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=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT 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 025F4C433DB for ; Thu, 25 Feb 2021 16:29:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BAA2464F19 for ; Thu, 25 Feb 2021 16:29:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233001AbhBYQ3E (ORCPT ); Thu, 25 Feb 2021 11:29:04 -0500 Received: from mx2.suse.de ([195.135.220.15]:40616 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233158AbhBYQ2z (ORCPT ); Thu, 25 Feb 2021 11:28:55 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1614270488; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=vxneo372S9t8nnxxBJftXS5hW6amgHXFT8XmT+FBdNw=; b=d16yxZ9Zd0YcS6BQLEgLbel9gO4iyMphIJTCHj28u/g/LkHzVn2tiStGRKgiG8PR2ECg5B SiuYYbYnXfgQB2ibCmEOSkmH2gHhSLloYfeVfHbeYsHAD/VoFPgu+3XaxubCBEC74pto2u ZOWy1y0ayRlDSnmD/maqMhKk7RqdXPA= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 71800ACE5; Thu, 25 Feb 2021 16:28:08 +0000 (UTC) From: =?UTF-8?q?Michal=20Koutn=C3=BD?= To: Vincent Guittot Cc: Phil Auld , Ingo Molnar , Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , linux-kernel@vger.kernel.org Subject: [RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance Date: Thu, 25 Feb 2021 17:27:57 +0100 Message-Id: <20210225162757.48858-1-mkoutny@suse.com> X-Mailer: git-send-email 2.30.1 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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). 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. 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. Signed-off-by: Michal Koutný --- kernel/sched/fair.c | 41 ++++++----------------------------------- kernel/sched/sched.h | 4 +--- 2 files changed, 7 insertions(+), 38 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 04a3ce20da67..634ba6637824 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4250,10 +4250,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) /* * When bandwidth control is enabled, cfs might have been removed - * because of a parent been throttled but cfs->nr_running > 1. Try to - * add it unconditionnally. + * because of a parent been throttled. We'll add it later (with + * complete branch up to se->on_rq/cfs_eq->on_list) in + * tg_unthrottle_up() and unthrottle_cfs_rq(). */ - if (cfs_rq->nr_running == 1 || cfs_bandwidth_used()) + if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq)) list_add_leaf_cfs_rq(cfs_rq); if (cfs_rq->nr_running == 1) @@ -4859,6 +4860,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) for_each_sched_entity(se) { if (se->on_rq) break; + // XXX: se->on_rq implies cfs_rq_of(se)->on_list (unless throttled_hierarchy) cfs_rq = cfs_rq_of(se); enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP); @@ -4896,17 +4898,6 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) add_nr_running(rq, task_delta); unthrottle_throttle: - /* - * The cfs_rq_throttled() breaks in the above iteration can result in - * incomplete leaf list maintenance, resulting in triggering the - * assertion below. - */ - for_each_sched_entity(se) { - cfs_rq = cfs_rq_of(se); - - if (list_add_leaf_cfs_rq(cfs_rq)) - break; - } assert_list_leaf_cfs_rq(rq); @@ -5518,6 +5509,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) for_each_sched_entity(se) { if (se->on_rq) break; + // XXX: se->on_rq implies cfs_rq_of(se)->on_list (unless throttled_hierarchy) cfs_rq = cfs_rq_of(se); enqueue_entity(cfs_rq, se, flags); @@ -5544,13 +5536,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) /* end evaluation on encountering a throttled cfs_rq */ if (cfs_rq_throttled(cfs_rq)) goto enqueue_throttle; - - /* - * One parent has been throttled and cfs_rq removed from the - * list. Add it back to not break the leaf list. - */ - if (throttled_hierarchy(cfs_rq)) - list_add_leaf_cfs_rq(cfs_rq); } /* At this point se is NULL and we are at root level*/ @@ -5574,20 +5559,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) update_overutilized_status(rq); enqueue_throttle: - if (cfs_bandwidth_used()) { - /* - * When bandwidth control is enabled; the cfs_rq_throttled() - * breaks in the above iteration can result in incomplete - * leaf list maintenance, resulting in triggering the assertion - * below. - */ - for_each_sched_entity(se) { - cfs_rq = cfs_rq_of(se); - - if (list_add_leaf_cfs_rq(cfs_rq)) - break; - } - } assert_list_leaf_cfs_rq(rq); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index bb09988451a0..f674d88920da 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -573,9 +573,7 @@ struct cfs_rq { struct rq *rq; /* CPU runqueue to which this cfs_rq is attached */ /* - * leaf cfs_rqs are those that hold tasks (lowest schedulable entity in - * a hierarchy). Non-leaf lrqs hold other higher schedulable entities - * (like users, containers etc.) + * leaf cfs_rqs are those that hold runnable entities. * * leaf_cfs_rq_list ties together list of leaf cfs_rq's in a CPU. * This list is used during load balance. -- 2.30.1