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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 E8DB3C04EB9 for ; Mon, 3 Dec 2018 16:11:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B7DB120834 for ; Mon, 3 Dec 2018 16:11:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B7DB120834 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 S1726687AbeLCQLJ (ORCPT ); Mon, 3 Dec 2018 11:11:09 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57172 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726560AbeLCQLJ (ORCPT ); Mon, 3 Dec 2018 11:11:09 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 517A1A7ED; Mon, 3 Dec 2018 16:11:01 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-55.bos.redhat.com [10.18.17.55]) by smtp.corp.redhat.com (Postfix) with ESMTP id 49AB01001944; Mon, 3 Dec 2018 16:10:58 +0000 (UTC) Subject: Re: [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg To: Davidlohr Bueso , 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, andrea.parri@amarulasolutions.com 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> <20181203053130.gwkw6kg72azt2npb@linux-r8p5> From: Waiman Long Openpgp: preference=signencrypt Autocrypt: addr=longman@redhat.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181203053130.gwkw6kg72azt2npb@linux-r8p5> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Mon, 03 Dec 2018 16:11:02 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/03/2018 12:31 AM, Davidlohr Bueso wrote: > 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); Acked-by: Waiman Long