Re: [PATCH v1] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting

[Vlastimil Babka <vbabka@suse.cz>]

On 7/29/25 05:51, Ruan Shiyang wrote:
> 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@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>
> Reported-by: Yasunori Gotou (Fujitsu) <y-goto@fujitsu.com>
> Suggested-by: Huang Ying <ying.huang@linux.alibaba.com>
> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
> Signed-off-by: Ruan Shiyang <ruansy.fnst@fujitsu.com>

Besides my nit, LGTM.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> 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",