From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2997554AbdD1Nej (ORCPT ); Fri, 28 Apr 2017 09:34:39 -0400 Received: from merlin.infradead.org ([205.233.59.134]:52346 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1162746AbdD1Ncg (ORCPT ); Fri, 28 Apr 2017 09:32:36 -0400 Message-Id: <20170428132502.849764343@infradead.org> User-Agent: quilt/0.63-1 Date: Fri, 28 Apr 2017 15:20:08 +0200 From: Peter Zijlstra To: mingo@kernel.org, lvenanci@redhat.com Cc: lwang@redhat.com, riel@redhat.com, efault@gmx.de, tglx@linutronix.de, linux-kernel@vger.kernel.org, peterz@infradead.org, stable@vger.kernel.org Subject: [PATCH 10/14] sched/topology: Fix overlapping sched_group_mask References: <20170428131958.893188882@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline; filename=peterz-sched-topo-sched_group_mask.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The point of sched_group_mask is to select those CPUs from sched_group_cpus that can actually arrive at this balance domain. The current code gets it wrong, as can be readily demonstrated with a topology like: node 0 1 2 3 0: 10 20 30 20 1: 20 10 20 30 2: 30 20 10 20 3: 20 30 20 10 Where (for example) domain 1 on CPU1 ends up with a mask that includes CPU0: [] CPU1 attaching sched-domain: [] domain 0: span 0-2 level NUMA [] groups: 1 (mask: 1), 2, 0 [] domain 1: span 0-3 level NUMA [] groups: 0-2 (mask: 0-2) (cpu_capacity: 3072), 0,2-3 (cpu_capacity: 3072) This causes sched_balance_cpu() to compute the wrong CPU and consequently should_we_balance() will terminate early resulting in missed load-balance opportunities. The fixed topology looks like: [] CPU1 attaching sched-domain: [] domain 0: span 0-2 level NUMA [] groups: 1 (mask: 1), 2, 0 [] domain 1: span 0-3 level NUMA [] groups: 0-2 (mask: 1) (cpu_capacity: 3072), 0,2-3 (cpu_capacity: 3072) (note: this relies on OVERLAP domains to always have children, this is true because the regular topology domains are still here -- this is before degenerate trimming) Cc: stable@vger.kernel.org Fixes: e3589f6c81e4 ("sched: Allow for overlapping sched_domain spans") Debugged-by: Lauro Ramos Venancio Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/topology.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -495,6 +495,9 @@ enum s_alloc { /* * Build an iteration mask that can exclude certain CPUs from the upwards * domain traversal. + * + * Only CPUs that can arrive at this group should be considered to continue + * balancing. */ static void build_group_mask(struct sched_domain *sd, struct sched_group *sg) { @@ -505,11 +508,24 @@ static void build_group_mask(struct sche for_each_cpu(i, sg_span) { sibling = *per_cpu_ptr(sdd->sd, i); - if (!cpumask_test_cpu(i, sched_domain_span(sibling))) + + /* + * Can happen in the asymmetric case, where these siblings are + * unused. The mask will not be empty because those CPUs that + * do have the top domain _should_ span the domain. + */ + if (!sibling->child) + continue; + + /* If we would not end up here, we can't continue from here */ + if (!cpumask_equal(sg_span, sched_domain_span(sibling->child))) continue; cpumask_set_cpu(i, sched_group_mask(sg)); } + + /* We must not have empty masks here */ + WARN_ON_ONCE(cpumask_empty(sched_group_mask(sg))); } /*