[sched, patch] better wake-balancing

[Ingo Molnar <mingo@elte.hu>]

another approach would be the patch below, to do wakeup-balancing only 
if the wakeup CPU or the task CPU is idle.

I've measured half-loaded tbench and unless total wakeup-balancing 
removal it does not degrade with this patch applied, while fully loaded 
tbench and other workloads clearly improve.

Ken, could you give this one a try? (It's against the current scheduler 
queue in -mm, but also applies fine to current Linus trees.)

	Ingo

---

do wakeup-balancing only if the wakeup-CPU or the task-CPU is idle.

this prevents excessive wakeup-balancing while the system is highly
loaded, but helps spread out the workload on partly idle systems.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

 kernel/sched.c |    6 ++++++
 1 files changed, 6 insertions(+)

Index: linux-sched-curr/kernel/sched.c
===================================================================
--- linux-sched-curr.orig/kernel/sched.c
+++ linux-sched-curr/kernel/sched.c
@@ -1252,7 +1252,13 @@ static int try_to_wake_up(task_t *p, uns
 	if (unlikely(task_running(rq, p)))
 		goto out_activate;
 
+	/*
+	 * If neither this CPU, nor the previous CPU the task was
+	 * running on is idle then skip wakeup-balancing:
+	 */
 	new_cpu = cpu;
+	if (!idle_cpu(this_cpu) && !idle_cpu(cpu))
+		goto out_set_cpu;
 
 	schedstat_inc(rq, ttwu_cnt);
 	if (cpu == this_cpu) {