Commit 337135e6 authored Jul 29, 2025 by Ruan Shiyang Committed by Andrew Morton Sep 13, 2025

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 these confusing statistics, introduce 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.

Link: https://lkml.kernel.org/r/20250901090122.124262-1-ruansy.fnst@fujitsu.com
Link: https://lkml.kernel.org/r/20250729035101.1601407-1-ruansy.fnst@fujitsu.com


Fixes: c6833e10 ("memory tiering: rate limit NUMA migration throughput")
Co-developed-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Ruan Shiyang <ruansy.fnst@fujitsu.com>
Reported-by: Yasunori Gotou (Fujitsu) <y-goto@fujitsu.com>
Suggested-by: Huang Ying <ying.huang@linux.alibaba.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent 915a4022

include/linux/mmzone.h

+15 −1

Original line number	Diff line number	Diff line
		@@ -234,7 +234,21 @@ 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,

kernel/sched/fair.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -1923,11 +1923,13 @@ 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;
		}

		@@ -1941,8 +1943,7 @@ 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);

mm/vmstat.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -1280,6 +1280,7 @@ const char * const vmstat_text[] = {
		#ifdef CONFIG_NUMA_BALANCING
		[I(PGPROMOTE_SUCCESS)] = "pgpromote_success",
		[I(PGPROMOTE_CANDIDATE)] = "pgpromote_candidate",
		[I(PGPROMOTE_CANDIDATE_NRL)] = "pgpromote_candidate_nrl",
		#endif
		[I(PGDEMOTE_KSWAPD)] = "pgdemote_kswapd",
		[I(PGDEMOTE_DIRECT)] = "pgdemote_direct",