From: Mel Gorman <mgorman@xxxxxxx>
Subject: mm, numa: really disable NUMA balancing by default on single node machines
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@xxxxxxx>
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.8+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mempolicy.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -puN mm/mempolicy.c~mm-numa-really-disable-numa-balancing-by-default-on-single-node-machines mm/mempolicy.c
--- a/mm/mempolicy.c~mm-numa-really-disable-numa-balancing-by-default-on-single-node-machines
+++ a/mm/mempolicy.c
@@ -2518,7 +2518,7 @@ 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",
_
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