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=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_NEOMUTT autolearn=ham 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 74F86C04EB9 for ; Mon, 3 Dec 2018 05:31:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F2ED320881 for ; Mon, 3 Dec 2018 05:31:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F2ED320881 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=stgolabs.net Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725840AbeLCFbo (ORCPT ); Mon, 3 Dec 2018 00:31:44 -0500 Received: from mx2.suse.de ([195.135.220.15]:34176 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725812AbeLCFbn (ORCPT ); Mon, 3 Dec 2018 00:31:43 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 4F26CACB1; Mon, 3 Dec 2018 05:31:39 +0000 (UTC) Date: Sun, 2 Dec 2018 21:31:30 -0800 From: Davidlohr Bueso To: Peter Zijlstra Cc: Yongji Xie , mingo@redhat.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, xieyongji@baidu.com, zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com, nixun@baidu.com, lilin24@baidu.com, longman@redhat.com, andrea.parri@amarulasolutions.com Subject: [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg Message-ID: <20181203053130.gwkw6kg72azt2npb@linux-r8p5> References: <20181129131232.GN2131@hirez.programming.kicks-ass.net> <5598cd71-c3c8-d6ef-eb30-777cf901a2ef@redhat.com> <20181129160627.GU2131@hirez.programming.kicks-ass.net> <8cc45695-b325-a219-8b46-d5da6ddfdd63@redhat.com> <20181129172700.GA11632@hirez.programming.kicks-ass.net> <20181129180828.GA11650@hirez.programming.kicks-ass.net> <729ceddb-dd9a-ec2a-f74e-03fa4d7e65e8@redhat.com> <20181129213017.v3eljor54lfpoug2@linux-r8p5> <20181129213421.wwvhsjql3m3lvtv4@linux-r8p5> <20181129221714.GF11632@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20181129221714.GF11632@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20180323 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The cmpxchg will fail when the task is already in the process of waking up, and as such is an extremely rare occurrence. Micro-optimize the call and put an unlikely() around it. To no surprise, when using CONFIG_PROFILE_ANNOTATED_BRANCHES under a number of workloads the incorrect rate was a mere 1-2%. Signed-off-by: Davidlohr Bueso --- kernel/sched/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 091e089063be..f7747cf6e427 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -408,7 +408,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task) * This cmpxchg() executes a full barrier, which pairs with the full * barrier executed by the wakeup in wake_up_q(). */ - if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) + if (unlikely(cmpxchg(&node->next, NULL, WAKE_Q_TAIL))) return; get_task_struct(task); -- 2.16.4