sched-Run-task-on-same-cpu-if-idle-and-allowed.patch

From 1954630a0637ad1ec2dcfe0324a36cd243f018ee Mon Sep 17 00:00:00 2001
From: Carsten Emde <C.Emde@osadl.org>
Date: Mon, 15 Feb 2010 22:30:34 +0100
Subject: [PATCH] sched: Run task on same cpu if idle and allowed

commit 1954630a0637ad1ec2dcfe0324a36cd243f018ee in tip.

The current RT balancer does not consider CPU topology and may
wakeup a process to run on another CPU although it might run
on the current.

This patch adds a check and runs the new task on the current CPU,
if it
    - is an RT task,
    - is allowed to run on the current CPU, and
    - current is not an RT task,

Signed-off-by: Carsten Emde <C.Emde@osadl.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 7a27ef6..9de5f18 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1030,6 +1030,17 @@ static int select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
 	}
 
 	/*
+	 * If the new task is an RT task, current is not an RT task
+	 * and the new one may run on the current CPU, run it here.
+	 * This avoids sending reschedule IPIs across CPUs.
+	 */
+	if (unlikely(rt_task(p)) && !rt_task(rq->curr)) {
+		int cpu = smp_processor_id();
+		if (cpumask_test_cpu(cpu, &p->cpus_allowed))
+			return cpu;
+	}
+
+	/*
 	 * Otherwise, just let it ride on the affined RQ and the
 	 * post-schedule router will push the preempted task away
 	 */
-- 
1.7.1.1