mm/compaction: do not count migratepages when unnecessary

During compaction, update_nr_listpages() has been used to count remaining
non-migrated and free pages after a call to migrage_pages().  The
freepages counting has become unneccessary, and it turns out that
migratepages counting is also unnecessary in most cases.

The only situation when it's needed to count cc->migratepages is when
migrate_pages() returns with a negative error code.  Otherwise, the
non-negative return value is the number of pages that were not migrated,
which is exactly the count of remaining pages in the cc->migratepages
list.

Furthermore, any non-zero count is only interesting for the tracepoint of
mm_compaction_migratepages events, because after that all remaining
unmigrated pages are put back and their count is set to 0.

This patch therefore removes update_nr_listpages() completely, and changes
the tracepoint definition so that the manual counting is done only when
the tracepoint is enabled, and only when migrate_pages() returns a
negative error code.

Furthermore, migrate_pages() and the tracepoints won't be called when
there's nothing to migrate.  This potentially avoids some wasted cycles
and reduces the volume of uninteresting mm_compaction_migratepages events
where "nr_migrated=0 nr_failed=0".  In the stress-highalloc mmtest, this
was about 75% of the events.  The mm_compaction_isolate_migratepages event
is better for determining that nothing was isolated for migration, and
this one was just duplicating the info.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 4 deletions

diff --git a/include/trace/events/compaction.h b/include/trace/events/compaction.h
index 06f544ef2f6..c6814b917bd 100644
--- a/include/trace/events/compaction.h
+++ b/include/trace/events/compaction.h
@@ -5,6 +5,7 @@
 #define _TRACE_COMPACTION_H
 
 #include <linux/types.h>
+#include <linux/list.h>
 #include <linux/tracepoint.h>
 #include <trace/events/gfpflags.h>
 
@@ -47,10 +48,11 @@ DEFINE_EVENT(mm_compaction_isolate_template, mm_compaction_isolate_freepages,
 
 TRACE_EVENT(mm_compaction_migratepages,
 
-	TP_PROTO(unsigned long nr_migrated,
-		unsigned long nr_failed),
+	TP_PROTO(unsigned long nr_all,
+		int migrate_rc,
+		struct list_head *migratepages),
 
-	TP_ARGS(nr_migrated, nr_failed),
+	TP_ARGS(nr_all, migrate_rc, migratepages),
 
 	TP_STRUCT__entry(
 		__field(unsigned long, nr_migrated)
@@ -58,7 +60,22 @@ TRACE_EVENT(mm_compaction_migratepages,
 	),
 
 	TP_fast_assign(
-		__entry->nr_migrated = nr_migrated;
+		unsigned long nr_failed = 0;
+		struct list_head *page_lru;
+
+		/*
+		 * migrate_pages() returns either a non-negative number
+		 * with the number of pages that failed migration, or an
+		 * error code, in which case we need to count the remaining
+		 * pages manually
+		 */
+		if (migrate_rc >= 0)
+			nr_failed = migrate_rc;
+		else
+			list_for_each(page_lru, migratepages)
+				nr_failed++;
+
+		__entry->nr_migrated = nr_all - nr_failed;
 		__entry->nr_failed = nr_failed;
 	),