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_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 38060C4321D for ; Mon, 20 Aug 2018 15:55:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E08A521581 for ; Mon, 20 Aug 2018 15:55:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E08A521581 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 S1726818AbeHTTMD (ORCPT ); Mon, 20 Aug 2018 15:12:03 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51452 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726033AbeHTTMD (ORCPT ); Mon, 20 Aug 2018 15:12:03 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0993A8780F; Mon, 20 Aug 2018 15:55:52 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-8.bos.redhat.com [10.18.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id AB0642166BA1; Mon, 20 Aug 2018 15:55:51 +0000 (UTC) Subject: Re: [PATCH] locking: Remove an insn from spin and write locks To: Matthew Wilcox Cc: Arnd Bergmann , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Will Deacon , Thomas Gleixner References: <20180820150652.29482-1-willy@infradead.org> <20180820155002.GB25153@bombadil.infradead.org> From: Waiman Long Organization: Red Hat Message-ID: Date: Mon, 20 Aug 2018 11:55:51 -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: <20180820155002.GB25153@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.1]); Mon, 20 Aug 2018 15:55:52 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.1]); Mon, 20 Aug 2018 15:55:52 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.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 08/20/2018 11:50 AM, Matthew Wilcox wrote: > On Mon, Aug 20, 2018 at 11:14:04AM -0400, Waiman Long wrote: >> On 08/20/2018 11:06 AM, Matthew Wilcox wrote: >>> Both spin locks and write locks currently do: >>> >>> f0 0f b1 17 lock cmpxchg %edx,(%rdi) >>> 85 c0 test %eax,%eax >>> 75 05 jne [slowpath] >>> >>> This 'test' insn is superfluous; the cmpxchg insn sets the Z flag >>> appropriately. Peter pointed out that using atomic_try_cmpxchg() >>> will let the compiler know this is true. Comparing before/after >>> disassemblies show the only effect is to remove this insn. > ... >>> static __always_inline int queued_spin_trylock(struct qspinlock *lock) >>> { >>> + u32 val = 0; >>> + >>> if (!atomic_read(&lock->val) && >>> - (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0)) >>> + (atomic_try_cmpxchg(&lock->val, &val, _Q_LOCKED_VAL))) >> Should you keep the _acquire suffix? > I don't know ;-) Probably. Peter didn't include it as part of his > suggested fix, but on reviewing the documentation, it seems likely that > it should be retained. I put them back in and (as expected) it changes > nothing on x86-64. We will certainly need to keep the _acquire suffix or it will likely regress performance for arm64. >> BTW, qspinlock and qrwlock are now also used by AArch64, mips and sparc. >> Have you tried to see what the effect will be for those architecture? > Nope! That's why I cc'd linux-arch, because I don't know who (other > than arm64 and x86) is using q-locks these days. I think both Sparc and mips are using qlocks now, though these architectures are not the ones that I am interested in. I do like to make sure that there will be no regression for arm64. Will should be able to answer that. Cheers, Longman