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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_PASS 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 0C76AC10F04
	for <linuxppc-dev@archiver.kernel.org>; Thu, 14 Feb 2019 14:56:00 +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 4CD6F2229F
	for <linuxppc-dev@archiver.kernel.org>; Thu, 14 Feb 2019 14:55:59 +0000 (UTC)
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4CD6F2229F
Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com
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 440fbK142QzDqRR
	for <linuxppc-dev@archiver.kernel.org>; Fri, 15 Feb 2019 01:55:57 +1100 (AEDT)
Authentication-Results: lists.ozlabs.org;
 spf=pass (mailfrom) smtp.mailfrom=redhat.com
 (client-ip=209.132.183.28; helo=mx1.redhat.com;
 envelope-from=longman@redhat.com; receiver=<UNKNOWN>)
Authentication-Results: lists.ozlabs.org;
 dmarc=pass (p=none dis=none) header.from=redhat.com
Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28])
 (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 440fXH1GYYzDqPZ
 for <linuxppc-dev@lists.ozlabs.org>; Fri, 15 Feb 2019 01:53:18 +1100 (AEDT)
Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com
 [10.5.11.11])
 (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by mx1.redhat.com (Postfix) with ESMTPS id 73A98C049D67;
 Thu, 14 Feb 2019 14:53:15 +0000 (UTC)
Received: from llong.remote.csb (dhcp-17-59.bos.redhat.com [10.18.17.59])
 by smtp.corp.redhat.com (Postfix) with ESMTP id 41521600C4;
 Thu, 14 Feb 2019 14:53:13 +0000 (UTC)
Subject: Re: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock()
To: Peter Zijlstra <peterz@infradead.org>
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>
From: Waiman Long <longman@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=longman@redhat.com; prefer-encrypt=mutual; keydata=
 xsFNBFgsZGsBEAC3l/RVYISY3M0SznCZOv8aWc/bsAgif1H8h0WPDrHnwt1jfFTB26EzhRea
 XQKAJiZbjnTotxXq1JVaWxJcNJL7crruYeFdv7WUJqJzFgHnNM/upZuGsDIJHyqBHWK5X9ZO
 jRyfqV/i3Ll7VIZobcRLbTfEJgyLTAHn2Ipcpt8mRg2cck2sC9+RMi45Epweu7pKjfrF8JUY
 r71uif2ThpN8vGpn+FKbERFt4hW2dV/3awVckxxHXNrQYIB3I/G6mUdEZ9yrVrAfLw5M3fVU
 CRnC6fbroC6/ztD40lyTQWbCqGERVEwHFYYoxrcGa8AzMXN9CN7bleHmKZrGxDFWbg4877zX
 0YaLRypme4K0ULbnNVRQcSZ9UalTvAzjpyWnlnXCLnFjzhV7qsjozloLTkZjyHimSc3yllH7
 VvP/lGHnqUk7xDymgRHNNn0wWPuOpR97J/r7V1mSMZlni/FVTQTRu87aQRYu3nKhcNJ47TGY
 evz/U0ltaZEU41t7WGBnC7RlxYtdXziEn5fC8b1JfqiP0OJVQfdIMVIbEw1turVouTovUA39
 Qqa6Pd1oYTw+Bdm1tkx7di73qB3x4pJoC8ZRfEmPqSpmu42sijWSBUgYJwsziTW2SBi4hRjU
 h/Tm0NuU1/R1bgv/EzoXjgOM4ZlSu6Pv7ICpELdWSrvkXJIuIwARAQABzR9Mb25nbWFuIExv
 bmcgPGxsb25nQHJlZGhhdC5jb20+wsF/BBMBAgApBQJYLGRrAhsjBQkJZgGABwsJCAcDAgEG
 FQgCCQoLBBYCAwECHgECF4AACgkQbjBXZE7vHeYwBA//ZYxi4I/4KVrqc6oodVfwPnOVxvyY
 oKZGPXZXAa3swtPGmRFc8kGyIMZpVTqGJYGD9ZDezxpWIkVQDnKM9zw/qGarUVKzElGHcuFN
