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, 7 insertions, 24 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index 56331f5124ba..3c60e3d5237e 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -822,22 +822,6 @@ static void compaction_free(struct page *page, unsigned long data)
         cc->nr_freepages++;
 }
 
-/*
- * We cannot control nr_migratepages fully when migration is running as
- * migrate_pages() has no knowledge of of compact_control.  When migration is
- * complete, we count the number of pages on the list by hand.
- */
-static void update_nr_listpages(struct compact_control *cc)
-{
-        int nr_migratepages = 0;
-        struct page *page;
-
-        list_for_each_entry(page, &cc->migratepages, lru)
-                nr_migratepages++;
-
-        cc->nr_migratepages = nr_migratepages;
-}
-
 /* possible outcome of isolate_migratepages */
 typedef enum {
         ISOLATE_ABORT,          /* Abort compaction now */
@@ -1032,7 +1016,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
         migrate_prep_local();
 
         while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
-                unsigned long nr_migrate, nr_remaining;
                 int err;
 
                 switch (isolate_migratepages(zone, cc)) {
@@ -1047,20 +1030,20 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
                         ;
                 }
 
-                nr_migrate = cc->nr_migratepages;
+                if (!cc->nr_migratepages)
+                        continue;
+
                 err = migrate_pages(&cc->migratepages, compaction_alloc,
                                 compaction_free, (unsigned long)cc, cc->mode,
                                 MR_COMPACTION);
-                update_nr_listpages(cc);
-                nr_remaining = cc->nr_migratepages;
 
-                trace_mm_compaction_migratepages(nr_migrate - nr_remaining,
-                                                nr_remaining);
+                trace_mm_compaction_migratepages(cc->nr_migratepages, err,
+                                                        &cc->migratepages);
 
-                /* Release isolated pages not migrated */
+                /* All pages were either migrated or will be released */
+                cc->nr_migratepages = 0;
                 if (err) {
                         putback_movable_pages(&cc->migratepages);
-                        cc->nr_migratepages = 0;
                         /*
                          * migrate_pages() may return -ENOMEM when scanners meet
                          * and we want compact_finished() to detect it