[PATCH 08/25] mm, compaction: Always finish scanning of a full pageblock

[Mel Gorman]

When compaction is finishing, it uses a flag to ensure the pageblock
is complete but it makes sense to always complete migration of a
pageblock. Minimally, skip information is based on a pageblock and
partially scanned pageblocks may incur more scanning in the future. The
pageblock skip handling also becomes more strict later in the series and
the hint is more useful if a complete pageblock was always scanned.

The potentially impacts latency as more scanning is done but it's not a
consistent win or loss as the scanning is not always a high percentage
of the pageblock and sometimes it is offset by future reductions
in scanning. Hence, the results are not presented this time due to a
misleading mix of gains/losses without any clear pattern. However, full
scanning of the pageblock is important for later patches.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/compaction.c | 19 ++++++++-----------
 mm/internal.h   |  1 -
 2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 94d1e5b062ea..8bf2090231a3 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1347,16 +1347,14 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 	if (is_via_compact_memory(cc->order))
 		return COMPACT_CONTINUE;
 
-	if (cc->finishing_block) {
-		/*
-		 * We have finished the pageblock, but better check again that
-		 * we really succeeded.
-		 */
-		if (IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
-			cc->finishing_block = false;
-		else
-			return COMPACT_CONTINUE;
-	}
+	/*
+	 * Always finish scanning a pageblock to reduce the possibility of
+	 * fallbacks in the future. This is particularly important when
+	 * migration source is unmovable/reclaimable but it's not worth
+	 * special casing.
+	 */
+	if (!IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
+		return COMPACT_CONTINUE;
 
 	/* Direct compactor: Is a suitable page free? */
 	for (order = cc->order; order < MAX_ORDER; order++) {
@@ -1398,7 +1396,6 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 				return COMPACT_SUCCESS;
 			}
 
-			cc->finishing_block = true;
 			return COMPACT_CONTINUE;
 		}
 	}
diff --git a/mm/internal.h b/mm/internal.h
index c6f794ad21a9..edb4029f64c8 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -202,7 +202,6 @@ struct compact_control {
 	bool direct_compaction;		/* False from kcompactd or /proc/... */
 	bool whole_zone;		/* Whole zone should/has been scanned */
 	bool contended;			/* Signal lock or sched contention */
-	bool finishing_block;		/* Finishing current pageblock */
 };
 
 unsigned long
-- 
2.16.4