linux-kernel - [PATCH] mm/vmscan: simplify the calculation of fractions for SCAN

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Date: Wed, 28 Feb 2024 10:55:00 +0900
From: Byungchul Park <byungchul@...com>
To: akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	kernel_team@...ynix.com,
	hannes@...xchg.org
Subject: [PATCH] mm/vmscan: simplify the calculation of fractions for SCAN_FRACT

The current way to calculate fractions for SACN_FRACT is little readable
and more complicated than it should be.  It also performs unnecessary
division and adjustment to avoid zero operands.  Prune away by
multiplying the fractions by 'anon_cost * file_cost / (3 * total_cost)':

where:
   total_cost = sc->anon_cost + sc->file_cost
   anon_cost = total_cost + sc->anon_cost
   file_cost = total_cost + sc->file_cost

before:
   fraction[0] = swappiness * 3 * total_cost / anon_cost
   fraction[1] = (200 - swappiness) * 3 * total_cost / file_cost

after:
   fraction[0] = swappiness * file_cost
   fraction[1] = (200 - swappiness) * anon_cost

Worth noting that this patch doesn't change the formula.

Signed-off-by: Byungchul Park <byungchul@...com>
---
 mm/vmscan.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4657440854db..7b33fcc1cbdc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2339,7 +2339,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	u64 fraction[ANON_AND_FILE];
 	u64 denominator = 0;	/* gcc */
 	enum scan_balance scan_balance;
-	unsigned long ap, fp;
 	enum lru_list lru;
 
 	/*
@@ -2416,17 +2415,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	total_cost = sc->anon_cost + sc->file_cost;
 	anon_cost = total_cost + sc->anon_cost;
 	file_cost = total_cost + sc->file_cost;
-	total_cost = anon_cost + file_cost;
 
-	ap = swappiness * (total_cost + 1);
-	ap /= anon_cost + 1;
-
-	fp = (200 - swappiness) * (total_cost + 1);
-	fp /= file_cost + 1;
-
-	fraction[0] = ap;
-	fraction[1] = fp;
-	denominator = ap + fp;
+	fraction[0] = swappiness * file_cost;
+	fraction[1] = (200 - swappiness) * anon_cost;
+	denominator = fraction[0] + fraction[1];
 out:
 	for_each_evictable_lru(lru) {
 		int file = is_file_lru(lru);
-- 
2.17.1