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

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Message-ID: <982da1b2-0024-4c01-b586-02c0b8a41e95@fujitsu.com>
Date: Fri, 25 Jul 2025 10:20:44 +0800
From: Shiyang Ruan <ruansy.fnst@...itsu.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.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



在 2025/7/24 15:36, Huang, Ying 写道:
> Shiyang Ruan <ruansy.fnst@...itsu.com> writes:
> 
>> 在 2025/7/23 11:09, Huang, Ying 写道:
>>> 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.
>>
>> OK. I'll move this part down below.>
>>>> 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.
>>
>> "NRL" Sounds good to me.
>>
>> I'm thinking another one: since it's not rate limited, it could be
>> migrated quickly/fast.  How about PGPROMOTE_CANDIDATE_FAST?
> 
> This sounds good to me, Thanks!

Gemini 2.5 gave me a more radical name for it:

/*
  * Candidate pages for promotion based on hint fault latency. This counter
  * is used by the feedback mechanism to control the promotion rate and
  * adjust the hot threshold.
  */
PGPROMOTE_CANDIDATE,
/*
  * Pages promoted aggressively to a fast-tier node when it has sufficient
  * free space. 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.
  */
PGPROMOTED_AGGRESSIVE,

I think this one is concise and easy to understand with the comments. What do 
you think?  If this one is not appropriate, then I will go with "_NRL" as you 
suggested.


--
Thanks,
Ruan.

> 
> ---
> Best Regards,
> Huang, Ying
> 
>>
>>>
>>>>
>>>>
>>> The empty line is unnecessary.
>>
>> OK.>
>>>> Cc: Huang Ying <ying.huang@...ux.alibaba.com>
>>> Suggested-by: Huang Ying <ying.huang@...ux.alibaba.com>
>>
>> OK.
>>
>>
>> --
>> Thanks,
>> Ruan.
>>
>>>
>>>> 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