[RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

["Barry Song (Xiaomi)" <baohua@kernel.org>]

From: lyugaofei <lyugaofei@xiaomi.com>

This partially restores the reclaim behavior introduced in Yu
Zhao's initial MGLRU commit, ac35a4902370 ("mm: multi-gen LRU:
minimal implementation"):
	/*
	 * It's also ideal to spread pages out evenly, i.e., 1/(MIN_NR_GENS+1)
	 * of the total number of pages for each generation. A reasonable range
	 * for this average portion is [1/MIN_NR_GENS, 1/(MIN_NR_GENS+2)]. The
	 * aging cares about the upper bound of hot pages, while the eviction
	 * cares about the lower bound of cold pages.
	 */
	if (young * MIN_NR_GENS > total)
		return true;
	if (old * (MIN_NR_GENS + 2) < total)
		return true;

But with a stricter condition: the younger generations must contain
at least four times as many folios as the older generations. This
allows aging to keep folios of the preferred type spread across the
reclaimable generations.

Signed-off-by: lyugaofei <lyugaofei@xiaomi.com>
Co-developed-by: Barry Song (Xiaomi) <baohua@kernel.org>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d6bd64b4dced..7c13dedb0b1f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4956,9 +4956,40 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	return scanned;
 }
 
+static bool lru_gen_imbalanced(struct lruvec *lruvec, int type,
+		unsigned long max_seq, unsigned long min_seq,
+		int swappiness)
+{
+	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	unsigned long young = 0, old = 0, seq;
+
+	/*
+	 * reclaim is forced to a single type in those cases, so there is
+	 * no need to consider the swappiness bias
+	 */
+	if (swappiness == MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
+		return false;
+
+	for (seq = min_seq; seq <= max_seq; seq++) {
+		int gen = lru_gen_from_seq(seq);
+		unsigned long size = 0;
+		int zone;
+
+		for (zone = 0; zone < MAX_NR_ZONES; zone++)
+			size += max(READ_ONCE(lrugen->nr_pages[gen][type][zone]), 0L);
+
+		if (seq + MIN_NR_GENS > max_seq)
+			young += size;
+		else
+			old += size;
+	}
+	return young > old * 4;
+}
+
 static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
 			     struct scan_control *sc, int swappiness)
 {
+	int type = get_type_to_scan(lruvec, swappiness);
 	DEFINE_MIN_SEQ(lruvec);
 
 	/* have to run aging, since eviction is not possible anymore */
@@ -4970,7 +5001,11 @@ static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
 		return false;
 
 	/* better to run aging even though eviction is still possible */
-	return evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS == max_seq;
+	if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS == max_seq)
+		return true;
+
+	/* Run aging if the preferred type is severely imbalanced across gens */
+	return lru_gen_imbalanced(lruvec, type, max_seq, min_seq[type], swappiness);
 }
 
 static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
-- 
2.39.3 (Apple Git-146)