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    Subject[PATCH 35/63] sched: numa: Do not trap hinting faults for shared libraries
    Date
    NUMA hinting faults will not migrate a shared executable page mapped by
    multiple processes on the grounds that the data is probably in the CPU
    cache already and the page may just bounce between tasks running on multipl
    nodes. Even if the migration is avoided, there is still the overhead of
    trapping the fault, updating the statistics, making scheduler placement
    decisions based on the information etc. If we are never going to migrate
    the page, it is overhead for no gain and worse a process may be placed on
    a sub-optimal node for shared executable pages. This patch avoids trapping
    faults for shared libraries entirely.

    Signed-off-by: Mel Gorman <mgorman@suse.de>
    ---
    kernel/sched/fair.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 9a2e68e..8760231 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1231,6 +1231,16 @@ void task_numa_work(struct callback_head *work)
    if (!vma_migratable(vma) || !vma_policy_mof(p, vma))
    continue;

    + /*
    + * Shared library pages mapped by multiple processes are not
    + * migrated as it is expected they are cache replicated. Avoid
    + * hinting faults in read-only file-backed mappings or the vdso
    + * as migrating the pages will be of marginal benefit.
    + */
    + if (!vma->vm_mm ||
    + (vma->vm_file && (vma->vm_flags & (VM_READ|VM_WRITE)) == (VM_READ)))
    + continue;
    +
    do {
    start = max(start, vma->vm_start);
    end = ALIGN(start + (pages << PAGE_SHIFT), HPAGE_SIZE);
    --
    1.8.4


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