Index: linux-2.6.31-bfs/kernel/sched_bfs.c
===================================================================
--- linux-2.6.31-bfs.orig/kernel/sched_bfs.c	2009-09-10 23:19:43.345014392 +1000
+++ linux-2.6.31-bfs/kernel/sched_bfs.c	2009-09-10 23:30:12.113014668 +1000
@@ -996,11 +996,23 @@
 	preempt_enable();
 }
 
+#ifdef CONFIG_SMP
+static int no_idle_cpus(void)
+{
+	return (cpus_empty(grq.cpu_idle_map));
+}
+#else
+static int no_idle_cpus(void)
+{
+	return 1;
+}
+#endif
+
 /***
  * try_to_wake_up - wake up a thread
  * @p: the to-be-woken-up thread
  * @state: the mask of task states that can be woken
- * sync is ignored on bfs
+ * @sync: do a synchronous wakeup?
  *
  * Put it on the run-queue if it's not already there. The "current"
  * thread is always on the run-queue (except when the actual
@@ -1010,7 +1022,7 @@
  *
  * returns failure only if the task is already active.
  */
-static int try_to_wake_up(struct task_struct *p, unsigned int state)
+static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)
 {
 	unsigned long flags;
 	int success = 0;
@@ -1026,7 +1038,14 @@
 		goto out_running;
 
 	activate_task(p, rq);
-	try_preempt(p, rq);
+	/*
+	 * Sync wakeups (i.e. those types of wakeups where the waker
+	 * has indicated that it will leave the CPU in short order)
+	 * don't trigger a preemption if there are no idle cpus,
+	 * instead waiting for current to deschedule.
+	 */
+	if (!sync || (sync && !no_idle_cpus()))
+		try_preempt(p, rq);
 	success = 1;
 
 out_running:
@@ -1050,13 +1069,13 @@
  */
 int wake_up_process(struct task_struct *p)
 {
-	return try_to_wake_up(p, TASK_ALL);
+	return try_to_wake_up(p, TASK_ALL, 0);
 }
 EXPORT_SYMBOL(wake_up_process);
 
 int wake_up_state(struct task_struct *p, unsigned int state)
 {
-	return try_to_wake_up(p, state);
+	return try_to_wake_up(p, state, 0);
 }
 
 /*
@@ -1124,18 +1143,6 @@
 	put_cpu();
 }
 
-#ifdef CONFIG_SMP
-static int no_idle_cpus(void)
-{
-	return (cpus_empty(grq.cpu_idle_map));
-}
-#else
-static int no_idle_cpus(void)
-{
-	return 1;
-}
-#endif
-
 /*
  * wake_up_new_task - wake up a newly created task for the first time.
  *
@@ -2271,7 +2278,7 @@
 int default_wake_function(wait_queue_t *curr, unsigned mode, int sync,
 			  void *key)
 {
-	return try_to_wake_up(curr->private, mode);
+	return try_to_wake_up(curr->private, mode, sync);
 }
 EXPORT_SYMBOL(default_wake_function);