From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 75F48C54FCB for ; Sun, 26 Apr 2020 16:32:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5ACBB20700 for ; Sun, 26 Apr 2020 16:32:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726223AbgDZQcI (ORCPT ); Sun, 26 Apr 2020 12:32:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55854 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1726144AbgDZQcI (ORCPT ); Sun, 26 Apr 2020 12:32:08 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EC8B2C061A0F for ; Sun, 26 Apr 2020 09:32:07 -0700 (PDT) Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jSkCC-0005h7-K4; Sun, 26 Apr 2020 18:31:56 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 7405C100605; Sun, 26 Apr 2020 18:31:55 +0200 (CEST) From: Thomas Gleixner To: Alex Shi Cc: Davidlohr Bueso , Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org, Peter Zijlstra Subject: Re: [PATCH 2/2] locking/rtmutex: optimize rt_mutex_cmpxchgs In-Reply-To: <1587135032-188866-2-git-send-email-alex.shi@linux.alibaba.com> References: <1587135032-188866-1-git-send-email-alex.shi@linux.alibaba.com> <1587135032-188866-2-git-send-email-alex.shi@linux.alibaba.com> Date: Sun, 26 Apr 2020 18:31:55 +0200 Message-ID: <87k122ryx0.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alex Shi writes: > Checking l->owner first to skip time cost cmpxchgs. I don't see what that buys. It actually adds an extra conditional in the non-contended case, which is the case we are optimizing for. In the contended case, i.e. when l->owner != c the cmpxchg cost is completely irrelevant compared to the slowpath costs. > #ifndef CONFIG_DEBUG_RT_MUTEXES > -# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c) > -# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c) > +# define rt_mutex_cmpxchg_acquire(l, c, n) \ > + (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c) > +# define rt_mutex_cmpxchg_release(l, c, n) \ > + (l->owner == c && cmpxchg_release(&l->owner, c, n) == c) This kind of micro-optimizing is more than dubious especially w/o numbers backing up the benefit. Thanks, tglx