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Message-Id: <20220416004104.4089743-1-roman.gushchin@linux.dev>
Date: Fri, 15 Apr 2022 17:41:04 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Dave Chinner <dchinner@...hat.com>,
linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Shakeel Butt <shakeelb@...gle.com>,
Yang Shi <shy828301@...il.com>,
Roman Gushchin <roman.gushchin@...ux.dev>
Subject: [PATCH] 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will get a potentially slightly outdated "new scan count" value, but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.
Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
---
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
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