From patchwork Wed Feb 13 20:32:12 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Waiman Long X-Patchwork-Id: 10810859 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id E4A8714E1 for ; Wed, 13 Feb 2019 20:32:45 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D1B802E4E9 for ; Wed, 13 Feb 2019 20:32:45 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id CF4FB2E1C2; Wed, 13 Feb 2019 20:32:45 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 622D52E506 for ; Wed, 13 Feb 2019 20:32:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2436662AbfBMUci (ORCPT ); Wed, 13 Feb 2019 15:32:38 -0500 Received: from mx1.redhat.com ([209.132.183.28]:52544 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728538AbfBMUch (ORCPT ); Wed, 13 Feb 2019 15:32:37 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A878A85376; Wed, 13 Feb 2019 20:32:35 +0000 (UTC) Received: from llong.com (ovpn-125-158.rdu2.redhat.com [10.10.125.158]) by smtp.corp.redhat.com (Postfix) with ESMTP id EE9581000239; Wed, 13 Feb 2019 20:32:30 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Will Deacon , Thomas Gleixner Cc: 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 , Waiman Long Subject: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock() Date: Wed, 13 Feb 2019 15:32:12 -0500 Message-Id: <1550089932-6888-3-git-send-email-longman@redhat.com> In-Reply-To: <1550089932-6888-1-git-send-email-longman@redhat.com> References: <1550089932-6888-1-git-send-email-longman@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Wed, 13 Feb 2019 20:32:36 +0000 (UTC) Sender: linux-sh-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sh@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP 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 For the uncontended case (1 thread), the new down_read_trylock() is a little bit faster. For the contended cases, the new down_read_trylock() perform pretty well in x86-64, but performance degrades at high contention level on ARM64. Suggested-by: Linus Torvalds Signed-off-by: Waiman Long --- kernel/locking/rwsem.h | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h index 067e265..e0bcc11 100644 --- a/kernel/locking/rwsem.h +++ b/kernel/locking/rwsem.h @@ -175,14 +175,17 @@ static inline int __down_read_killable(struct rw_semaphore *sem) static inline int __down_read_trylock(struct rw_semaphore *sem) { - long tmp; + /* + * Optimize for the case when the rwsem is not locked at all. + */ + long tmp = RWSEM_UNLOCKED_VALUE; - while ((tmp = atomic_long_read(&sem->count)) >= 0) { - if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp, - tmp + RWSEM_ACTIVE_READ_BIAS)) { + do { + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, + tmp + RWSEM_ACTIVE_READ_BIAS)) { return 1; } - } + } while (tmp >= 0); return 0; }