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    SubjectRe: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely
    On Fri, 24 Jun 2011 15:44:54 +0100
    Mel Gorman <> wrote:

    > During allocator-intensive workloads, kswapd will be woken frequently
    > causing free memory to oscillate between the high and min watermark.
    > This is expected behaviour.
    > A problem occurs if the highest zone is small. balance_pgdat()
    > only considers unreclaimable zones when priority is DEF_PRIORITY
    > but sleeping_prematurely considers all zones. It's possible for this
    > sequence to occur
    > 1. kswapd wakes up and enters balance_pgdat()
    > 2. At DEF_PRIORITY, marks highest zone unreclaimable
    > 3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
    > 4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
    > highest zone, clearing all_unreclaimable. Highest zone
    > is still unbalanced
    > 5. kswapd returns and calls sleeping_prematurely
    > 6. sleeping_prematurely looks at *all* zones, not just the ones
    > being considered by balance_pgdat. The highest small zone
    > has all_unreclaimable cleared but but the zone is not
    > balanced. all_zones_ok is false so kswapd stays awake
    > This patch corrects the behaviour of sleeping_prematurely to check
    > the zones balance_pgdat() checked.

    But kswapd is making progress: it's reclaiming slab. Eventually that
    won't work any more and all_unreclaimable will not be cleared and the
    condition will fix itself up?


    if (!sleeping_prematurely(...))

    hurts my brain. My brain would prefer

    if (kswapd_should_sleep(...))


    > Reported-and-tested-by: Pádraig Brady <>

    But what were the before-and-after observations? I don't understand
    how this can cause a permanent cpuchew by kswapd.

    > --- a/mm/vmscan.c
    > +++ b/mm/vmscan.c
    > @@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
    > return true;
    > /* Check the watermark levels */
    > - for (i = 0; i < pgdat->nr_zones; i++) {
    > + for (i = 0; i <= classzone_idx; i++) {
    > struct zone *zone = pgdat->node_zones + i;
    > if (!populated_zone(zone))

    The patch looks sensible.
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      Last update: 2011-06-28 23:53    [W:0.033 / U:3.772 seconds]
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