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=-3.8 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 5F753C433E0 for ; Fri, 8 Jan 2021 09:51:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 14D4B236F9 for ; Fri, 8 Jan 2021 09:51:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728214AbhAHJvB convert rfc822-to-8bit (ORCPT ); Fri, 8 Jan 2021 04:51:01 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:51317 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725791AbhAHJvA (ORCPT ); Fri, 8 Jan 2021 04:51:00 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-219-OQYpK6Q3PCSSKUP3lf-peg-1; Fri, 08 Jan 2021 09:49:21 +0000 X-MC-Unique: OQYpK6Q3PCSSKUP3lf-peg-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; Fri, 8 Jan 2021 09:49:20 +0000 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; Fri, 8 Jan 2021 09:49:20 +0000 From: David Laight To: 'Al Viro' , Linus Torvalds CC: kernel test robot , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Peter Zijlstra , LKML , "lkp@lists.01.org" , kernel test robot , "Huang, Ying" , Feng Tang , "zhengjun.xing@intel.com" Subject: RE: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression Thread-Topic: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression Thread-Index: AQHW5SP2JRv8nkj1EkuGw6TTCMykjqodevww Date: Fri, 8 Jan 2021 09:49:20 +0000 Message-ID: References: <20210107134723.GA28532@xsang-OptiPlex-9020> <20210107183358.GG3579531@ZenIV.linux.org.uk> In-Reply-To: <20210107183358.GG3579531@ZenIV.linux.org.uk> Accept-Language: en-GB, 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 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro > Sent: 07 January 2021 18:34 > > On Thu, Jan 07, 2021 at 09:43:54AM -0800, Linus Torvalds wrote: > > > Before, it would do the whole CLAC/STAC dance inside that loop for > > every entry (and with that commit d55564cfc22 it would be a function > > call, of course). > > > > Can you verify that this fixes the regression (and in fact I'd expect > > it to improve that test-case)? > > I'm not sure it's the best approach, TBH. How about simply > for (walk = head; walk; ufds += walk->len, walk = walk->next) { > if (copy_to_user(ufds, walk->entries, > walk->len * sizeof(struct pollfd)) > goto out_fds; > } > in there? It's both simpler (obviously matches the copyin side) and > might very well be faster... I bet it isn't, especially if 'user copy hardening' is enabled. You also have to 'negotiate' all the conditionals in copy_to_user() for misaligned lengths, very short copies and alternate algorithms for long copies. I'll bet there is a measurable gain from having copy_to/from_user_u64() that is optimised for copying a structure to/from userspace in 64bit chunks. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)