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    Subject[PATCH 3/4] Count the number of times zone_reclaim() scans and fails
    Date
    On NUMA machines, the administrator can configure zone_reclaim_mode that
    is a more targetted form of direct reclaim. On machines with large NUMA
    distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
    unmapped pages will be reclaimed if the zone watermarks are not being met.

    There is a heuristic that determines if the scan is worthwhile but it is
    possible that the heuristic will fail and the CPU gets tied up scanning
    uselessly. Detecting the situation requires some guesswork and experimentation
    so this patch adds a counter "zreclaim_failed" to /proc/vmstat. If during
    high CPU utilisation this counter is increasing rapidly, then the resolution
    to the problem may be to set /proc/sys/vm/zone_reclaim_mode to 0.

    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    ---
    include/linux/vmstat.h | 3 +++
    mm/vmscan.c | 4 ++++
    mm/vmstat.c | 3 +++
    3 files changed, 10 insertions(+), 0 deletions(-)

    diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
    index ff4696c..416f748 100644
    --- a/include/linux/vmstat.h
    +++ b/include/linux/vmstat.h
    @@ -36,6 +36,9 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
    FOR_ALL_ZONES(PGSTEAL),
    FOR_ALL_ZONES(PGSCAN_KSWAPD),
    FOR_ALL_ZONES(PGSCAN_DIRECT),
    +#ifdef CONFIG_NUMA
    + PGSCAN_ZONERECLAIM_FAILED,
    +#endif
    PGINODESTEAL, SLABS_SCANNED, KSWAPD_STEAL, KSWAPD_INODESTEAL,
    PAGEOUTRUN, ALLOCSTALL, PGROTATED,
    #ifdef CONFIG_HUGETLB_PAGE
    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index e862fc9..8be4582 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -2489,6 +2489,10 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
    ret = __zone_reclaim(zone, gfp_mask, order);
    zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);

    + if (!ret) {
    + count_vm_events(PGSCAN_ZONERECLAIM_FAILED, 1);
    + }
    +
    return ret;
    }
    #endif
    diff --git a/mm/vmstat.c b/mm/vmstat.c
    index 1e3aa81..02677d1 100644
    --- a/mm/vmstat.c
    +++ b/mm/vmstat.c
    @@ -673,6 +673,9 @@ static const char * const vmstat_text[] = {
    TEXTS_FOR_ZONES("pgscan_kswapd")
    TEXTS_FOR_ZONES("pgscan_direct")

    +#ifdef CONFIG_NUMA
    + "zreclaim_failed",
    +#endif
    "pginodesteal",
    "slabs_scanned",
    "kswapd_steal",
    --
    1.5.6.5


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