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

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Message-ID: <20250703051836.1759-2-yunjeong.mun@sk.com>
Date: Thu,  3 Jul 2025 14:18:34 +0900
From: Yunjeong Mun <yunjeong.mun@...com>
To: sj@...nel.org
Cc: 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 SeongJae,

On Tue,  1 Jul 2025 17:08:25 -0700 SeongJae Park <sj@...nel.org> wrote:
> 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.

Thanks for explanation. I understand the intention of mtier module.
As you mentioned, mtier is helpful for leraning how to configure and 
run DAMON.

> 
> 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.
> 

I also think that fast node 0 and the slow node 1 is intuitive and easy to
understand the code.

> > 
> > 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.
> 

Ok, I will drop this patch in next version RFC. As you mentiond, mtier
module should be simple and intuitive enough. I will consider this
kind of tiering module to mm/damon. I'll send you RFC v2 soon.

Best Regards,
Yunjeong Mun

> 
> Thanks,
> SJ
> 
> [...]