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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 0365BC282CE for ; Tue, 4 Jun 2019 17:28:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D3D95208E4 for ; Tue, 4 Jun 2019 17:28:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726352AbfFDR2q (ORCPT ); Tue, 4 Jun 2019 13:28:46 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38834 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725932AbfFDR2p (ORCPT ); Tue, 4 Jun 2019 13:28:45 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A1E2330860B5; Tue, 4 Jun 2019 17:28:37 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id 41CB15D9CC; Tue, 4 Jun 2019 17:28:36 +0000 (UTC) Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190520205918.22251-1-longman@redhat.com> <20190520205918.22251-16-longman@redhat.com> <20190604091000.GH3402@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: Date: Tue, 4 Jun 2019 13:28:35 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190604091000.GH3402@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Tue, 04 Jun 2019 17:28:45 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/4/19 5:10 AM, Peter Zijlstra wrote: > On Mon, May 20, 2019 at 04:59:14PM -0400, Waiman Long wrote: >> Reader optimistic spinning is helpful when the reader critical section >> is short and there aren't that many readers around. It makes readers >> relatively more preferred than writers. When a writer times out spinning >> on a reader-owned lock and set the nospinnable bits, there are two main >> reasons for that. >> >> 1) The reader critical section is long, perhaps the task sleeps after >> acquiring the read lock. >> 2) There are just too many readers contending the lock causing it to >> take a while to service all of them. >> >> In the former case, long reader critical section will impede the progress >> of writers which is usually more important for system performance. >> In the later case, reader optimistic spinning tends to make the reader >> groups that contain readers that acquire the lock together smaller >> leading to more of them. That may hurt performance in some cases. In >> other words, the setting of nonspinnable bits indicates that reader >> optimistic spinning may not be helpful for those workloads that cause it. >> >> Therefore, any writers that have observed the setting of the writer >> nonspinnable bit for a given rwsem after they fail to acquire the lock >> via optimistic spinning will set the reader nonspinnable bit once they >> acquire the write lock. Similarly, readers that observe the setting >> of reader nonspinnable bit at slowpath entry will also set the reader >> nonspinnable bit when they acquire the read lock via the wakeup path. > So both cases set the _reader_ nonspinnable bit? Yes. -Longman