mm/compaction: stop the isolation when we isolate enough freepage

Currently, freepage isolation in one pageblock doesn't consider how many
freepages we isolate. When I traced flow of compaction, compaction
sometimes isolates more than 256 freepages to migrate just 32 pages.

In this patch, freepage isolation is stopped at the point that we
have more isolated freepage than isolated page for migration. This
results in slowing down free page scanner and make compaction success
rate higher.

stress-highalloc test in mmtests with non movable order 7 allocation shows
increase of compaction success rate.

Compaction success rate (Compaction success * 100 / Compaction stalls, %)
27.13 : 31.82

pfn where both scanners meets on compaction complete
(separate test due to enormous tracepoint buffer)
(zone_start=4096, zone_end=1048576)
586034 : 654378

In fact, I didn't fully understand why this patch results in such good
result. There was a guess that not used freepages are released to pcp list
and on next compaction trial we won't isolate them again so compaction
success rate would decrease. To prevent this effect, I tested with adding
pcp drain code on release_freepages(), but, it has no good effect.

Anyway, this patch reduces waste time to isolate unneeded freepages so
seems reasonable.

Vlastimil said:

: I briefly tried it on top of the pivot-changing series and with order-9
: allocations it reduced free page scanned counter by almost 10%.  No effect
: on success rates (maybe because pivot changing already took care of the
: scanners meeting problem) but the scanning reduction is good on its own.
:
: It also explains why e14c720e ("mm, compaction: remember position
: within pageblock in free pages scanner") had less than expected
: improvements.  It would only actually stop within pageblock in case of
: async compaction detecting contention.  I guess that's also why the
: infinite loop problem fixed by 1d5bfe1f affected so relatively few
: people.
Signed-off-by: NJoonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Tested-by: NVlastimil Babka <vbabka@suse.cz>
Reviewed-by: NZhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -490,6 +490,13 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
 
 		/* If a page was split, advance to the end of it */
 		if (isolated) {
+			cc->nr_freepages += isolated;
+			if (!strict &&
+				cc->nr_migratepages <= cc->nr_freepages) {
+				blockpfn += isolated;
+				break;
+			}
+
 			blockpfn += isolated - 1;
 			cursor += isolated - 1;
 			continue;
@@ -899,7 +906,6 @@ static void isolate_freepages(struct compact_control *cc)
 	unsigned long isolate_start_pfn; /* exact pfn we start at */
 	unsigned long block_end_pfn;	/* end of current pageblock */
 	unsigned long low_pfn;	     /* lowest pfn scanner is able to scan */
-	int nr_freepages = cc->nr_freepages;
 	struct list_head *freelist = &cc->freepages;
 
 	/*
@@ -924,11 +930,11 @@ static void isolate_freepages(struct compact_control *cc)
 	 * pages on cc->migratepages. We stop searching if the migrate
 	 * and free page scanners meet or enough free pages are isolated.
 	 */
-	for (; block_start_pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
+	for (; block_start_pfn >= low_pfn &&
+			cc->nr_migratepages > cc->nr_freepages;
 				block_end_pfn = block_start_pfn,
 				block_start_pfn -= pageblock_nr_pages,
 				isolate_start_pfn = block_start_pfn) {
-		unsigned long isolated;
 
 		/*
 		 * This can iterate a massively long zone without finding any
@@ -953,9 +959,8 @@ static void isolate_freepages(struct compact_control *cc)
 			continue;
 
 		/* Found a block suitable for isolating free pages from. */
-		isolated = isolate_freepages_block(cc, &isolate_start_pfn,
+		isolate_freepages_block(cc, &isolate_start_pfn,
 					block_end_pfn, freelist, false);
-		nr_freepages += isolated;
 
 		/*
 		 * Remember where the free scanner should restart next time,
@@ -987,8 +992,6 @@ static void isolate_freepages(struct compact_control *cc)
 	 */
 	if (block_start_pfn < low_pfn)
 		cc->free_pfn = cc->migrate_pfn;
-
-	cc->nr_freepages = nr_freepages;
 }
 
 /*