mm: page_alloc: reduce cost of the fair zone allocation policy

The fair zone allocation policy round-robins allocations between zones
within a node to avoid age inversion problems during reclaim.  If the
first allocation fails, the batch counts are reset and a second attempt
made before entering the slow path.

One assumption made with this scheme is that batches expire at roughly
the same time and the resets each time are justified.  This assumption
does not hold when zones reach their low watermark as the batches will
be consumed at uneven rates.  Allocation failure due to watermark
depletion result in additional zonelist scans for the reset and another
watermark check before hitting the slowpath.

On UMA, the benefit is negligible -- around 0.25%.  On 4-socket NUMA
machine it's variable due to the variability of measuring overhead with
the vmstat changes.  The system CPU overhead comparison looks like

          3.16.0-rc3  3.16.0-rc3  3.16.0-rc3
             vanilla   vmstat-v5 lowercost-v5
User          746.94      774.56      802.00
System      65336.22    32847.27    40852.33
Elapsed     27553.52    27415.04    27368.46

However it is worth noting that the overall benchmark still completed
faster and intuitively it makes sense to take as few passes as possible
through the zonelists.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0bd77f730b38..318df7051850 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -534,6 +534,7 @@ typedef enum {
 	ZONE_WRITEBACK,			/* reclaim scanning has recently found
 					 * many pages under writeback
 					 */
+	ZONE_FAIR_DEPLETED,		/* fair zone policy batch depleted */
 } zone_flags_t;
 
 static inline void zone_set_flag(struct zone *zone, zone_flags_t flag)
@@ -571,6 +572,11 @@ static inline int zone_is_reclaim_locked(const struct zone *zone)
 	return test_bit(ZONE_RECLAIM_LOCKED, &zone->flags);
 }
 
+static inline int zone_is_fair_depleted(const struct zone *zone)
+{
+	return test_bit(ZONE_FAIR_DEPLETED, &zone->flags);
+}
+
 static inline int zone_is_oom_locked(const struct zone *zone)
 {
 	return test_bit(ZONE_OOM_LOCKED, &zone->flags);