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=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 9A3CAC433E0 for ; Thu, 23 Jul 2020 08:29:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7FFF920768 for ; Thu, 23 Jul 2020 08:29:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727781AbgGWI3H convert rfc822-to-8bit (ORCPT ); Thu, 23 Jul 2020 04:29:07 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:24673 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726177AbgGWI3H (ORCPT ); Thu, 23 Jul 2020 04:29:07 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-273-yLbMdh2jNnOD6IwmQddN9g-1; Thu, 23 Jul 2020 09:29:03 +0100 X-MC-Unique: yLbMdh2jNnOD6IwmQddN9g-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Thu, 23 Jul 2020 09:29:02 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Thu, 23 Jul 2020 09:29:02 +0100 From: David Laight To: 'Al Viro' CC: Linus Torvalds , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" Subject: RE: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum Thread-Topic: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum Thread-Index: AQHWX51MlcPCEWebQUuN/OB/armWnKkTU0FggABJU4CAABlpkP//+uQAgAAU8xCAAAgogIABBOPg Date: Thu, 23 Jul 2020 08:29:02 +0000 Message-ID: References: <20200721202425.GA2786714@ZenIV.linux.org.uk> <20200721202549.4150745-1-viro@ZenIV.linux.org.uk> <20200721202549.4150745-4-viro@ZenIV.linux.org.uk> <2d85ebb8ea2248c8a14f038a0c60297e@AcuMS.aculab.com> <20200722144213.GE2786714@ZenIV.linux.org.uk> <4e03cce8ed184d40bb0ea40fd3d51000@AcuMS.aculab.com> <20200722155452.GF2786714@ZenIV.linux.org.uk> <20200722173903.GG2786714@ZenIV.linux.org.uk> In-Reply-To: <20200722173903.GG2786714@ZenIV.linux.org.uk> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro > Sent: 22 July 2020 18:39 > On Wed, Jul 22, 2020 at 04:17:02PM +0000, David Laight wrote: > > > David, do you *ever* bother to RTFS? I mean, competent supercilious twits > > > are annoying, but at least with those you can generally assume that what > > > they say makes sense and has some relation to reality. You, OTOH, keep > > > spewing utter bollocks, without ever lowering yourself to checking if your > > > guesses have anything to do with the reality. With supercilious twit part > > > proudly on the display - you do speak with confidence, and the way you > > > dispense the oh-so-valuable advice to everyone around... > > > > Yes, I do look at the code. > > I've actually spent a lot of time looking at the x86 checksum code. > > I've posted a patch for a version that is about twice as fast as the > > current one on a large range of x86 cpus. > > > > Possibly I meant the 32bit reduction inside csum_add() > > rather than what csum_fold() does. > > Really? > static inline unsigned add32_with_carry(unsigned a, unsigned b) > { > asm("addl %2,%0\n\t" > "adcl $0,%0" > : "=r" (a) > : "0" (a), "rm" (b)); > return a; > } I agree it isn't much, but both those instructions almost certainly get replicated with the initial value fed into the checksum function. Everything except x86, sparc/64 and powerpc/64 uses the C code from include/net/checksum.h which is the longer sequences: csum += addend; csum += csum < addend; That's three instructions on something like MIPS - not too bad. I'm not sure about ARM - ARM could probably use adc. Some architectures may end up with an actual conditional jump. Quite how the instructions get scheduled probably makes more difference. The sequence is a register dependency chain, and the checksum register could easily be limiting the execution speed. On x86 the 'adc' loop runs at two clocks per adc on a wide range of Intel cpus. Actually there is lot more to be gained in the code that reads the iovec[] from userspace. The calling sequences for the two nexted functions used are horrid. Fixing that does make a measurable difference to semdmsg(). David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)