linux-kernel - [012/151] sched: Check for an idle shared cache in select_task_rq

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Date:	Wed, 16 Dec 2009 19:55:09 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: [012/151] sched: Check for an idle shared cache in select_task_rq_fair()

2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Mike Galbraith <efault@....de>

commit a1f84a3ab8e002159498814eaa7e48c33752b04b upstream.

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@....de>
Cc: Arjan van de Ven <arjan@...radead.org>
Cc: Peter Zijlstra <peterz@...radead.org>
LKML-Reference: <1256654138.17752.7.camel@...ge.simson.net>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/sched_fair.c |   33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1398,11 +1398,36 @@ static int select_task_rq_fair(struct ta
 				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;
 
-			affine_sd = tmp;
-			want_affine = 0;
+			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)


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