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

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Message-ID: <878qiya0os.fsf@DESKTOP-5N7EMDA>
Date: Mon, 01 Sep 2025 19:09:55 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Ruan Shiyang <ruansy.fnst@...itsu.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>,  Vlastimil Babka <vbabka@...e.cz>,  Ben Segall
 <bsegall@...gle.com>,  stable@...r.kernel.org
Subject: Re: [PATCH v3] mm: memory-tiering: fix PGPROMOTE_CANDIDATE counting

Ruan Shiyang <ruansy.fnst@...itsu.com> writes:

> 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
> Co-developed-by: Li Zhijian <lizhijian@...itsu.com>
> Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
> Signed-off-by: Ruan Shiyang <ruansy.fnst@...itsu.com>
> Reported-by: Yasunori Gotou (Fujitsu) <y-goto@...itsu.com>
> Suggested-by: Huang Ying <ying.huang@...ux.alibaba.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 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>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

LGTM, feel free to add my

Reviewed-by: Huang Ying <ying.huang@...ux.alibaba.com>

in the future versions.

[snip]

---
Best Regards,
Huang, Ying