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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D2BCBC77B7C for ; Thu, 11 May 2023 10:49:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237718AbjEKKtG convert rfc822-to-8bit (ORCPT ); Thu, 11 May 2023 06:49:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39982 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237891AbjEKKtC (ORCPT ); Thu, 11 May 2023 06:49:02 -0400 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 21FC47692 for ; Thu, 11 May 2023 03:48:41 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-202-iCHHTVxWNbqcZ3-yAECREg-1; Thu, 11 May 2023 11:48:39 +0100 X-MC-Unique: iCHHTVxWNbqcZ3-yAECREg-1 Received: from AcuMS.Aculab.com (10.202.163.4) by AcuMS.aculab.com (10.202.163.4) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Thu, 11 May 2023 11:48:37 +0100 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Thu, 11 May 2023 11:48:37 +0100 From: David Laight To: 'Tony Lindgren' , Geert Uytterhoeven CC: Stephen Boyd , Tomasz Figa , Sylwester Nawrocki , Will Deacon , Arnd Bergmann , Wolfram Sang , Dejin Zheng , Kai-Heng Feng , Nicholas Piggin , Heiko Carstens , Peter Zijlstra , Russell King , John Stultz , Thomas Gleixner , "Krzysztof Kozlowski" , Tero Kristo , "Ulf Hansson" , "Rafael J . Wysocki" , Vincent Guittot , "linux-arm-kernel@lists.infradead.org" , "linux-renesas-soc@vger.kernel.org" , "linux-pm@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops Thread-Topic: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops Thread-Index: AQHZg9SqgvUK/klkyEy9n2eMPFqMgK9U5DUg Date: Thu, 11 May 2023 10:48:37 +0000 Message-ID: <494a000774b546e4aae00ae0a7dfeae4@AcuMS.aculab.com> References: <20230511064839.GG14287@atomide.com> In-Reply-To: <20230511064839.GG14287@atomide.com> 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 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-pm@vger.kernel.org From: Tony Lindgren > Sent: 11 May 2023 07:49 > > * Geert Uytterhoeven [230510 13:23]: > > It is considered good practice to call cpu_relax() in busy loops, see > > Documentation/process/volatile-considered-harmful.rst. This can not > > only lower CPU power consumption or yield to a hyperthreaded twin > > processor, but also allows an architecture to mitigate hardware issues > > (e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the > > architecture-specific cpu_relax() implementation. Don't you also need to call cond_resched() (at least some times). Otherwise the process can't be pre-empted and a RT process that last ran on that cpu will never be scheduled. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)