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Message-ID: <85d83be2-02f8-4ef6-91c7-ff920e47d834@fujitsu.com>
Date: Thu, 24 Jul 2025 10:39:40 +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 2025/7/23 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?
>
>>
>>
>
> 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
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