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Subject[PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is any eligible zone
Date
kswapd scans from highest to lowest for a zone that requires balancing.
This was necessary when reclaim was per-zone to fairly age pages on lower
zones. Now that we are reclaiming on a per-node basis, any eligible zone
can be used and pages will still be aged fairly. This patch avoids
reclaiming excessively unless buffer_heads are over the limit and it's
necessary to reclaim from a higher zone than requested by the waker of
kswapd to relieve low memory pressure.

[hillf.zj@alibaba-inc.com: Force kswapd reclaim no more than needed]
Link: http://lkml.kernel.org/r/1466518566-30034-12-git-send-email-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/vmscan.c | 56 ++++++++++++++++++++++++--------------------------------
1 file changed, 24 insertions(+), 32 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 911142d25de2..2f898ba2ee2e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3141,31 +3141,36 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)

sc.nr_reclaimed = 0;

- /* Scan from the highest requested zone to dma */
- for (i = classzone_idx; i >= 0; i--) {
- zone = pgdat->node_zones + i;
- if (!populated_zone(zone))
- continue;
-
- /*
- * If the number of buffer_heads in the machine
- * exceeds the maximum allowed level and this node
- * has a highmem zone, force kswapd to reclaim from
- * it to relieve lowmem pressure.
- */
- if (buffer_heads_over_limit && is_highmem_idx(i)) {
- classzone_idx = i;
- break;
- }
+ /*
+ * If the number of buffer_heads in the machine exceeds the
+ * maximum allowed level then reclaim from all zones. This is
+ * not specific to highmem as highmem may not exist but it is
+ * it is expected that buffer_heads are stripped in writeback.
+ */
+ if (buffer_heads_over_limit) {
+ for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
+ zone = pgdat->node_zones + i;
+ if (!populated_zone(zone))
+ continue;

- if (!zone_balanced(zone, order, 0)) {
classzone_idx = i;
break;
}
}

- if (i < 0)
- goto out;
+ /*
+ * Only reclaim if there are no eligible zones. Check from
+ * high to low zone to avoid prematurely clearing pgdat
+ * congested state.
+ */
+ for (i = classzone_idx; i >= 0; i--) {
+ zone = pgdat->node_zones + i;
+ if (!populated_zone(zone))
+ continue;
+
+ if (zone_balanced(zone, sc.order, classzone_idx))
+ goto out;
+ }

/*
* Do some background aging of the anon list, to give
@@ -3211,19 +3216,6 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
break;

/*
- * Stop reclaiming if any eligible zone is balanced and clear
- * node writeback or congested.
- */
- for (i = 0; i <= classzone_idx; i++) {
- zone = pgdat->node_zones + i;
- if (!populated_zone(zone))
- continue;
-
- if (zone_balanced(zone, sc.order, classzone_idx))
- goto out;
- }
-
- /*
* Raise priority if scanning rate is too low or there was no
* progress in reclaiming pages
*/
--
2.6.4
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