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 43D24C433E7 for ; Tue, 1 Sep 2020 09:29:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2E9C520866 for ; Tue, 1 Sep 2020 09:29:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726212AbgIAJ3T convert rfc822-to-8bit (ORCPT ); Tue, 1 Sep 2020 05:29:19 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:43474 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725989AbgIAJ3S (ORCPT ); Tue, 1 Sep 2020 05:29:18 -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-67-waiQy7scMTWHtZ1SLW3FRA-1; Tue, 01 Sep 2020 10:29:14 +0100 X-MC-Unique: waiQy7scMTWHtZ1SLW3FRA-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; Tue, 1 Sep 2020 10:29:13 +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; Tue, 1 Sep 2020 10:29:13 +0100 From: David Laight To: 'Al Viro' , Shaokun Zhang CC: "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Yuqi Jin , kernel test robot , Will Deacon , Mark Rutland , Peter Zijlstra , Boqun Feng Subject: RE: [NAK] Re: [PATCH] fs: Optimized fget to improve performance Thread-Topic: [NAK] Re: [PATCH] fs: Optimized fget to improve performance Thread-Index: AQHWf0XaPFsGRwWaJkSlxP9k8oAd3qlThXHg Date: Tue, 1 Sep 2020 09:29:13 +0000 Message-ID: References: <1598523584-25601-1-git-send-email-zhangshaokun@hisilicon.com> <20200827142848.GZ1236603@ZenIV.linux.org.uk> <20200831032127.GW1236603@ZenIV.linux.org.uk> In-Reply-To: <20200831032127.GW1236603@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: 0.002 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro > Sent: 31 August 2020 04:21 > > On Mon, Aug 31, 2020 at 09:43:31AM +0800, Shaokun Zhang wrote: > > > How about this? We try to replace atomic_cmpxchg with atomic_add to improve > > performance. The atomic_add does not check the current f_count value. > > Therefore, the number of online CPUs is reserved to prevent multi-core > > competition. > > No. Really, really - no. Not unless you can guarantee that process on another > CPU won't lose its timeslice, ending up with more than one increment happening on > the same CPU - done by different processes scheduled there, one after another. > > If you have some change of atomic_long_add_unless(), do it there. And get it > past the arm64 folks. get_file_rcu() is nothing special in that respect *AND* > it has to cope with any architecture out there. > > BTW, keep in mind that there's such thing as a KVM - race windows are much > wider there, since a thread representing a guest CPU might lose its timeslice > whenever the host feels like that. At which point you get a single instruction > on a guest CPU taking longer than many thousands of instructions on another > CPU of the same guest. The same thing can happen if a hardware interrupt occurs. Not only the delays for the interrupt itself, but all the softint processing that happens as well. That can take a long time - even milliseconds. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)