ZONE_CONGESTED is only cleared in kswapd, but pages can be freed in any task.
It's possible ZONE_CONGESTED isn't cleared in some cases:
1. the zone is already balanced just entering balance_pgdat() for order-0 because
concurrent tasks free memory. In this case, later check will skip the zone as
it's balanced so the flag isn't cleared.
2. high order balance fallbacks to order-0. quote from Mel:
At the end of balance_pgdat(), kswapd uses the following logic;
If reclaiming at high order {
for each zone {
if all_unreclaimable
skip
if watermark is not met
order = 0
loop again
/* watermark is met */
clear congested
}
}
i.e. it clears ZONE_CONGESTED if it the zone is balanced. if not,
it restarts balancing at order-0. However, if the higher zones are
balanced for order-0, kswapd will miss clearing ZONE_CONGESTED
as that only happens after a zone is shrunk.
This can mean that wait_iff_congested() stalls unnecessarily. This patch
makes kswapd clear ZONE_CONGESTED during its initial highmem->dma scan
for zones that are already balanced.
Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
---
mm/vmscan.c | 3 +++
1 file changed, 3 insertions(+)
Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c 2011-08-11 09:26:37.000000000 +0800
+++ linux/mm/vmscan.c 2011-08-22 09:01:19.000000000 +0800
@@ -2529,6 +2529,9 @@ loop_again:
high_wmark_pages(zone), 0, 0)) {
end_zone = i;
break;
+ } else {
+ /* If balanced, clear the congested flag */
+ zone_clear_flag(zone, ZONE_CONGESTED);
}
}
if (i < 0)
--
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