From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=RHZ4=QV=lists.ozlabs.org=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@kernel.org>
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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED,
	HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT
	autolearn=unavailable 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 44F8AC43381
	for <linuxppc-dev@archiver.kernel.org>; Thu, 14 Feb 2019 10:35:34 +0000 (UTC)
Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2])
	(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
	(No client certificate requested)
	by mail.kernel.org (Postfix) with ESMTPS id 6A6392229F
	for <linuxppc-dev@archiver.kernel.org>; Thu, 14 Feb 2019 10:35:33 +0000 (UTC)
Authentication-Results: mail.kernel.org;
	dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="bSpx1tff"
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6A6392229F
Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org
Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3])
	by lists.ozlabs.org (Postfix) with ESMTP id 440Xpp3LGkzDqVd
	for <linuxppc-dev@archiver.kernel.org>; Thu, 14 Feb 2019 21:35:30 +1100 (AEDT)
Authentication-Results: lists.ozlabs.org;
 spf=none (mailfrom) smtp.mailfrom=infradead.org
 (client-ip=2607:7c80:54:e::133; helo=bombadil.infradead.org;
 envelope-from=peterz@infradead.org; receiver=<UNKNOWN>)
Authentication-Results: lists.ozlabs.org;
 dmarc=none (p=none dis=none) header.from=infradead.org
Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key;
 unprotected) header.d=infradead.org header.i=@infradead.org
 header.b="bSpx1tff"; dkim-atps=neutral
Received: from bombadil.infradead.org (bombadil.infradead.org
 [IPv6:2607:7c80:54:e::133])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (No client certificate requested)
 by lists.ozlabs.org (Postfix) with ESMTPS id 440Xmn0zr7zDqVM
 for <linuxppc-dev@lists.ozlabs.org>; Thu, 14 Feb 2019 21:33:44 +1100 (AEDT)
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed;
 d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version
 :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To:
 Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date:
 Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id:
 List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive;
 bh=vDu94xn1oAM+sMbizgi+8P4bASl0cSHLy/8cGHt1spU=; b=bSpx1tffFFjQ9XwCly5V2uy4l
 OurixB2Qe39xoEcivRbLI3UxKpTc+00+w5+01aU9ifKS9RKoyRoQIPHDpROL4xqW7+P4/kHwWGfQ+
 hmLlI3Luim8bpDRlNPI7solsQ21Q6LGuqzjbWj3Ybb22aGzkBKrGiQGijzMYNe8pT8Dt0afUuwryr
 mamxbtcd7/ujoShuZ6Uvb5LBEBdwbWifNfrUoGg8pa5W9BWCtUHvqz1uLRsy279KcvEuW8CZDl2YW
 HlJ7+fJ/drKoaDw6HFZawRWWy4hT9BYYC8vGxn2pXM+Qb4wPUsJ1PsoGHHuqpIGASa2ogGT+juR0o
 35gHo1c+w==;
Received: from j217100.upc-j.chello.nl ([24.132.217.100]
 helo=hirez.programming.kicks-ass.net)
 by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux))
 id 1guEKk-0004r6-UD; Thu, 14 Feb 2019 10:33:35 +0000
Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000)
 id 6F3DE20298375; Thu, 14 Feb 2019 11:33:33 +0100 (CET)
Date: Thu, 14 Feb 2019 11:33:33 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Subject: Re: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock()
Message-ID: <20190214103333.GH32494@hirez.programming.kicks-ass.net>
References: <1550089932-6888-1-git-send-email-longman@redhat.com>
 <1550089932-6888-3-git-send-email-longman@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1550089932-6888-3-git-send-email-longman@redhat.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
X-BeenThere: linuxppc-dev@lists.ozlabs.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Linux on PowerPC Developers Mail List <linuxppc-dev.lists.ozlabs.org>
List-Unsubscribe: <https://lists.ozlabs.org/options/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=unsubscribe>
List-Archive: <http://lists.ozlabs.org/pipermail/linuxppc-dev/>
List-Post: <mailto:linuxppc-dev@lists.ozlabs.org>
List-Help: <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=help>
List-Subscribe: <https://lists.ozlabs.org/listinfo/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=subscribe>
Cc: linux-arch@vger.kernel.org, linux-xtensa@linux-xtensa.org,
 Davidlohr Bueso <dave@stgolabs.net>, linux-ia64@vger.kernel.org,
 Tim Chen <tim.c.chen@linux.intel.com>, Arnd Bergmann <arnd@arndb.de>,
 linux-sh@vger.kernel.org, linux-hexagon@vger.kernel.org, x86@kernel.org,
 Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
 Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
 linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org,
 Thomas Gleixner <tglx@linutronix.de>, linuxppc-dev@lists.ozlabs.org,
 Andrew Morton <akpm@linux-foundation.org>,
 linux-arm-kernel@lists.infradead.org
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

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 <down_read_trylock+5>
>    0x0000000000000005 <+5>:     jmp    0x18 <down_read_trylock+24>
>    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 <down_read_trylock+35>
>    0x0000000000000018 <+24>:    mov    (%rdi),%rdx
>    0x000000000000001b <+27>:    test   %rdx,%rdx
>    0x000000000000001e <+30>:    jns    0x7 <down_read_trylock+7>
>    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 <down_read_trylock+5>
>    0x0000000000000005 <+5>:	xor    %eax,%eax
>    0x0000000000000007 <+7>:	lea    0x1(%rax),%rdx
>    0x000000000000000b <+11>:	lock cmpxchg %rdx,(%rdi)
>    0x0000000000000010 <+16>:	jne    0x29 <down_read_trylock+41>
>    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 <down_read_trylock+7>
>    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?