linux-kernel - Re: [RFC PATCH 2/2] samples/damon: add `migrate_hot` and `migrate

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Message-Id: <20250702000825.57587-1-sj@kernel.org>
Date: Tue,  1 Jul 2025 17:08:25 -0700
From: SeongJae Park <sj@...nel.org>
To: Yunjeong Mun <yunjeong.mun@...com>
Cc: SeongJae Park <sj@...nel.org>,
	akpm@...ux-foundation.org,
	damon@...ts.linux.dev,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	kernel_team@...ynix.com,
	honggyu.kim@...com
Subject: Re: [RFC PATCH 2/2] samples/damon: add `migrate_hot` and `migrate_cold` knobs

Hello Yunjeong,

On Tue,  1 Jul 2025 17:54:17 +0900 Yunjeong Mun <yunjeong.mun@...com> wrote:

> This patch introduces two new konbs for promotion/demotion:
> `migrate_hot` and `migrate_cold`. It receives node ids for migration in
> a comma-separated format as `<src,dst>`. The usage is as follows:
> 
>     # demote pages from nid 0 to nid 1
>     $ echo 0,1 > /sys/module/mtier/parameters/migrate_cold
>     # promote pages from nid 1 to nid 0
>     $ echo 1,0 > /sys/module/mtier/parameters/migrate_hot

I understand you want to support some setups such as having a fast node of id 1
and a slow node of id 0.

Because mtier is a sample code, I think it's primary goal is to help developers
know how they can use DAMON API functions for writing kernel code that is
required for a situation similar to the sample.  Hence the sample should be
clean and simple enough to be understood.

The assumption of the two nodes (the fast node 0 and the slow node 1) is
arguably intended for making the code simple, in my opinion.  We could of
course make this kind of changes if it helps more experiments for understanding
the code, as sample code is not only for reading but also running.

> 
> Susggested-by: Honggyu Kim <honggyu.kim@...com>

checkpatch.pl found a typo: s/Susggest/Suggest/

> Signed-off-by: Yunjeong Mun <yunjeong.mun@...com>
> ---
>  samples/damon/mtier.c | 68 +++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 66 insertions(+), 2 deletions(-)

But, I feel like this change is rather making code too longer and complicated.
Hence I would suggest dropping this patch if you agree.  Please let me know if
you have different opinions, or I'm missing something.

Thanks,
SJ

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