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Message-ID: <23765f70-e869-4ef6-8ddd-24c0b1548fb2@suse.cz>
Date: Fri, 29 Aug 2025 11:08:06 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Ruan Shiyang <ruansy.fnst@...itsu.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, lkp@...el.com,
ying.huang@...ux.alibaba.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, Ben Segall <bsegall@...gle.com>,
Li Zhijian <lizhijian@...itsu.com>
Subject: Re: [PATCH v1] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting
On 7/29/25 05:51, Ruan Shiyang wrote:
A process nit: your RFC v3 had:
From: Li Zhijian <lizhijian@...itsu.com>
and this one doesn't.
> 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@...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>
> Suggested-by: Huang Ying <ying.huang@...ux.alibaba.com>
> Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
So the S-o-b from Li doesn't match anything now.
You can either reinstate that "From: Li ..." or add a "Co-developed-by: Li
..." right above the "S-o-b: Li ..." - that's for you two to decide who is
the main author.
More details in Documentation/process/submitting-patches.rst
> Signed-off-by: Ruan Shiyang <ruansy.fnst@...itsu.com>
> ---
> 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",
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