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=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 1B40EC433E0 for ; Mon, 8 Mar 2021 08:18:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CCD4564FD4 for ; Mon, 8 Mar 2021 08:18:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231867AbhCHISW (ORCPT ); Mon, 8 Mar 2021 03:18:22 -0500 Received: from foss.arm.com ([217.140.110.172]:33774 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231592AbhCHISN (ORCPT ); Mon, 8 Mar 2021 03:18:13 -0500 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 0282D31B; Mon, 8 Mar 2021 00:18:13 -0800 (PST) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 51CAC3F70D; Mon, 8 Mar 2021 00:18:11 -0800 (PST) Subject: Re: [RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance To: =?UTF-8?Q?Michal_Koutn=c3=bd?= , Vincent Guittot Cc: Phil Auld , Ingo Molnar , Peter Zijlstra , Juri Lelli , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , linux-kernel@vger.kernel.org References: <20210225162757.48858-1-mkoutny@suse.com> From: Dietmar Eggemann Message-ID: <09e6a72d-5bdf-abab-b45d-cad733e93100@arm.com> Date: Mon, 8 Mar 2021 09:18:10 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210225162757.48858-1-mkoutny@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Michal, On 25/02/2021 17:27, Michal Koutný wrote: > leaf_cfs_rq_list should contain cfs_rqs that have runnable entities in > them. When we're operating under a throttled hierarchy we always update > the leaf_cfs_rq_list in order no to break list_add_leaf_cfs_rq invariant > of adding complete branches. So this patch replaces in enqueue_entity() the unconditional list_add_leaf_cfs_rq(cfs_rq) in case cfs->nr_running > 1 (parent had been throttled) - if (cfs_rq->nr_running == 1 || cfs_bandwidth_used()) ^^^^^^^^^^^^^^^^^^^^^^^ with + if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ and removes the leaf_cfs_rq_list maintenance code after the xxx_throttle label in enqueue_task_fair() and unthrottle_cfs_rq from commit f6783319737f ("sched/fair: Fix insertion in rq->leaf_cfs_rq_list") and fe61468b2cb ("sched/fair: Fix enqueue_task_fair warning"). > This caused troubles when an entity became runnable (enqueue_entity) > under a throttled hierarchy (see commit b34cb07dde7c ("sched/fair: Fix > enqueue_task_fair() warning some more")). While it proved well, this > new change ignores updating leaf_cfs_rq_list when we're operating under > the throttled hierarchy and defers the leaf_cfs_rq_list update to the > point when whole hiearchy is unthrottled (tg_unthrottle_up). IMHO, f6783319737f gives the explanation why throttled cfs_rq's have to be added to rq->leaf_cfs_rq_list. IIRC, fe61468b2cb was fixing a use case in which a cfs-rq with on_list=0 and nr_running > 1 within the cgroup hierarchy wasn't added back to rq->leaf_cfs_rq_list: https://lkml.kernel.org/r/15252de5-9a2d-19ae-607a-594ee88d1ba1@de.ibm.com In enqueue_task_fair() just before the assert_list_leaf_cfs_rq(rq), Iterate through the se heriarchy of p=[CPU 2/KVM 2662] in case the assert will hit: ... CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10.scope/vcpuX on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662] CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10.scope on_list=0 nr_running=3 throttled=0 p=[CPU 2/KVM 2662] ^^^^^^^^^ ^^^^^^^^^^^^ CPU23 path=/machine.slice/machine-test.slice on_list=1 nr_running=1 throttled=1 p=[CPU 2/KVM 2662] CPU23 path=/machine.slice on_list=1 nr_running=0 throttled=0 p=[CPU 2/KVM 2662] CPU23 path=/ on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662] ... > The code is now simpler and leaf_cfs_rq_list contains only cfs_rqs that > are truly runnable. > > Why is this RFC? > - Primarily, I'm not sure I interpreted the purpose of leaf_cfs_rq_list > right. The removal of throttled cfs_rqs from it would exclude them > from __update_blocked_fair() calculation and I can't see past it now. The leaf_cfs_rq_list should contain all cfs_rqs with cfs_rq->avg.load/runnable/util_avg != 0 so that in case there are no runnable entities on them anymore this (blocked) load cfs_rq->avg.xxx_avg can be decayed. IMHO. the "leaf_" is misleading here since it can also contain non-leaf cfs_rqs. > If it's wrong assumption, I'd like this to help clarify what the > proper definition of leaf_cfs_rq_list would be. > - Additionally, I didn't check thoroughly for corner cases when > se->on_rq => cfs_rq_of(se)->on_list wouldn't hold, so the patch > certainly isn't finished. [...]