[PATCH 6/6] mm, compaction: decouple updating pageblock_skip and cached pfn

[Vlastimil Babka]

The pageblock_skip bitmap and cached scanner pfn's are two mechanisms in
compaction to prevent rescanning pages where isolation has recently failed
or they were scanned during the previous compaction attempt.

Currently, both kinds of information are updated via update_pageblock_skip(),
which is suboptimal for the cached scanner pfn's:

- The condition "isolation has failed in the pageblock" checked by
  update_pageblock_skip() may be valid for the pageblock_skip bitmap, but makes
  less sense for cached pfn's. There's little point for the next compaction
  attempt to scan again a pageblock where all pages that could be isolated were
  already processed.

- whole pageblocks can be skipped at the level of isolate_migratepages() or
  isolate_freepages() before going into the corresponding _block() function.
  Not updating cached scanner positions at the higher level may again result
  in extra iterations.

This patch moves updating cached scanner pfn's from update_pageblock_skip()
to dedicated functions, which are called directly from isolate_migratepages()
and isolate_freepages(), resolving both inefficiencies.

During testing, the observed differences in compact_migrate_scanned and
compact_free_scanned were lost in the noise.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/compaction.c | 48 +++++++++++++++++++++++++-----------------------
 1 file changed, 25 insertions(+), 23 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 4a14084..c326607 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -261,17 +261,31 @@ void reset_isolation_suitable(pg_data_t *pgdat)
 	}
 }
 
+static inline void
+update_cached_migrate_pfn(struct zone *zone, unsigned long pfn,
+						enum migrate_mode mode)
+{
+	if (pfn > zone->compact_cached_migrate_pfn[0])
+		zone->compact_cached_migrate_pfn[0] = pfn;
+	if (mode != MIGRATE_ASYNC &&
+	    pfn > zone->compact_cached_migrate_pfn[1])
+		zone->compact_cached_migrate_pfn[1] = pfn;
+}
+
+static inline void
+update_cached_free_pfn(struct zone *zone, unsigned long pfn)
+{
+	if (pfn < zone->compact_cached_free_pfn)
+		zone->compact_cached_free_pfn = pfn;
+}
+
 /*
  * If no pages were isolated then mark this pageblock to be skipped in the
  * future. The information is later cleared by __reset_isolation_suitable().
  */
 static void update_pageblock_skip(struct compact_control *cc,
-			struct page *page, unsigned long nr_isolated,
-			bool migrate_scanner)
+			struct page *page, unsigned long nr_isolated)
 {
-	struct zone *zone = cc->zone;
-	unsigned long pfn;
-
 	if (cc->ignore_skip_hint)
 		return;
 
@@ -282,20 +296,6 @@ static void update_pageblock_skip(struct compact_control *cc,
 		return;
 
 	set_pageblock_skip(page);
-
-	pfn = page_to_pfn(page);
-
-	/* Update where async and sync compaction should restart */
-	if (migrate_scanner) {
-		if (pfn > zone->compact_cached_migrate_pfn[0])
-			zone->compact_cached_migrate_pfn[0] = pfn;
-		if (cc->mode != MIGRATE_ASYNC &&
-		    pfn > zone->compact_cached_migrate_pfn[1])
-			zone->compact_cached_migrate_pfn[1] = pfn;
-	} else {
-		if (pfn < zone->compact_cached_free_pfn)
-			zone->compact_cached_free_pfn = pfn;
-	}
 }
 #else
 static inline bool isolation_suitable(struct compact_control *cc,
@@ -305,8 +305,7 @@ static inline bool isolation_suitable(struct compact_control *cc,
 }
 
 static void update_pageblock_skip(struct compact_control *cc,
-			struct page *page, unsigned long nr_isolated,
-			bool migrate_scanner)
+			struct page *page, unsigned long nr_isolated)
 {
 }
 #endif /* CONFIG_COMPACTION */
@@ -540,7 +539,7 @@ isolate_fail:
 
 	/* Update the pageblock-skip if the whole pageblock was scanned */
 	if (blockpfn == end_pfn)
-		update_pageblock_skip(cc, valid_page, total_isolated, false);
+		update_pageblock_skip(cc, valid_page, total_isolated);
 
 	count_compact_events(COMPACTFREE_SCANNED, nr_scanned);
 	if (total_isolated)
@@ -843,7 +842,7 @@ isolate_success:
 	 * if the whole pageblock was scanned without isolating any page.
 	 */
 	if (low_pfn == end_pfn)
-		update_pageblock_skip(cc, valid_page, nr_isolated, true);
+		update_pageblock_skip(cc, valid_page, nr_isolated);
 
 	trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn,
 						nr_scanned, nr_isolated);
@@ -1043,6 +1042,7 @@ static void isolate_freepages(struct compact_control *cc)
 	 * and the loop terminated due to isolate_start_pfn < low_pfn
 	 */
 	cc->free_pfn = isolate_start_pfn;
+	update_cached_free_pfn(zone, isolate_start_pfn);
 }
 
 /*
@@ -1177,6 +1177,8 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
 	acct_isolated(zone, cc);
 	/* Record where migration scanner will be restarted. */
 	cc->migrate_pfn = low_pfn;
+	update_cached_migrate_pfn(zone, low_pfn, cc->mode);
+
 
 	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
 }
-- 
2.1.4

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