linux-kernel - Re: [RFC PATCH 0/4] mm/damon/sysfs: support periodic and automated stats update

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Message-ID: <923d9fe1-b959-4fba-9da7-10d2b3126858@sk.com>
Date: Wed, 16 Jul 2025 07:20:57 +0900
From: Honggyu Kim <honggyu.kim@...com>
To: SeongJae Park <sj@...nel.org>
Cc: kernel_team@...ynix.com, Andrew Morton <akpm@...ux-foundation.org>,
 Jonathan Corbet <corbet@....net>, damon@...ts.linux.dev,
 kernel-team@...a.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/4] mm/damon/sysfs: support periodic and automated
 stats update

Hi SeongJae,

On 7/13/2025 5:46 AM, SeongJae Park wrote:
> DAMON sysfs interface provides files for reading DAMON internal status
> including DAMOS stats.  The content of the files are not automatically
> updated, though.  Users should manually request updates of the contents
> by writing a special command to 'state' file of each kdamond directory.
> This interface is good for minimizing overhead, but causes the below
> problems.
> 
> First, the usage is cumbersome.  This is arguably not a big problem,
> since the user-space tool (damo) can do this instead of the user.
> 
> Second, it can be too slow.  The update request is not directly handled
> by the sysfs interface but kdamond thread.  And kdamond threads wake up
> only once per the sampling interval.  Hence if sampling interval is not
> short, each update request could take too long time.  The recommended
> sampling interval setup is asking DAMON to automatically tune it, within
> a range between 5 milliseconds and 10 seconds.  On production systems it
> is not very rare to have a few seconds sampling interval as a result of
> the auto-tuning, so this can disturb observing DAMON internal status.
> 
> Finally, parallel update requests can conflict with each other.  When
> parallel update requests are received, DAMON sysfs interface simply
> returns -EBUSY to one of the requests.  DAMON user-space tool is hence
> implementing its own backoff mechanism, but this can make the operation
> even slower.
> 
> Introduce a new sysfs file, namely refresh_ms, for asking DAMON sysfs
> interface to repeat the essential contents update with a user-specified
> time delay.

Thanks for working on this, but I have a few questions.
1. Could you please list up what are the "essential contents"?
2. Does it mean that it is different from writing "commit" to "state"?
3. If not, then is there equivalent action to writing something to "state"?

If possible, then this kind of information is better to be documented because
users might get confused if something isn't udpated when "refresh_ms" is set.

Thanks,
Honggyu

> If non-zero value is written to the file, DAMON sysfs
> interface does the updates for essential DAMON internal status including
> auto-tuned monitoring intervals, DAMOS stats, and auto-tuned DAMOS
> quotas using the user-written value as the time delay.  If zero is
> written to the file, the automatic refresh is disabled.
> 
> SeongJae Park (4):
>    mm/damon/sysfs: implement refresh_ms file under kdamond directory
>    mm/damon/sysfs: implement refresh_ms file internal work
>    Docs/admin-guide/mm/damon/usage: document refresh_ms file
>    Docs/ABI/damon: update for refresh_ms
> 
>   .../ABI/testing/sysfs-kernel-mm-damon         |  7 +++
>   Documentation/admin-guide/mm/damon/usage.rst  | 13 ++++-
>   mm/damon/sysfs.c                              | 58 +++++++++++++++++++
>   3 files changed, 75 insertions(+), 3 deletions(-)
> 
> 
> base-commit: 982b86e9191292ffcd0f30018981cb16f9fac5c1
> --
> 2.39.5
>