linux-kernel - [PATCH v1] mm: memory-tiering: Fix PGPROMOTE

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Message-ID: <20250729035101.1601407-1-ruansy.fnst@fujitsu.com>
Date: Tue, 29 Jul 2025 11:51:01 +0800
From: Ruan Shiyang <ruansy.fnst@...itsu.com>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	lkp@...el.com,
	ying.huang@...ux.alibaba.com,
	akpm@...ux-foundation.org,
	y-goto@...itsu.com,
	mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	vincent.guittot@...aro.org,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	mgorman@...e.de,
	vschneid@...hat.com,
	Ben Segall <bsegall@...gle.com>,
	Li Zhijian <lizhijian@...itsu.com>
Subject: [PATCH v1] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting

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_NRL
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.

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Valentin Schneider <vschneid@...hat.com>
Reported-by: Yasunori Gotou (Fujitsu) <y-goto@...itsu.com>
Suggested-by: Huang Ying <ying.huang@...ux.alibaba.com>
Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
Signed-off-by: Ruan Shiyang <ruansy.fnst@...itsu.com>
---
Changes since RFC v3:
  1. change the naming of new added stat to PGPROMOTE_CANDIDATE_NRL.
  2. improve the description of the two stats.
---
 include/linux/mmzone.h | 16 +++++++++++++++-
 kernel/sched/fair.c    |  5 +++--
 mm/vmstat.c            |  1 +
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 283913d42d7b..4345996a7d5a 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -230,7 +230,21 @@ enum node_stat_item {
 #endif
 #ifdef CONFIG_NUMA_BALANCING
 	PGPROMOTE_SUCCESS,	/* promote successfully */
-	PGPROMOTE_CANDIDATE,	/* candidate pages to promote */
+	/**
+	 * Candidate pages for promotion based on hint fault latency.  This
+	 * counter is used to control the promotion rate and adjust the hot
+	 * threshold.
+	 */
+	PGPROMOTE_CANDIDATE,
+	/**
+	 * Not rate-limited (NRL) candidate pages for those can be promoted
+	 * without considering hot threshold because of enough free pages in
+	 * fast-tier node.  These promotions bypass the regular hotness checks
+	 * and do NOT influence the promotion rate-limiter or
+	 * threshold-adjustment logic.
+	 * This is for statistics/monitoring purposes.
+	 */
+	PGPROMOTE_CANDIDATE_NRL,
 #endif
 	/* PGDEMOTE_*: pages demoted */
 	PGDEMOTE_KSWAPD,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a14da5396fb..4022c9c1f346 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1940,11 +1940,13 @@ 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_NRL, nr);
 			return true;
 		}
 
@@ -1958,8 +1960,7 @@ 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..bb0d2b330dd5 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_nrl",
 #endif
 	"pgdemote_kswapd",
 	"pgdemote_direct",
-- 
2.43.0