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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=ham 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 122FBC54FCB for ; Mon, 27 Apr 2020 14:17:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EA30D206B6 for ; Mon, 27 Apr 2020 14:17:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727842AbgD0ORf (ORCPT ); Mon, 27 Apr 2020 10:17:35 -0400 Received: from mail.sssup.it ([193.205.80.98]:21733 "EHLO mail.santannapisa.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726651AbgD0ORe (ORCPT ); Mon, 27 Apr 2020 10:17:34 -0400 Received: from [151.41.75.232] (account l.abeni@santannapisa.it HELO nowhere) by santannapisa.it (CommuniGate Pro SMTP 6.1.11) with ESMTPSA id 147663101; Mon, 27 Apr 2020 16:17:31 +0200 Date: Mon, 27 Apr 2020 16:17:15 +0200 From: luca abeni To: Juri Lelli Cc: Dietmar Eggemann , Ingo Molnar , Peter Zijlstra , Vincent Guittot , Steven Rostedt , Daniel Bristot de Oliveira , Wei Wang , Quentin Perret , Alessio Balsini , Pavan Kondeti , Patrick Bellasi , Morten Rasmussen , Valentin Schneider , Qais Yousef , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 6/6] sched/deadline: Implement fallback mechanism for !fit case Message-ID: <20200427161715.3dd3a148@nowhere> In-Reply-To: <20200427133438.GA6469@localhost.localdomain> References: <20200427083709.30262-1-dietmar.eggemann@arm.com> <20200427083709.30262-7-dietmar.eggemann@arm.com> <20200427133438.GA6469@localhost.localdomain> Organization: Scuola Superiore S.Anna X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Juri, On Mon, 27 Apr 2020 15:34:38 +0200 Juri Lelli wrote: > Hi, > > On 27/04/20 10:37, Dietmar Eggemann wrote: > > From: Luca Abeni > > > > When a task has a runtime that cannot be served within the > > scheduling deadline by any of the idle CPU (later_mask) the task is > > doomed to miss its deadline. > > > > This can happen since the SCHED_DEADLINE admission control > > guarantees only bounded tardiness and not the hard respect of all > > deadlines. In this case try to select the idle CPU with the largest > > CPU capacity to minimize tardiness. > > > > Signed-off-by: Luca Abeni > > Signed-off-by: Dietmar Eggemann [...] > > - if (!cpumask_empty(later_mask)) > > - return 1; > > + if (cpumask_empty(later_mask)) > > + cpumask_set_cpu(max_cpu, later_mask); > > Think we touched upon this during v1 review, but I'm (still?) > wondering if we can do a little better, still considering only free > cpus. > > Can't we get into a situation that some of the (once free) big cpus > have been occupied by small tasks and now a big task enters the > system and it only finds small cpus available, were it could have fit > into bigs if small tasks were put onto small cpus? > > I.e., shouldn't we always try to best fit among free cpus? Yes; there was an additional patch that tried schedule each task on the slowest core where it can fit, to address this issue. But I think it will go in a second round of patches. Luca