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=-0.8 required=3.0 tests=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 14010C2BA1A for ; Wed, 8 Apr 2020 15:01:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DF74F208E4 for ; Wed, 8 Apr 2020 15:01:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729276AbgDHPBw (ORCPT ); Wed, 8 Apr 2020 11:01:52 -0400 Received: from foss.arm.com ([217.140.110.172]:39654 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727906AbgDHPBv (ORCPT ); Wed, 8 Apr 2020 11:01:51 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E80431045; Wed, 8 Apr 2020 08:01:50 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EAF5E3F68F; Wed, 8 Apr 2020 08:01:48 -0700 (PDT) References: <20200408095012.3819-1-dietmar.eggemann@arm.com> <20200408095012.3819-3-dietmar.eggemann@arm.com> <20200408153032.447e098d@nowhere> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: luca abeni 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 , Qais Yousef , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities In-reply-to: <20200408153032.447e098d@nowhere> Date: Wed, 08 Apr 2020 16:01:43 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/04/20 14:30, luca abeni wrote: >> >> I don't think this is strictly equivalent to what we have now for the >> SMP case. 'cpus' used to come from dl_bw_cpus(), which is an ugly way >> of writing >> >> cpumask_weight(rd->span AND cpu_active_mask); >> >> The rd->cpu_capacity_orig field you added gets set once per domain >> rebuild, so it also happens in sched_cpu_(de)activate() but is >> separate from touching cpu_active_mask. AFAICT this mean we can >> observe a CPU as !active but still see its capacity_orig accounted in >> a root_domain. > > Sorry, I suspect this is my fault, because the bug comes from my > original patch. > When I wrote the original code, I believed that when a CPU is > deactivated it is also removed from its root domain. > > I now see that I was wrong. > Well it is indeed the case, but sadly it's not an atomic step - AFAICT with cpusets we do hold some cpuset lock when calling __dl_overflow() and when rebuilding the domains, but not when fiddling with the active mask. I just realized it's even more obvious for dl_cpu_busy(): IIUC it is meant to prevent the removal of a CPU if it would lead to a DL overflow - it works now because the active mask is modified before it gets called, but here it breaks because it's called before the sched_domain rebuild. Perhaps re-computing the root domain capacity sum at every dl_bw_cpus() call would be simpler. It's a bit more work, but then we already have a for_each_cpu_*() loop, and we only rely on the masks being correct. > > Luca