[PATCH v2 6/8] mm: Count requests to free & nr freed per shrinker

[Kent Overstreet <kent.overstreet@gmail.com>]

The next step in this patch series for improving debugging of shrinker
related issues: keep counts of number of objects we request to free vs.
actually freed, and prints them in shrinker_to_text().

Shrinkers won't necessarily free all objects requested for a variety of
reasons, but if the two counts are wildly different something is likely
amiss.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
---
 include/linux/shrinker.h |  3 +++
 mm/vmscan.c              | 16 +++++++++++++---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index b5f411768b..12967748f9 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -79,6 +79,9 @@ struct shrinker {
 #endif
 	/* objs pending delete, per node */
 	atomic_long_t *nr_deferred;
+
+	atomic_long_t objects_requested_to_free;
+	atomic_long_t objects_freed;
 };
 #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 905bc23800..12562546a7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -711,16 +711,22 @@ void shrinker_to_text(struct printbuf *out, struct shrinker *shrinker)
 		pr_buf(out, "%s", shrinker->name);
 	else
 		pr_buf(out, "%ps:", shrinker->scan_objects);
+	pr_newline(out);
+	pr_indent_push(out, 2);
 
-	pr_buf(out, " objects: %lu", shrinker->count_objects(shrinker, &sc));
+	pr_buf(out, "objects:           %lu", shrinker->count_objects(shrinker, &sc));
+	pr_newline(out);
+	pr_buf(out, "requested to free: %lu", atomic_long_read(&shrinker->objects_requested_to_free));
+	pr_newline(out);
+	pr_buf(out, "objects freed:     %lu", atomic_long_read(&shrinker->objects_freed));
 	pr_newline(out);
 
 	if (shrinker->to_text) {
-		pr_indent_push(out, 2);
 		shrinker->to_text(out, shrinker);
-		pr_indent_pop(out, 2);
 		pr_newline(out);
 	}
+
+	pr_indent_pop(out, 2);
 }
 
 /**
@@ -846,12 +852,16 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		unsigned long ret;
 		unsigned long nr_to_scan = min(batch_size, total_scan);
 
+		atomic_long_add(nr_to_scan, &shrinker->objects_requested_to_free);
+
 		shrinkctl->nr_to_scan = nr_to_scan;
 		shrinkctl->nr_scanned = nr_to_scan;
 		ret = shrinker->scan_objects(shrinker, shrinkctl);
 		if (ret == SHRINK_STOP)
 			break;
+
 		freed += ret;
+		atomic_long_add(ret, &shrinker->objects_freed);
 
 		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
 		total_scan -= shrinkctl->nr_scanned;
-- 
2.35.2