[PATCH RFC v3] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting

[Ruan Shiyang]

From: Li Zhijian <lizhijian@xxxxxxxxxxx>

===
Changes since v2:
  1. According to Huang's suggestion, add a new stat to not count these
  pages into PGPROMOTE_CANDIDATE, to avoid changing the rate limit
  mechanism.
===

Goto-san reported confusing pgpromote statistics where the
pgpromote_success count significantly exceeded pgpromote_candidate.

On a system with three nodes (nodes 0-1: DRAM 4GB, node 2: NVDIMM 4GB):
 # Enable demotion only
 echo 1 > /sys/kernel/mm/numa/demotion_enabled
 numactl -m 0-1 memhog -r200 3500M >/dev/null &
 pid=$!
 sleep 2
 numactl memhog -r100 2500M >/dev/null &
 sleep 10
 kill -9 $pid # terminate the 1st memhog
 # Enable promotion
 echo 2 > /proc/sys/kernel/numa_balancing

After a few seconds, we observeed `pgpromote_candidate < pgpromote_success`
$ grep -e pgpromote /proc/vmstat
pgpromote_success 2579
pgpromote_candidate 0

In this scenario, after terminating the first memhog, the conditions for
pgdat_free_space_enough() are quickly met, and triggers promotion.
However, these migrated pages are only counted for in PGPROMOTE_SUCCESS,
not in PGPROMOTE_CANDIDATE.

To solve this confusing statistics, introduce this
PGPROMOTE_CANDIDATE_NOLIMIT to count the missed promotion pages.  And
also, not counting these pages into PGPROMOTE_CANDIDATE is to avoid
changing the existing algorithm or performance of the promotion rate
limit.

Perhaps PGPROMOTE_CANDIDATE_NOLIMIT is not well named, please comment if
you have a better idea.


Cc: Huang Ying <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Reported-by: Yasunori Gotou (Fujitsu) <y-goto@xxxxxxxxxxx>
Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
Signed-off-by: Ruan Shiyang <ruansy.fnst@xxxxxxxxxxx>
---
 include/linux/mmzone.h | 2 ++
 kernel/sched/fair.c    | 6 ++++--
 mm/vmstat.c            | 1 +
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 283913d42d7b..6216e2eecf3b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -231,6 +231,8 @@ enum node_stat_item {
 #ifdef CONFIG_NUMA_BALANCING
 	PGPROMOTE_SUCCESS,	/* promote successfully */
 	PGPROMOTE_CANDIDATE,	/* candidate pages to promote */
+	PGPROMOTE_CANDIDATE_NOLIMIT,	/* candidate pages without considering
+					 * hot threshold */
 #endif
 	/* PGDEMOTE_*: pages demoted */
 	PGDEMOTE_KSWAPD,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a14da5396fb..12dac3519c49 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1940,11 +1940,14 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 		struct pglist_data *pgdat;
 		unsigned long rate_limit;
 		unsigned int latency, th, def_th;
+		long nr = folio_nr_pages(folio);
 
 		pgdat = NODE_DATA(dst_nid);
 		if (pgdat_free_space_enough(pgdat)) {
 			/* workload changed, reset hot threshold */
 			pgdat->nbp_threshold = 0;
+			mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE_NOLIMIT,
+					    nr);
 			return true;
 		}
 
@@ -1958,8 +1961,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
 		if (latency >= th)
 			return false;
 
-		return !numa_promotion_rate_limit(pgdat, rate_limit,
-						  folio_nr_pages(folio));
+		return !numa_promotion_rate_limit(pgdat, rate_limit, nr);
 	}
 
 	this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a78d70ddeacd..ca44a2dd5497 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1272,6 +1272,7 @@ const char * const vmstat_text[] = {
 #ifdef CONFIG_NUMA_BALANCING
 	"pgpromote_success",
 	"pgpromote_candidate",
+	"pgpromote_candidate_nolimit",
 #endif
 	"pgdemote_kswapd",
 	"pgdemote_direct",
-- 
2.43.0