[sched, patch] better wake-balancing, #2

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

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

there's an even simpler way: only do wakeup-balancing if this_cpu is 
idle. (tbench results are still OK, and other workloads improved.)

	Ingo

--------
do wakeup-balancing only if the wakeup-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
@@ -1253,7 +1253,13 @@ static int try_to_wake_up(task_t *p, uns
 	if (unlikely(task_running(rq, p)))
 		goto out_activate;
 
+	/*
+	 * Only do wakeup-balancing (= potentially migrate the task)
+	 * if this CPU is idle:
+	 */
 	new_cpu = cpu;
+	if (!idle_cpu(this_cpu))
+		goto out_set_cpu;
 
 	schedstat_inc(rq, ttwu_cnt);
 	if (cpu = this_cpu) {