[sched, patch] better wake-balancing, #3

From: Ingo Molnar
Date: Fri Jul 29 2005 - 11:26:31 EST

* Ingo Molnar <mingo@xxxxxxx> wrote:

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

here's an updated patch. It handles one more detail: on SCHED_SMT we
should check the idleness of siblings too. Benchmark numbers still look


do wakeup-balancing only if the wakeup-CPU (or any of its siblings)
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@xxxxxxx>

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 (or any SMT sibling) is idle:
+ */
new_cpu = cpu;
+ if (!idle_cpu(this_cpu) && this_cpu == wake_idle(this_cpu, p))
+ goto out_set_cpu;

schedstat_inc(rq, ttwu_cnt);
if (cpu == this_cpu) {
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