[PATCH 1/2] mm/compaction: do not count migratepages when unnecessary

Vlastimil Babka
2014-05-02T15:29:33Z

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>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
---
include/trace/events/compaction.h | 26 ++++++++++++++++++++++----
mm/compaction.c | 30 ++++++++----------------------
2 files changed, 30 insertions(+), 26 deletions(-)

diff --git a/include/trace/events/compaction.h b/include/trace/events/compaction.h
index 06f544e..bbd5e1f 100644
--- a/include/trace/events/compaction.h
+++ b/include/trace/events/compaction.h
@@ -5,7 +5,9 @@
#define _TRACE_COMPACTION_H

#include <linux/types.h>
+#include <linux/list.h>
#include <linux/tracepoint.h>
+#include <linux/mm_types.h>
#include <trace/events/gfpflags.h>

DECLARE_EVENT_CLASS(mm_compaction_isolate_template,
@@ -47,10 +49,11 @@ 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 +61,22 @@ TRACE_EVENT(mm_compaction_migratepages,
),

TP_fast_assign(
- __entry->nr_migrated = nr_migrated;
+ unsigned long nr_failed = 0;
+ struct page *page;
+
+ /*
+ * 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_entry(page, migratepages, lru)
+ nr_failed++;
+
+ __entry->nr_migrated = nr_all - nr_failed;
__entry->nr_failed = nr_failed;
),

diff --git a/mm/compaction.c b/mm/compaction.c
index ae1d0ae..873d7de 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -799,22 +799,6 @@ 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 */
@@ -1006,7 +990,6 @@ 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)) {
@@ -1021,16 +1004,16 @@ 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->sync ? MIGRATE_SYNC_LIGHT : MIGRATE_ASYNC,
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 */
if (err) {
@@ -1044,6 +1027,9 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
ret = COMPACT_PARTIAL;
goto out;
}
+ } else {
+ /* All pages were successfully migrated */
+ cc->nr_migratepages = 0;
}
}

--
1.8.4.5