linux-kernel - [PATCH 3.12 074/111] mm, numa: really disable NUMA balancing by default on single node machines

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Message-Id: <caf4b0e71fe36cdfb9cafd01fd55da3a086f82cd.1433943052.git.jslaby@suse.cz>
Date:	Wed, 10 Jun 2015 17:27:03 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 074/111] mm, numa: really disable NUMA balancing by default on single node machines

From: Mel Gorman <mgorman@...e.de>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit b0dc2b9bb4ab782115b964310518ee0b17784277 upstream.

NUMA balancing is meant to be disabled by default on UMA machines but
the check is using nr_node_ids (highest node) instead of
num_online_nodes (online nodes).

The consequences are that a UMA machine with a node ID of 1 or higher
will enable NUMA balancing.  This will incur useless overhead due to
minor faults with the impact depending on the workload.  These are the
impact on the stats when running a kernel build on a single node machine
whose node ID happened to be 1:

  			       vanilla     patched
  NUMA base PTE updates          5113158           0
  NUMA huge PMD updates              643           0
  NUMA page range updates        5442374           0
  NUMA hint faults               2109622           0
  NUMA hint local faults         2109622           0
  NUMA hint local percent            100         100
  NUMA pages migrated                  0           0

Signed-off-by: Mel Gorman <mgorman@...e.de>
Reviewed-by: Rik van Riel <riel@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 mm/mempolicy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index e63ade49ea24..51cd7d066e0f 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2589,7 +2589,7 @@ static void __init check_numabalancing_enable(void)
 	if (numabalancing_override)
 		set_numabalancing_state(numabalancing_override == 1);
 
-	if (nr_node_ids > 1 && !numabalancing_override) {
+	if (num_online_nodes() > 1 && !numabalancing_override) {
 		printk(KERN_INFO "%s automatic NUMA balancing. "
 			"Configure with numa_balancing= or sysctl",
 			numabalancing_default ? "Enabling" : "Disabling");
-- 
2.4.2

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