 ddtwX64yxDhA+3Og8MTy8+8ZucM4oNsbM9Dx171bFnHjWSka8o6qhK5siBAf9WXcPNogUk4S
 fMNYKxexcUayv750GK5E8RouG0DrjtIMYVJwu+p3X1bRHHDoieVfE1i380YydPd7mXa7FrRl
 7unTlrxUyJSiBc83HgKCdFC8+ggmRVisbs+1clMsK++ehz08dmGlbQD8Fv2VK5KR2+QXYLU0
 rRQjXk/gJ8wcMasuUcywnj8dqqO3kIS1EfshrfR/xCNSREcv2fwHvfJjprpoE9tiL1qP7Jrq
 4tUYazErOEQJcE8Qm3fioh40w8YrGGYEGNA4do/jaHXm1iB9rShXE2jnmy3ttdAh3M8W2OMK
 4B/Rlr+Awr2NlVdvEF7iL70kO+aZeOu20Lq6mx4Kvq/WyjZg8g+vYGCExZ7sd8xpncBSl7b3
 99AIyT55HaJjrs5F3Rl8dAklaDyzXviwcxs+gSYvRCr6AMzevmfWbAILN9i1ZkfbnqVdpaag
 QmWlmPuKzqKhJP+OMYSgYnpd/vu5FBbc+eXpuhydKqtUVOWjtp5hAERNnSpD87i1TilshFQm
 TFxHDzbOwU0EWCxkawEQALAcdzzKsZbcdSi1kgjfce9AMjyxkkZxcGc6Rhwvt78d66qIFK9D
 Y9wfcZBpuFY/AcKEqjTo4FZ5LCa7/dXNwOXOdB1Jfp54OFUqiYUJFymFKInHQYlmoES9EJEU
 yy+2ipzy5yGbLh3ZqAXyZCTmUKBU7oz/waN7ynEP0S0DqdWgJnpEiFjFN4/ovf9uveUnjzB6
 lzd0BDckLU4dL7aqe2ROIHyG3zaBMuPo66pN3njEr7IcyAL6aK/IyRrwLXoxLMQW7YQmFPSw
 drATP3WO0x8UGaXlGMVcaeUBMJlqTyN4Swr2BbqBcEGAMPjFCm6MjAPv68h5hEoB9zvIg+fq
 M1/Gs4D8H8kUjOEOYtmVQ5RZQschPJle95BzNwE3Y48ZH5zewgU7ByVJKSgJ9HDhwX8Ryuia
 79r86qZeFjXOUXZjjWdFDKl5vaiRbNWCpuSG1R1Tm8o/rd2NZ6l8LgcK9UcpWorrPknbE/pm
 MUeZ2d3ss5G5Vbb0bYVFRtYQiCCfHAQHO6uNtA9IztkuMpMRQDUiDoApHwYUY5Dqasu4ZDJk
 bZ8lC6qc2NXauOWMDw43z9He7k6LnYm/evcD+0+YebxNsorEiWDgIW8Q/E+h6RMS9kW3Rv1N
 qd2nFfiC8+p9I/KLcbV33tMhF1+dOgyiL4bcYeR351pnyXBPA66ldNWvABEBAAHCwWUEGAEC
 AA8FAlgsZGsCGwwFCQlmAYAACgkQbjBXZE7vHeYxSQ/+PnnPrOkKHDHQew8Pq9w2RAOO8gMg
 9Ty4L54CsTf21Mqc6GXj6LN3WbQta7CVA0bKeq0+WnmsZ9jkTNh8lJp0/RnZkSUsDT9Tza9r
 GB0svZnBJMFJgSMfmwa3cBttCh+vqDV3ZIVSG54nPmGfUQMFPlDHccjWIvTvyY3a9SLeamaR
 jOGye8MQAlAD40fTWK2no6L1b8abGtziTkNh68zfu3wjQkXk4kA4zHroE61PpS3oMD4AyI9L
 7A4Zv0Cvs2MhYQ4Qbbmafr+NOhzuunm5CoaRi+762+c508TqgRqH8W1htZCzab0pXHRfywtv
 0P+BMT7vN2uMBdhr8c0b/hoGqBTenOmFt71tAyyGcPgI3f7DUxy+cv3GzenWjrvf3uFpxYx4
 yFQkUcu06wa61nCdxXU/BWFItryAGGdh2fFXnIYP8NZfdA+zmpymJXDQeMsAEHS0BLTVQ3+M
 7W5Ak8p9V+bFMtteBgoM23bskH6mgOAw6Cj/USW4cAJ8b++9zE0/4Bv4iaY5bcsL+h7TqQBH
 Lk1eByJeVooUa/mqa2UdVJalc8B9NrAnLiyRsg72Nurwzvknv7anSgIkL+doXDaG21DgCYTD
 wGA5uquIgb8p3/ENgYpDPrsZ72CxVC2NEJjJwwnRBStjJOGQX4lV1uhN1XsZjBbRHdKF2W9g
 weim8xU=
Organization: Red Hat
Message-ID: <9e01d4ef-56df-7af8-a0f5-b49644e298bf@redhat.com>
Date: Thu, 14 Feb 2019 09:53:12 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <20190214103333.GH32494@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Content-Language: en-US
X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16
 (mx1.redhat.com [10.5.110.31]); Thu, 14 Feb 2019 14:53:16 +0000 (UTC)
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 02/14/2019 05:33 AM, 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 <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?

The ARM64 result is what I would have expected given that the change was
to optimize for the uncontended case. The x86-64 result is kind of an
anomaly to me, but I haven't bothered to dig into that.

Cheers,
Longman