linux-kernel - Re: [PATCH RFC v3] mm: memory-tiering: Fix PGPROMOTE

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Message-ID: <87cy9r38ny.fsf@DESKTOP-5N7EMDA>
Date: Wed, 23 Jul 2025 11:09:21 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Ruan Shiyang <ruansy.fnst@...itsu.com>
Cc: linux-mm@...ck.org,  linux-kernel@...r.kernel.org,  lkp@...el.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,  Li Zhijian
 <lizhijian@...itsu.com>,  Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH RFC v3] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE
 counting

Ruan Shiyang <ruansy.fnst@...itsu.com> writes:

> From: Li Zhijian <lizhijian@...itsu.com>
>
> ===
> 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.
> ===

This isn't the popular place for changelog, please refer to other patch
email.

> 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.

Yes.  Naming is hard.  I guess that the name comes from the promotion
that isn't rate limited.  I have asked Deepseek that what is the good
abbreviation for "not rate limited".  Its answer is "NRL".  I don't know
whether it's good.  However, "NOT_RATE_LIMITED" appears too long.

>
>

The empty line is unnecessary.

> Cc: Huang Ying <ying.huang@...ux.alibaba.com>

Suggested-by: Huang Ying <ying.huang@...ux.alibaba.com>

> 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>
> Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
> Signed-off-by: Ruan Shiyang <ruansy.fnst@...itsu.com>
> ---
>  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",

---
Best Regards,
Huang, Ying