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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 00B57C43381 for ; Thu, 14 Feb 2019 18:02:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CC6AF2080A for ; Thu, 14 Feb 2019 18:02:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2406142AbfBNSCq (ORCPT ); Thu, 14 Feb 2019 13:02:46 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:48778 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729935AbfBNSCp (ORCPT ); Thu, 14 Feb 2019 13:02:45 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 65562EBD; Thu, 14 Feb 2019 10:02:45 -0800 (PST) Received: from fuggles.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D806D3F589; Thu, 14 Feb 2019 10:02:41 -0800 (PST) Date: Thu, 14 Feb 2019 18:02:39 +0000 From: Will Deacon To: Peter Zijlstra Cc: Waiman Long , Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org, x86@kernel.org, Arnd Bergmann , Borislav Petkov , "H. Peter Anvin" , Davidlohr Bueso , Linus Torvalds , Andrew Morton , Tim Chen Subject: Re: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock() Message-ID: <20190214180239.GI2475@fuggles.cambridge.arm.com> References: <1550089932-6888-1-git-send-email-longman@redhat.com> <1550089932-6888-3-git-send-email-longman@redhat.com> <20190214103333.GH32494@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190214103333.GH32494@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) () Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 14, 2019 at 11:33:33AM +0100, Peter Zijlstra wrote: > On Wed, Feb 13, 2019 at 03:32:12PM -0500, Waiman Long wrote: > > Modify __down_read_trylock() to optimize for an unlocked rwsem and make > > it generate slightly better code. > > > > Before this patch, down_read_trylock: > > > > 0x0000000000000000 <+0>: callq 0x5 > > 0x0000000000000005 <+5>: jmp 0x18 > > 0x0000000000000007 <+7>: lea 0x1(%rdx),%rcx > > 0x000000000000000b <+11>: mov %rdx,%rax > > 0x000000000000000e <+14>: lock cmpxchg %rcx,(%rdi) > > 0x0000000000000013 <+19>: cmp %rax,%rdx > > 0x0000000000000016 <+22>: je 0x23 > > 0x0000000000000018 <+24>: mov (%rdi),%rdx > > 0x000000000000001b <+27>: test %rdx,%rdx > > 0x000000000000001e <+30>: jns 0x7 > > 0x0000000000000020 <+32>: xor %eax,%eax > > 0x0000000000000022 <+34>: retq > > 0x0000000000000023 <+35>: mov %gs:0x0,%rax > > 0x000000000000002c <+44>: or $0x3,%rax > > 0x0000000000000030 <+48>: mov %rax,0x20(%rdi) > > 0x0000000000000034 <+52>: mov $0x1,%eax > > 0x0000000000000039 <+57>: retq > > > > After patch, down_read_trylock: > > > > 0x0000000000000000 <+0>: callq 0x5 > > 0x0000000000000005 <+5>: xor %eax,%eax > > 0x0000000000000007 <+7>: lea 0x1(%rax),%rdx > > 0x000000000000000b <+11>: lock cmpxchg %rdx,(%rdi) > > 0x0000000000000010 <+16>: jne 0x29 > > 0x0000000000000012 <+18>: mov %gs:0x0,%rax > > 0x000000000000001b <+27>: or $0x3,%rax > > 0x000000000000001f <+31>: mov %rax,0x20(%rdi) > > 0x0000000000000023 <+35>: mov $0x1,%eax > > 0x0000000000000028 <+40>: retq > > 0x0000000000000029 <+41>: test %rax,%rax > > 0x000000000000002c <+44>: jns 0x7 > > 0x000000000000002e <+46>: xor %eax,%eax > > 0x0000000000000030 <+48>: retq > > > > By using a rwsem microbenchmark, the down_read_trylock() rate (with a > > load of 10 to lengthen the lock critical section) on a x86-64 system > > before and after the patch were: > > > > Before Patch After Patch > > # of Threads rlock rlock > > ------------ ----- ----- > > 1 14,496 14,716 > > 2 8,644 8,453 > > 4 6,799 6,983 > > 8 5,664 7,190 > > > > On a ARM64 system, the performance results were: > > > > Before Patch After Patch > > # of Threads rlock rlock > > ------------ ----- ----- > > 1 23,676 24,488 > > 2 7,697 9,502 > > 4 4,945 3,440 > > 8 2,641 1,603 > > Urgh, yes LL/SC is the obvious exception that can actually do better > here :/ > > Will, what say you? What machine were these numbers generated on and is it using LL/SC or LSE atomics for arm64? If you stick the microbenchmark somewhere, I can go play with a broader variety of h/w. Will