mm: vmscan: limit the range of LRU type balancing

When LRU cost only shows up on one list, we abruptly stop scanning that
list altogether.  That's an extreme reaction: by the time the other list
starts thrashing and the pendulum swings back, we may have no recent age
information on the first list anymore, and we could have significant
latencies until the scanner has caught up.

Soften this change in the feedback system by ensuring that no list
receives less than a third of overall pressure, and only distribute the
other 66% according to LRU cost.  This ensures that we maintain a minimum
rate of aging on the entire workingset while it's being pressured, while
still allowing a generous rate of convergence when the relative sizes of
the lists need to adjust.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Link: http://lkml.kernel.org/r/20200520232525.798933-15-hannes@cmpxchg.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2237,12 +2237,11 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 			   unsigned long *nr)
 {
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+	unsigned long anon_cost, file_cost, total_cost;
 	int swappiness = mem_cgroup_swappiness(memcg);
 	u64 fraction[2];
 	u64 denominator = 0;	/* gcc */
-	unsigned long anon_prio, file_prio;
 	enum scan_balance scan_balance;
-	unsigned long totalcost;
 	unsigned long ap, fp;
 	enum lru_list lru;
 
@@ -2301,17 +2300,22 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	 * the relative IO cost of bringing back a swapped out
 	 * anonymous page vs reloading a filesystem page (swappiness).
 	 *
+	 * Although we limit that influence to ensure no list gets
+	 * left behind completely: at least a third of the pressure is
+	 * applied, before swappiness.
+	 *
 	 * With swappiness at 100, anon and file have equal IO cost.
 	 */
-	anon_prio = swappiness;
-	file_prio = 200 - anon_prio;
+	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;
 
-	totalcost = sc->anon_cost + sc->file_cost;
-	ap = anon_prio * (totalcost + 1);
-	ap /= sc->anon_cost + 1;
+	ap = swappiness * (total_cost + 1);
+	ap /= anon_cost + 1;
 
-	fp = file_prio * (totalcost + 1);
-	fp /= sc->file_cost + 1;
+	fp = (200 - swappiness) * (total_cost + 1);
+	fp /= file_cost + 1;
 
 	fraction[0] = ap;
 	fraction[1] = fp;