[PATCH 25/27] mm: page_alloc: Cache the last node whose dirty limit is reached

[Mel Gorman <mgorman@techsingularity.net>]

If a page is about to be dirtied then the page allocator attempts to limit
the total number of dirty pages that exists in any given zone. The call
to node_dirty_ok is expensive so this patch records if the last pgdat
examined hit the dirty limits. In some cases, this reduces the number
of calls to node_dirty_ok().

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/page_alloc.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 54cfe26dcc66..a6e6184d3e38 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2607,6 +2607,7 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 	struct zoneref *z;
 	struct page *page = NULL;
 	struct zone *zone;
+	struct pglist_data *last_pgdat_dirty_limit = NULL;
 
 	/*
 	 * Scan zonelist, looking for a zone with enough free.
@@ -2639,8 +2640,15 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 		 * will require awareness of nodes in the
 		 * dirty-throttling and the flusher threads.
 		 */
-		if (ac->spread_dirty_pages && !node_dirty_ok(zone->zone_pgdat))
-			continue;
+		if (ac->spread_dirty_pages) {
+			if (last_pgdat_dirty_limit == zone->zone_pgdat)
+				continue;
+
+			if (!node_dirty_ok(zone->zone_pgdat)) {
+				last_pgdat_dirty_limit = zone->zone_pgdat;
+				continue;
+			}
+		}
 
 		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
 		if (!zone_watermark_ok(zone, order, mark,
-- 
2.6.4

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