[PATCH v3 17/17] --- kernel/sched_rt.c | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-)

[Steven Rostedt <rostedt@goodmis.org>]

[-- Attachment #1: rt-balance-avoid-overloading.patch --]
[-- Type: text/plain, Size: 1390 bytes --]

Index: linux-compile.git/kernel/sched_rt.c
===================================================================
--- linux-compile.git.orig/kernel/sched_rt.c	2007-11-17 00:18:27.000000000 -0500
+++ linux-compile.git/kernel/sched_rt.c	2007-11-17 00:20:31.000000000 -0500
@@ -52,11 +52,23 @@ static int select_task_rq_rt(struct task
 	struct rq *rq = task_rq(p);
 
 	/*
-	 * If the task will not preempt the RQ, try to find a better RQ
-	 * before we even activate the task
+	 * If the current task is an RT task, then
+	 * try to see if we can wake this RT task up on another
+	 * runqueue. Otherwise simply start this RT task
+	 * on its current runqueue.
+	 *
+	 * We want to avoid overloading runqueues. Even if
+	 * the RT task is of higher priority than the current RT task.
+	 * RT tasks behave differently than other tasks. If
+	 * one gets preempted, we try to push it off to another queue.
+	 * So trying to keep a preempting RT task on the same
+	 * cache hot CPU will force the running RT task to
+	 * a cold CPU. So we waste all the cache for the lower
+	 * RT task in hopes of saving some of a RT task
+	 * that is just being woken and probably will have
+	 * cold cache anyway.
 	 */
-	if ((p->prio >= rq->rt.highest_prio)
-	    && (p->nr_cpus_allowed > 1)) {
+	if (unlikely(rt_task(rq->curr))) {
 		int cpu = find_lowest_rq(p);
 
 		return (cpu == -1) ? task_cpu(p) : cpu;

--