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_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 EFE13ECDFB0 for ; Fri, 13 Jul 2018 18:32:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AA5A32084E for ; Fri, 13 Jul 2018 18:32:48 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AA5A32084E 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 S1731634AbeGMSsd (ORCPT ); Fri, 13 Jul 2018 14:48:33 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:49314 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730028AbeGMSsd (ORCPT ); Fri, 13 Jul 2018 14:48:33 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B6D6540583DB; Fri, 13 Jul 2018 18:32:45 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-175.bos.redhat.com [10.18.17.175]) by smtp.corp.redhat.com (Postfix) with ESMTP id 7C37623140; Fri, 13 Jul 2018 18:32:45 +0000 (UTC) Subject: Re: [PATCH] locking/rwsem: Take read lock immediate if empty queue with no writer To: Will Deacon Cc: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, Mark Ray , Joe Mario , Scott Norton References: <1531247490-26852-1-git-send-email-longman@redhat.com> <20180713100257.GE32020@arm.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Fri, 13 Jul 2018 14:32:45 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <20180713100257.GE32020@arm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Fri, 13 Jul 2018 18:32:45 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Fri, 13 Jul 2018 18:32:45 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'longman@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/13/2018 06:02 AM, Will Deacon wrote: > On Tue, Jul 10, 2018 at 02:31:30PM -0400, Waiman Long wrote: >> It was found that a constant stream of readers might cause the count to >> go negative most of the time after an initial trigger by a writer even >> if no writer was present afterward. As a result, most of the readers >> would have to go through the slowpath reducing their performance. >> >> To avoid that from happening, an additional check is added to detect >> the special case that the reader in the critical section is the only >> one in the wait queue and no writer is present. When that happens, it >> can just have the lock and return immediately without further action. >> Other incoming readers won't see a waiter is present and be forced >> into the slowpath. >> >> After the list_empty() calls, the CPU should have the lock cacheline >> anyway, so an additional semaphore count check shouldn't have any >> performance impact. >> >> Signed-off-by: Waiman Long >> --- >> kernel/locking/rwsem-xadd.c | 16 +++++++++++++++- >> 1 file changed, 15 insertions(+), 1 deletion(-) > This looks ok to me, but it would be nice to include some performance > figures in the commit log. Do you have any? Phrases such as "shouldn't have > any performance impact" and "probably generate better code" don't fill me > with good feelings ;) > > Will I just got the customer testing results back. I had posted a v2 patch with the customer data. I also remove wordings that may cause confusion. Cheers, Longman