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 176F7C388F9 for ; Sat, 21 Nov 2020 11:50:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C3B1222226 for ; Sat, 21 Nov 2020 11:50:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727593AbgKULu3 convert rfc822-to-8bit (ORCPT ); Sat, 21 Nov 2020 06:50:29 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:39614 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727337AbgKULu3 (ORCPT ); Sat, 21 Nov 2020 06:50:29 -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-45-zOLMDCtGMdWjyngmXZIzpw-1; Sat, 21 Nov 2020 11:50:23 +0000 X-MC-Unique: zOLMDCtGMdWjyngmXZIzpw-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; Sat, 21 Nov 2020 11:50:23 +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; Sat, 21 Nov 2020 11:50:22 +0000 From: David Laight To: 'Davidlohr Bueso' CC: 'Waiman Long' , Peter Zijlstra , Ingo Molnar , Will Deacon , "linux-kernel@vger.kernel.org" , Phil Auld Subject: RE: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning Thread-Topic: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning Thread-Index: AQHWvqN0O6hwIV/IOUmlkB4qEdd62KnQ/N3QgACQ0wCAAOypEA== Date: Sat, 21 Nov 2020 11:50:22 +0000 Message-ID: <45f36a7545c84b62b49aaff95f28a49a@AcuMS.aculab.com> References: <20201118030429.23017-1-longman@redhat.com> <20201118030429.23017-6-longman@redhat.com> <20201118053556.3fmmtat7upv6dtvd@linux-p48b.lan> <5fe76531782f4a8492b341d5f381147b@AcuMS.aculab.com> <20201120213828.n7f23qy75hduommo@linux-p48b.lan> In-Reply-To: <20201120213828.n7f23qy75hduommo@linux-p48b.lan> 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: Davidlohr Bueso > Sent: 20 November 2020 21:38 > > On Fri, 20 Nov 2020, David Laight wrote: > >I got massive performance improvements from changing a driver > >we have to use mutex instead of the old semaphores (the driver > >was written a long time ago). > > > >While these weren't 'rw' the same issue will apply. > > > >The problem was that the semaphore/mutex was typically only held over > >a few instructions (eg to add an item to a list). > >But with semaphore if you got contention the process always slept. > >OTOH mutex spin 'for a while' before sleeping so the code rarely slept. > > The caveat here is if you are using trylock/unlock from irq, which > is the only reason why regular semaphores are still around today. If > not, indeed a mutex is better. Unless you want to timeout the lock request. Timeouts are particularly useful in code paths that might run after an 'oops' or other deadlock. Typically for reporting status information. You get to choose whether to error the status request or carry on knowing that the data is unlikely to change. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)