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

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Message-Id: <20250901125917.e9792e5d0df12ba1c552c537@linux-foundation.org>
Date: Mon, 1 Sep 2025 12:59:17 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ruan Shiyang <ruansy.fnst@...itsu.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, lkp@...el.com,
 ying.huang@...ux.alibaba.com, 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

On Mon,  1 Sep 2025 17:01:22 +0800 Ruan Shiyang <ruansy.fnst@...itsu.com> wrote:

> 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.
> 
> ...
>

It would be good to have a Fixes: here, to tell people how far back to
backport it.

Could be either c6833e10008f or c959924b0dc5 afaict.  I'll go with
c6833e10008f, OK?