[PATCH v2] mm/compaction : do optimazition when the migration scanner gets no page

[Huang Shijie <b32955@freescale.com>]

In the real tests, there are maybe many times the cc->nr_migratepages is zero,
but isolate_migratepages() returns ISOLATE_SUCCESS.

Memory in our mx6q board:
	2G memory, 8192 pages per page block

We use the following command to test in two types system loads:
	#echo 1 > /proc/sys/vm/compact_memory

Test Result:
	[1] little load(login in the ubuntu):
		all the scanned pageblocks	: 79
		pageblocks which get no pages	: 46

		The ratio of `get no pages` pageblock is 58.2%.

	[2] heavy load(start thunderbird, firefox, ..etc):
		all the scanned pageblocks	: 89
		pageblocks which get no pages	: 36

		The ratio of `get no pages` pageblock is 40.4%.

In order to get better performance, we should check the number of the
really isolated pages. And do the optimazition for this case.

Also fix the confused comments(from Mel Gorman).

Tested this patch in MX6Q board.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Mel Gorman <mgorman@suse.de>
---
 mm/compaction.c |   28 ++++++++++++++++------------
 1 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index f4f514d..41d1b72a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -246,8 +246,8 @@ static bool too_many_isolated(struct zone *zone)
 /* possible outcome of isolate_migratepages */
 typedef enum {
 	ISOLATE_ABORT,		/* Abort compaction now */
-	ISOLATE_NONE,		/* No pages isolated, continue scanning */
-	ISOLATE_SUCCESS,	/* Pages isolated, migrate */
+	ISOLATE_NONE,		/* No pages scanned, consider next pageblock*/
+	ISOLATE_SUCCESS,	/* Pages scanned and maybe isolated, migrate */
 } isolate_migrate_t;
 
 /*
@@ -542,7 +542,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
 
 	while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
 		unsigned long nr_migrate, nr_remaining;
-		int err;
+		int err = 0;
 
 		switch (isolate_migratepages(zone, cc)) {
 		case ISOLATE_ABORT:
@@ -554,17 +554,21 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
 			;
 		}
 
-		nr_migrate = cc->nr_migratepages;
-		err = migrate_pages(&cc->migratepages, compaction_alloc,
-				(unsigned long)cc, false,
-				cc->sync);
-		update_nr_listpages(cc);
-		nr_remaining = cc->nr_migratepages;
+		nr_migrate = nr_remaining = cc->nr_migratepages;
+		if (nr_migrate) {
+			err = migrate_pages(&cc->migratepages, compaction_alloc,
+					(unsigned long)cc, false,
+					cc->sync);
+			update_nr_listpages(cc);
+			nr_remaining = cc->nr_migratepages;
+			count_vm_events(COMPACTPAGES,
+					nr_migrate - nr_remaining);
+			if (nr_remaining)
+				count_vm_events(COMPACTPAGEFAILED,
+						nr_remaining);
+		}
 
 		count_vm_event(COMPACTBLOCKS);
-		count_vm_events(COMPACTPAGES, nr_migrate - nr_remaining);
-		if (nr_remaining)
-			count_vm_events(COMPACTPAGEFAILED, nr_remaining);
 		trace_mm_compaction_migratepages(nr_migrate - nr_remaining,
 						nr_remaining);
 
-- 
1.7.3.2


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