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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 <mgorman@suse.de> 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?



btw,

if (!sleeping_prematurely(...))
sleep();
hurts my brain. My brain would prefer

if (kswapd_should_sleep(...))
sleep();
no?

> Reported-and-tested-by: Pádraig Brady <P@draigBrady.com>

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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