mm, numa: really disable NUMA balancing by default on single node machines

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>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index e8fff0fa1202e17ea3a9491954c7e92591ef07ab..936866e72b1d9c28522aca317be83d2d2caff0fd 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2663,7 +2663,7 @@ 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) {
                pr_info("%s automatic NUMA balancing. "
                        "Configure with numa_balancing= or the "
                        "kernel.numa_balancing sysctl",