The patch titled Subject: mm: memory-tiering: fix PGPROMOTE_CANDIDATE counting has been added to the -mm mm-new branch. Its filename is mm-memory-tiering-fix-pgpromote_candidate-counting.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
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------------------------------------------------------ From: Ruan Shiyang ruansy.fnst@fujitsu.com Subject: mm: memory-tiering: fix PGPROMOTE_CANDIDATE counting Date: Tue, 29 Jul 2025 11:51:01 +0800
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 these confusing statistics, introduce 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.
Link: https://lkml.kernel.org/r/20250729035101.1601407-1-ruansy.fnst@fujitsu.com Signed-off-by: Li Zhijian lizhijian@fujitsu.com Signed-off-by: Ruan Shiyang ruansy.fnst@fujitsu.com Reported-by: Yasunori Gotou (Fujitsu) y-goto@fujitsu.com Suggested-by: Huang Ying ying.huang@linux.alibaba.com 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 Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
include/linux/mmzone.h | 16 +++++++++++++++- kernel/sched/fair.c | 5 +++-- mm/vmstat.c | 1 + 3 files changed, 19 insertions(+), 3 deletions(-)
--- a/include/linux/mmzone.h~mm-memory-tiering-fix-pgpromote_candidate-counting +++ a/include/linux/mmzone.h @@ -234,7 +234,21 @@ 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, --- a/kernel/sched/fair.c~mm-memory-tiering-fix-pgpromote_candidate-counting +++ a/kernel/sched/fair.c @@ -1940,11 +1940,13 @@ bool should_numa_migrate_memory(struct t 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 t 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); --- a/mm/vmstat.c~mm-memory-tiering-fix-pgpromote_candidate-counting +++ a/mm/vmstat.c @@ -1280,6 +1280,7 @@ const char * const vmstat_text[] = { #ifdef CONFIG_NUMA_BALANCING [I(PGPROMOTE_SUCCESS)] = "pgpromote_success", [I(PGPROMOTE_CANDIDATE)] = "pgpromote_candidate", + [I(PGPROMOTE_CANDIDATE_NRL)] = "pgpromote_candidate_nrl", #endif [I(PGDEMOTE_KSWAPD)] = "pgdemote_kswapd", [I(PGDEMOTE_DIRECT)] = "pgdemote_direct", _
Patches currently in -mm which might be from ruansy.fnst@fujitsu.com are
mm-memory-tiering-fix-pgpromote_candidate-counting.patch