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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 02E30C2BB86 for ; Thu, 9 Apr 2020 13:00:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D84E220771 for ; Thu, 9 Apr 2020 13:00:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726723AbgDINAX (ORCPT ); Thu, 9 Apr 2020 09:00:23 -0400 Received: from ms01.santannapisa.it ([193.205.80.98]:48186 "EHLO mail.santannapisa.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726609AbgDINAX (ORCPT ); Thu, 9 Apr 2020 09:00:23 -0400 Received: from [151.41.75.232] (account l.abeni@santannapisa.it HELO sweethome) by santannapisa.it (CommuniGate Pro SMTP 6.1.11) with ESMTPSA id 147267237; Thu, 09 Apr 2020 15:00:21 +0200 Date: Thu, 9 Apr 2020 15:00:10 +0200 From: luca abeni To: Qais Yousef Cc: Dietmar Eggemann , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Steven Rostedt , Daniel Bristot de Oliveira , Wei Wang , Quentin Perret , Alessio Balsini , Pavan Kondeti , Patrick Bellasi , Morten Rasmussen , Valentin Schneider , linux-kernel@vger.kernel.org Subject: Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for !fit case Message-ID: <20200409150010.468951df@sweethome> In-Reply-To: <20200409102557.h4humnsa5dlwvlym@e107158-lin.cambridge.arm.com> References: <20200408095012.3819-1-dietmar.eggemann@arm.com> <20200408095012.3819-5-dietmar.eggemann@arm.com> <20200409102557.h4humnsa5dlwvlym@e107158-lin.cambridge.arm.com> X-Mailer: Claws Mail 3.17.4 (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, On Thu, 9 Apr 2020 11:25:58 +0100 Qais Yousef wrote: > On 04/08/20 11:50, 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 > > --- > > Outside of the scope of this series. But does it make sense to make > sched_setattr() fail to create a new deadline task if the system will > be overcommitted, hence causing some dl tasks to miss their deadlines? The problem is that with multiple processors/cores it is not easy to know in advance if any task will miss a deadline (see section 3.3 of Documentation/scheduler/sched-deadline.rst). The admission control we are currently using should prevent SCHED_DEADLINE tasks from overloading the system (starving non-deadline tasks); proving hard deadline guarantees with global EDF scheduling is much more difficult (and could be probably done in user-space, I think). > If some overcommitting is fine (some deadlines are soft and are okay > to fail every once in a while), does it make sense for this to be a > tunable of how much the system can be overcommitted before > disallowing new DL tasks to be created? There is already a tunable for the SCHED_DEADLINE admission test (/proc/sys/kernel/sched_rt_{runtime,period}_us, if I understand well what you are suggesting). The problem is that it is not easy to find a value for this tunable that guarantees the hard respect of all deadlines. But IMHO if someone really wants hard deadline guarantees it is better to use partitioned scheduling (see Section 5 of the SCHED_DEADLINE documentation). Luca > > Just thinking out loudly. This fallback is fine, but it made me think > why did we have to end up in a situation that we can fail in the > first place since the same info is available when a new DL task is > created, and being preventative might be a better approach.. > > Thanks > > -- > Qais Yousef > > > kernel/sched/cpudeadline.c | 19 +++++++++++++++---- > > 1 file changed, 15 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c > > index 8630f2a40a3f..8525d73e3de4 100644 > > --- a/kernel/sched/cpudeadline.c > > +++ b/kernel/sched/cpudeadline.c > > @@ -121,19 +121,30 @@ int cpudl_find(struct cpudl *cp, struct > > task_struct *p, > > if (later_mask && > > cpumask_and(later_mask, cp->free_cpus, p->cpus_ptr)) { > > - int cpu; > > + unsigned long cap, max_cap = 0; > > + int cpu, max_cpu = -1; > > > > if > > (!static_branch_unlikely(&sched_asym_cpucapacity)) return 1; > > > > /* Ensure the capacity of the CPUs fits the task. > > */ for_each_cpu(cpu, later_mask) { > > - if (!dl_task_fits_capacity(p, cpu)) > > + if (!dl_task_fits_capacity(p, cpu)) { > > cpumask_clear_cpu(cpu, later_mask); > > + > > + cap = arch_scale_cpu_capacity(cpu); > > + > > + if (cap > max_cap) { > > + max_cap = cap; > > + max_cpu = cpu; > > + } > > + } > > } > > > > - if (!cpumask_empty(later_mask)) > > - return 1; > > + if (cpumask_empty(later_mask)) > > + cpumask_set_cpu(max_cpu, later_mask); > > + > > + return 1; > > } else { > > int best_cpu = cpudl_maximum(cp); > > > > -- > > 2.17.1 > >