Re: [PATCH RFC v3] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting

[Shiyang Ruan <ruansy.fnst@fujitsu.com>]

在 2025/7/23 11:09, Huang, Ying 写道:
> Ruan Shiyang <ruansy.fnst@fujitsu.com> writes:
> 
>> From: Li Zhijian <lizhijian@fujitsu.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@linux.alibaba.com>
> 
> Suggested-by: Huang Ying <ying.huang@linux.alibaba.com>

OK.


--
Thanks,
Ruan.

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