+ mm-do-not-call-add_nr_deferred-with-zero-deferred.patch added to -mm tree

+ mm-do-not-call-add_nr_deferred-with-zero-deferred.patch added to -mm tree

[Andrew Morton]

The patch titled
     Subject: mm: do not call add_nr_deferred() with zero deferred
has been added to the -mm tree.  Its filename is
     mm-do-not-call-add_nr_deferred-with-zero-deferred.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-do-not-call-add_nr_deferred-with-zero-deferred.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-do-not-call-add_nr_deferred-with-zero-deferred.patch

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------------------------------------------------------
From: Roman Gushchin <roman.gushchin@linux.dev>
Subject: mm: do not call add_nr_deferred() with zero deferred

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get a potentially slightly outdated "new scan count" value, but it's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.

Link: https://lkml.kernel.org/r/20220416004104.4089743-1-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vmscan.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/vmscan.c~mm-do-not-call-add_nr_deferred-with-zero-deferred
+++ a/mm/vmscan.c
@@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(stru
 	 * 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;
_

Patches currently in -mm which might be from roman.gushchin@linux.dev are

mm-do-not-call-add_nr_deferred-with-zero-deferred.patch