sched: Check for an idle shared cache in select_task_rq_fair()

When waking affine, check for an idle shared cache, and if
found, wake to that CPU/sibling instead of the waker's CPU.

This improves pgsql+oltp ramp up by roughly 8%. Possibly more
for other loads, depending on overlap. The trade-off is a
roughly 1% peak downturn if tasks are truly synchronous.
Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <stable@kernel.org>
LKML-Reference: <1256654138.17752.7.camel@marge.simson.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -1372,11 +1372,36 @@ static int select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flag
 				want_sd = 0;
 		}
 
-		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
-		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
+		if (want_affine && (tmp->flags & SD_WAKE_AFFINE)) {
+			int candidate = -1, i;
 
+			if (cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
+				candidate = cpu;
+
+			/*
+			 * Check for an idle shared cache.
+			 */
+			if (tmp->flags & SD_PREFER_SIBLING) {
+				if (candidate == cpu) {
+					if (!cpu_rq(prev_cpu)->cfs.nr_running)
+						candidate = prev_cpu;
+				}
+
+				if (candidate == -1 || candidate == cpu) {
+					for_each_cpu(i, sched_domain_span(tmp)) {
+						if (!cpu_rq(i)->cfs.nr_running) {
+							candidate = i;
+							break;
+						}
+					}
+				}
+			}
+
+			if (candidate >= 0) {
 				affine_sd = tmp;
 				want_affine = 0;
+				cpu = candidate;
+			}
 		}
 
 		if (!want_sd && !want_affine)