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    Subject[PATCH 3/6] mm: kswapd: Use the order that kswapd was reclaiming at for sleeping_prematurely()
    Date
    Before kswapd goes to sleep, it uses sleeping_prematurely() to check if
    there was a race pushing a zone below its watermark. If the race
    happened, it stays awake. However, balance_pgdat() can decide to reclaim
    at a lower order if it decides that high-order reclaim is not working as
    expected. This information is not passed back to sleeping_prematurely().
    The impact is that kswapd remains awake reclaiming pages long after it
    should have gone to sleep. This patch passes the adjusted order to
    sleeping_prematurely and uses the same logic as balance_pgdat to decide
    if it's ok to go to sleep.

    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    ---
    mm/vmscan.c | 14 ++++++++++----
    1 files changed, 10 insertions(+), 4 deletions(-)

    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index b4472a1..52e229e 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -2132,7 +2132,7 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced)
    }

    /* is kswapd sleeping prematurely? */
    -static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
    +static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
    {
    int i;
    unsigned long balanced = 0;
    @@ -2142,7 +2142,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
    if (remaining)
    return 1;

    - /* If after HZ/10, a zone is below the high mark, it's premature */
    + /* Check the watermark levels */
    for (i = 0; i < pgdat->nr_zones; i++) {
    struct zone *zone = pgdat->node_zones + i;

    @@ -2427,7 +2427,13 @@ out:
    }
    }

    - return sc.nr_reclaimed;
    + /*
    + * Return the order we were reclaiming at so sleeping_prematurely()
    + * makes a decision on the order we were last reclaiming at. However,
    + * if another caller entered the allocator slow path while kswapd
    + * was awake, order will remain at the higher level
    + */
    + return order;
    }

    /*
    @@ -2537,7 +2543,7 @@ static int kswapd(void *p)
    */
    if (!ret) {
    trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
    - balance_pgdat(pgdat, order, classzone_idx);
    + order = balance_pgdat(pgdat, order, classzone_idx);
    }
    }
    return 0;
    --
    1.7.1


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