From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755868AbdJJLDr (ORCPT ); Tue, 10 Oct 2017 07:03:47 -0400 Received: from terminus.zytor.com ([65.50.211.136]:48039 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751420AbdJJLDp (ORCPT ); Tue, 10 Oct 2017 07:03:45 -0400 Date: Tue, 10 Oct 2017 03:59:06 -0700 From: tip-bot for Brendan Jackman Message-ID: Cc: efault@gmx.de, mingo@kernel.org, pjt@google.com, tglx@linutronix.de, morten.rasmussen@arm.com, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, brendan.jackman@arm.com, hpa@zytor.com, peterz@infradead.org Reply-To: morten.rasmussen@arm.com, tglx@linutronix.de, pjt@google.com, mingo@kernel.org, efault@gmx.de, peterz@infradead.org, hpa@zytor.com, brendan.jackman@arm.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org In-Reply-To: <20170807163900.25180-1-brendan.jackman@arm.com> References: <20170807163900.25180-1-brendan.jackman@arm.com> To: linux-tip-commits@vger.kernel.org Subject: [tip:sched/core] sched/fair: Force balancing on NOHZ balance if local group has capacity Git-Commit-ID: 583ffd99d7657755736d831bbc182612d1d2697d X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 583ffd99d7657755736d831bbc182612d1d2697d Gitweb: https://git.kernel.org/tip/583ffd99d7657755736d831bbc182612d1d2697d Author: Brendan Jackman AuthorDate: Thu, 5 Oct 2017 11:58:54 +0100 Committer: Ingo Molnar CommitDate: Tue, 10 Oct 2017 11:45:32 +0200 sched/fair: Force balancing on NOHZ balance if local group has capacity The "goto force_balance" here is intended to mitigate the fact that avg_load calculations can result in bad placement decisions when priority is asymmetrical. The original commit that adds it: fab476228ba3 ("sched: Force balancing on newidle balance if local group has capacity") explains: Under certain situations, such as a niced down task (i.e. nice = -15) in the presence of nr_cpus NICE0 tasks, the niced task lands on a sched group and kicks away other tasks because of its large weight. This leads to sub-optimal utilization of the machine. Even though the sched group has capacity, it does not pull tasks because sds.this_load >> sds.max_load, and f_b_g() returns NULL. A similar but inverted issue also affects ARM big.LITTLE (asymmetrical CPU capacity) systems - consider 8 always-running, same-priority tasks on a system with 4 "big" and 4 "little" CPUs. Suppose that 5 of them end up on the "big" CPUs (which will be represented by one sched_group in the DIE sched_domain) and 3 on the "little" (the other sched_group in DIE), leaving one CPU unused. Because the "big" group has a higher group_capacity its avg_load may not present an imbalance that would cause migrating a task to the idle "little". The force_balance case here solves the problem but currently only for CPU_NEWLY_IDLE balances, which in theory might never happen on the unused CPU. Including CPU_IDLE in the force_balance case means there's an upper bound on the time before we can attempt to solve the underutilization: after DIE's sd->balance_interval has passed the next nohz balance kick will help us out. Signed-off-by: Brendan Jackman Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Morten Rasmussen Cc: Paul Turner Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20170807163900.25180-1-brendan.jackman@arm.com Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index c04a425..cf3e816 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8186,8 +8186,11 @@ static struct sched_group *find_busiest_group(struct lb_env *env) if (busiest->group_type == group_imbalanced) goto force_balance; - /* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */ - if (env->idle == CPU_NEWLY_IDLE && group_has_capacity(env, local) && + /* + * When dst_cpu is idle, prevent SMP nice and/or asymmetric group + * capacities from resulting in underutilization due to avg_load. + */ + if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && busiest->group_no_capacity) goto force_balance;