[PATCH] mm: do not call add_nr_deferred() with zero deferred

[Roman Gushchin]

add_nr_deferred() is often called with next_deferred equal to 0.
For instance, it's happening under low memory pressure for any
shrinkers with a low number of cached objects. A corresponding trace
looks like:
  <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \
  super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1	     \
  unused scan count 0 new scan count 0 total_scan 0	     \
  last shrinker return val 0

  <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \
  super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1	     \
  unused scan count 0 new scan count 0 total_scan 0	     \
  last shrinker return val 0

  <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \
  super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \
  scan count 0 new scan count 0 total_scan 0	             \
  last shrinker return val 0

This lead to unnecessary checks and atomic operations, which can be
avoided by checking next_deferred for not being zero before calling
add_nr_deferred(). In this case the mm_shrink_slab_end trace point
will get a potentially slightly outdated "new scan count" value, but
it's totally fine.

Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
---
 mm/vmscan.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d4a7d2bd276d..19d3d4fa1aad 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	 * move the unused scan count back into the shrinker in a
 	 * manner that handles concurrent updates.
 	 */
-	new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
+	if (next_deferred)
+		new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
+	else
+		new_nr = nr;
 
 	trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan);
 	return freed;
-- 
2.35.1