[RFC PATCH v4 15/16] mm/mglru: dynamically scale aging threshold in lru_gen_imbalanced()

Currently, lru_gen_imbalanced() uses a fixed threshold (MAX_NR_GENS)
to determine whether the young-to-old folio ratio warrants aging the
preferred LRU type.

This fixed threshold may not scale well with large memory systems.
Borrow the adaptive ratio calculation from inactive_is_low() by
scaling the threshold with the square root of the memory size in GB.

Suggested-by: Zicheng Wang <[email protected]>
Signed-off-by: Barry Song (Xiaomi) <[email protected]>
---
 mm/vmscan.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d6fac5b91ac1..c0350a8b61d1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5105,6 +5105,7 @@ static bool lru_gen_imbalanced(struct lruvec *lruvec, unsigned long max_seq,
 {
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	unsigned long young = 0, old = 0, lag = 0;
+	unsigned long inactive_ratio, gb;
 	DEFINE_MIN_SEQ(lruvec);
 
 	/* we still have enough generations to reclaim */
@@ -5127,7 +5128,13 @@ static bool lru_gen_imbalanced(struct lruvec *lruvec, unsigned long max_seq,
 	if (min_seq[!type] + MAX_NR_GENS == max_seq + 1)
 		lag += lruvec_gen_size(lrugen, !type, min_seq[!type]);
 
-	return young > old * MAX_NR_GENS && (lag < MAX_LRU_BATCH ||
+	/*
+	 * Borrow the adaptive ratio from inactive_is_low(), and scale
+	 * it by sqrt(MAX_NR_GENS) to make aging less aggressive
+	 */
+	gb = (young + old) >> (30 - PAGE_SHIFT);
+	inactive_ratio = gb ? int_sqrt(10 * gb * MAX_NR_GENS) : MAX_NR_GENS;
+	return young > old * inactive_ratio && (lag < MAX_LRU_BATCH ||
 	       (is_extreme_swappiness(swappiness) && sc->priority > 2));
 }
 
-- 
2.34.1