From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753190AbaCXMSv (ORCPT ); Mon, 24 Mar 2014 08:18:51 -0400 Received: from hofr.at ([212.69.189.236]:59826 "EHLO mail.hofr.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752767AbaCXMSt (ORCPT ); Mon, 24 Mar 2014 08:18:49 -0400 Date: Mon, 24 Mar 2014 13:18:48 +0100 From: Nicholas Mc Guire To: linux-rt-users@vger.kernel.org Cc: linux-kernel@vger.kernel.org, rostedt@goodmis.org, tglx@linutronix.de, Sebastian Andrzej Siewior , Peter Zijlstra Subject: [PATCH RT] dont calculate hweight in update_migrate_disable Message-ID: <20140324121848.GA18300@opentech.at> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi ! Proposal for a minor optimization in update_migrate_disable - its only a few instructions saved but those are in the hot path of locks so it might be worth it When being scheduled out while migrate_disable > 0 and migrate_disabled_updated is not yet set we end up here (kernel/sched/core.c): static inline void update_migrate_disable(struct task_struct *p) { ... mask = tsk_cpus_allowed(p); if (p->sched_class->set_cpus_allowed) p->sched_class->set_cpus_allowed(p, mask); p->nr_cpus_allowed = cpumask_weight(mask); as we only can get here if migrate_disable > 0 there is no need to calculate the cpumask_weight(mask) as tsk_cpus_allowed in that case will return cpumask_of(task_cpu(p)) which only can have a hamming weight of 1 anyway. So we can simply do: p->nr_cpus_allowed = 1; without changing the behavior. Patch is on top of 3.12.13-rt21 Signed-off-by: Nicholas Mc Guire --- kernel/sched/core.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index bf93f63..6c07fe9 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2405,7 +2405,8 @@ static inline void update_migrate_disable(struct task_struct *p) if (p->sched_class->set_cpus_allowed) p->sched_class->set_cpus_allowed(p, mask); - p->nr_cpus_allowed = cpumask_weight(mask); + /* mask==cpumask_of(task_cpu(p)) which has a cpumask_weight==1 */ + p->nr_cpus_allowed = 1; /* Let migrate_enable know to fix things back up */ p->migrate_disable |= MIGRATE_DISABLE_SET_AFFIN; -- 1.7.10.4