[PATCH 4/5] mm/mglru: avoid redundant rotations of young memcgs

[Max Kellermann <max.kellermann@ionos.com>]

Concurrent reclaimers can request promotion after another reclaimer
has already moved a memcg into the young generation.  Requeueing it
into a random bin again adds list mutations without advancing the
old-generation scan.

Therefore, skip `MEMCG_LRU_YOUNG` when the memcg is already in the
default segment of the current young generation.

In my micro benchmark, this increases the request rate by 36% and
reduces the average duration of each write request by 27%.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 mm/vmscan.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 69976df5d537..922c59eb9629 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4445,6 +4445,14 @@ static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
 	else
 		VM_WARN_ON_ONCE(true);
 
+	if (op == MEMCG_LRU_YOUNG && old == new && lruvec->lrugen.seg == seg)
+		/*
+		 * This memcg is already in the young generation and
+		 * reshuffling it again cannot help the scan of the
+		 * old generation.
+		 */
+		goto unlock;
+
 	WRITE_ONCE(lruvec->lrugen.seg, seg);
 	WRITE_ONCE(lruvec->lrugen.gen, new);
 
@@ -4458,6 +4466,7 @@ static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
 	if (!pgdat->memcg_lru.nr_memcgs[old] && old == get_memcg_gen(pgdat->memcg_lru.seq))
 		WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
 
+unlock:
 	spin_unlock_irqrestore(&pgdat->memcg_lru.lock, flags);
 }
 
-- 
2.47.3