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    Subject[PATCH 3.19.y-ckt 068/146] mm, numa: really disable NUMA balancing by default on single node machines
    Date
    3.19.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

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

    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: Kamal Mostafa <kamal@canonical.com>
    ---
    mm/mempolicy.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/mm/mempolicy.c b/mm/mempolicy.c
    index 0e0961b..bf20260 100644
    --- a/mm/mempolicy.c
    +++ b/mm/mempolicy.c
    @@ -2537,7 +2537,7 @@ 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",
    --
    1.9.1


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