[PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely

Mel Gorman
2011-06-24T14:45:07Z

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: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
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))
--
1.7.3.4