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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 452F4C43441 for ; Wed, 21 Nov 2018 13:44:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0C60D21479 for ; Wed, 21 Nov 2018 13:44:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0C60D21479 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 S1729682AbeKVASy (ORCPT ); Wed, 21 Nov 2018 19:18:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:49998 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728243AbeKVASx (ORCPT ); Wed, 21 Nov 2018 19:18:53 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9C52030024F5; Wed, 21 Nov 2018 13:44:25 +0000 (UTC) Received: from [10.43.17.120] (unknown [10.43.17.120]) by smtp.corp.redhat.com (Postfix) with ESMTP id 93B62662D2; Wed, 21 Nov 2018 13:44:22 +0000 (UTC) Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes To: Jens Axboe , Ingo Molnar Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , the arch/x86 maintainers , Linus Torvalds , Andrew Morton , Andy Lutomirski , Peter Zijlstra , Brian Gerst , linux-kernel@vger.kernel.org, pabeni@redhat.com References: <02bfc577-32a5-66be-64bf-d476b7d447d2@kernel.dk> <20181121063609.GA109082@gmail.com> <48e27a3a-2bb2-ff41-3512-8aeb3fd59e57@kernel.dk> From: Denys Vlasenko Message-ID: <26eff539-7de7-784c-0c88-f1d30753299d@redhat.com> Date: Wed, 21 Nov 2018 14:44:21 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <48e27a3a-2bb2-ff41-3512-8aeb3fd59e57@kernel.dk> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Wed, 21 Nov 2018 13:44:25 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/21/2018 02:32 PM, Jens Axboe wrote: > On 11/20/18 11:36 PM, Ingo Molnar wrote: >> * Jens Axboe wrote: >>> So this is a fun one... While I was doing the aio polled work, I noticed >>> that the submitting process spent a substantial amount of time copying >>> data to/from userspace. For aio, that's iocb and io_event, which are 64 >>> and 32 bytes respectively. Looking closer at this, and it seems that >>> ERMS rep movsb is SLOWER for smaller copies, due to a higher startup >>> cost. >>> >>> I came up with this hack to test it out, and low and behold, we now cut >>> the time spent in copying in half. 50% less. >>> >>> Since these kinds of patches tend to lend themselves to bike shedding, I >>> also ran a string of kernel compilations out of RAM. Results are as >>> follows: >>> >>> Patched : 62.86s avg, stddev 0.65s >>> Stock : 63.73s avg, stddev 0.67s >>> >>> which would also seem to indicate that we're faster punting smaller >>> (< 128 byte) copies. >>> >>> CPU: Intel(R) Xeon(R) CPU E5-2650 v4 @ 2.20GHz >>> >>> Interestingly, text size is smaller with the patch as well?! >>> >>> I'm sure there are smarter ways to do this, but results look fairly >>> conclusive. FWIW, the behaviorial change was introduced by: >>> >>> commit 954e482bde20b0e208fd4d34ef26e10afd194600 >>> Author: Fenghua Yu >>> Date: Thu May 24 18:19:45 2012 -0700 >>> >>> x86/copy_user_generic: Optimize copy_user_generic with CPU erms feature >>> >>> which contains nothing in terms of benchmarking or results, just claims >>> that the new hotness is better. >>> >>> Signed-off-by: Jens Axboe >>> --- >>> >>> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h >>> index a9d637bc301d..7dbb78827e64 100644 >>> --- a/arch/x86/include/asm/uaccess_64.h >>> +++ b/arch/x86/include/asm/uaccess_64.h >>> @@ -29,16 +29,27 @@ copy_user_generic(void *to, const void *from, unsigned len) >>> { >>> unsigned ret; >>> >>> + /* >>> + * For smaller copies, don't use ERMS as it's slower. >>> + */ >>> + if (len < 128) { >>> + alternative_call(copy_user_generic_unrolled, >>> + copy_user_generic_string, X86_FEATURE_REP_GOOD, >>> + ASM_OUTPUT2("=a" (ret), "=D" (to), "=S" (from), >>> + "=d" (len)), >>> + "1" (to), "2" (from), "3" (len) >>> + : "memory", "rcx", "r8", "r9", "r10", "r11"); >>> + return ret; >>> + } >>> + >>> /* >>> * If CPU has ERMS feature, use copy_user_enhanced_fast_string. >>> * Otherwise, if CPU has rep_good feature, use copy_user_generic_string. >>> * Otherwise, use copy_user_generic_unrolled. >>> */ >>> alternative_call_2(copy_user_generic_unrolled, >>> - copy_user_generic_string, >>> - X86_FEATURE_REP_GOOD, >>> - copy_user_enhanced_fast_string, >>> - X86_FEATURE_ERMS, >>> + copy_user_generic_string, X86_FEATURE_REP_GOOD, >>> + copy_user_enhanced_fast_string, X86_FEATURE_ERMS, >>> ASM_OUTPUT2("=a" (ret), "=D" (to), "=S" (from), >>> "=d" (len)), >>> "1" (to), "2" (from), "3" (len) >> >> So I'm inclined to do something like yours, because clearly the changelog >> of 954e482bde20 was at least partly false: Intel can say whatever they >> want, it's a fact that ERMS has high setup costs for low buffer sizes - >> ERMS is optimized for large size, cache-aligned copies mainly. > > I'm actually surprised that something like that was accepted, I guess > 2012 was a simpler time :-) > >> But the result is counter-intuitive in terms of kernel text footprint, >> plus the '128' is pretty arbitrary - we should at least try to come up >> with a break-even point where manual copy is about as fast as ERMS - on >> at least a single CPU ... > > I did some more investigation yesterday, and found this: > > commit 236222d39347e0e486010f10c1493e83dbbdfba8 > Author: Paolo Abeni > Date: Thu Jun 29 15:55:58 2017 +0200 > > x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings > > which does attempt to rectify it, but only using ERMS for >= 64 byte copies. > At least for me, looks like the break even point is higher than that, which > would mean that something like the below would be more appropriate. I also tested this while working for string ops code in musl. I think at least 128 bytes would be the minimum where "REP insn" are more efficient. In my testing, it's more like 256 bytes...