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=-6.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,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 6ED0AC04EB9 for ; Thu, 29 Nov 2018 21:30:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3C895214DB for ; Thu, 29 Nov 2018 21:30:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3C895214DB 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 S1727196AbeK3IhP (ORCPT ); Fri, 30 Nov 2018 03:37:15 -0500 Received: from mx2.suse.de ([195.135.220.15]:52086 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726555AbeK3IhO (ORCPT ); Fri, 30 Nov 2018 03:37:14 -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 182D0B0D6; Thu, 29 Nov 2018 21:30:25 +0000 (UTC) Date: Thu, 29 Nov 2018 13:30:17 -0800 From: Davidlohr Bueso Cc: Peter Zijlstra , 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 Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil Message-ID: <20181129213017.v3eljor54lfpoug2@linux-r8p5> References: <1543495830-2644-1-git-send-email-xieyongji@baidu.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <729ceddb-dd9a-ec2a-f74e-03fa4d7e65e8@redhat.com> 1;5002;0cTo: Waiman Long User-Agent: NeoMutt/20180323 To: unlisted-recipients:; (no To-header on input) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 29 Nov 2018, Waiman Long wrote: >That can be costly for x86 which will now have 2 locked instructions. Yeah, and when used as an actual queue we should really start to notice. Some users just have a single task in the wake_q because avoiding the cost of wake_up_process() with locks held is significant. How about instead of adding the barrier before the cmpxchg, we do it in the failed branch, right before we return. This is the uncommon path. Thanks, Davidlohr diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 091e089063be..0d844a18a9dc 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -408,8 +408,14 @@ 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 (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) { + /* + * Ensure, that when the cmpxchg() fails, the corresponding + * wake_up_q() will observe our prior state. + */ + smp_mb__after_atomic(); return; + } get_task_struct(task);