[PATCH 3/3] mm, lru_gen: try to prefetch next page when canning LRU

[Kairui Song <ryncsn@xxxxxxxxx>]

From: Kairui Song <kasong@xxxxxxxxxxx>

Prefetch for inactive/active LRU have been long exiting, apply the same
optimization for MGLRU.

Tested in a 4G memcg on a EPYC 7K62 with:

  memcached -u nobody -m 16384 -s /tmp/memcached.socket \
    -a 0766 -t 16 -B binary &

  memtier_benchmark -S /tmp/memcached.socket \
    -P memcache_binary -n allkeys \
    --key-minimum=1 --key-maximum=16000000 -d 1024 \
    --ratio=1:0 --key-pattern=P:P -c 2 -t 16 --pipeline 8 -x 6

Average result of 18 test runs:

Before:           44017.78 Ops/sec
After patch 1-3:  44890.50 Ops/sec (+1.8%)

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/vmscan.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index af1266129c1b..1e9d69e18443 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3764,10 +3764,12 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, bool can_swap)
 			VM_WARN_ON_ONCE_FOLIO(folio_is_file_lru(folio) != type, folio);
 			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
 
-			if (unlikely(list_is_first(&folio->lru, head)))
+			if (unlikely(list_is_first(&folio->lru, head))) {
 				prev = NULL;
-			else
+			} else {
 				prev = lru_to_folio(&folio->lru);
+				prefetchw(&prev->flags);
+			}
 
 			new_gen = folio_inc_gen(lruvec, folio, false, &batch);
 			lru_gen_try_inc_bulk(lrugen, folio, bulk_gen, new_gen, type, zone, &batch);
@@ -4434,10 +4436,12 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
 			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
 
 			scanned += delta;
-			if (unlikely(list_is_first(&folio->lru, head)))
+			if (unlikely(list_is_first(&folio->lru, head))) {
 				prev = NULL;
-			else
+			} else {
 				prev = lru_to_folio(&folio->lru);
+				prefetchw(&prev->flags);
+			}
 
 			if (sort_folio(lruvec, folio, sc, tier, bulk_gen, &batch))
 				sorted += delta;
-- 
2.43.0