[PATCH 4.0 113/148] mm, numa: really disable NUMA balancing by default on single node machines

Greg Kroah-Hartman
2015-06-03T12:25:47Z

From: Mel Gorman <mgorman@suse.de>

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@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/mempolicy.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2517,7 +2517,7 @@ static void __init check_numabalancing_e
if (numabalancing_override)
set_numabalancing_state(numabalancing_override == 1);

- if (nr_node_ids > 1 && !numabalancing_override) {
+ if (num_online_nodes() > 1 && !numabalancing_override) {
pr_info("%s automatic NUMA balancing. "
"Configure with numa_balancing= or the "
"kernel.numa_balancing sysctl",