Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping insleeping_prematurely

[KOSAKI Motohiro]

(2011/06/24 23:44), 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.
> 
> Reported-and-tested-by: PÃdraig Brady <P@xxxxxxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
>  mm/vmscan.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8ff834e..841e3bf 100644
> --- 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))

sorry for the delay.
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>




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