on an AMD system. Indeed, the fences are usually slower than locked RMWs, in particular, if you do not need to add an instruction. I originally missed that amazing stunt the GCC pulled off with replacing the branch with carry flag magic. It seems that two twisted minds have found each other here :) One of my concerns was adding a branch in here... so that is settled, and if everybody else feels like this is easier to reason about... go ahead :) (I'll keep my itch to myself then.) Stephan -- Stephan Diestelhorst, AMD Operating System Research Center stephan.diestelhorst@amd.com, Tel. +49 (0)351 448 356 719 Advanced Micro Devices GmbH Einsteinring 24 85609 Aschheim Germany Geschaeftsfuehrer: Alberto Bozzo; Sitz: Dornach, Gemeinde Aschheim, Landkreis Muenchen Registergericht Muenchen, HRB Nr. 43632, WEEE-Reg-Nr: DE 12919551