Re: [RFC PATCH v1 0/4] mm/damon: Introduce a huge page collapsing mechanism using auto tuning

[SeongJae Park <sj@kernel.org>]

Hello Asier,

On Thu, 30 Apr 2026 13:41:35 +0000 <gutierrez.asier@huawei-partners.com> wrote:

> From: Asier Gutierrez <gutierrez.asier@huawei-partners.com>
> 
> Overview
> ================

Let's align the length of underline with the text.

> 
> This patch set introduces a new autotuning which allows to collapse
> hot regions into hugepages.
> 
> Motivation
> ================
> 
> Since TLB is a bottleneck for many systems, a way to optimize TLB
> misses (or hits) is to use huge pages. Unfortunately, using "always"
> in THP leads to memory fragmentation and memory waste. For this reason,
> most application guides and system administrators suggest to disable THP.

I think the motivation should further explain why existing DAMOS features
(access pattern-based targetting and quota auto-tuning with existing quota
goals) are insufficient.  If that makes the text too long, I think the
explanation for well-known THP benefit and problems can be reduced or skipped.

> 
> Solution
> ================
> 
> A new autotuning quota goal[1], damos_get_used_hugepage_mem_bp, is
> introduced, which checks the huge page consumption to total anonymous
> memory consumption. This new quota mechanism reuses current autotuning
> architecture.

The idea makes sense to me.

The name sounds bit redundant, though.  How about DAMOS_QUOTA_HUGEPAGE_BP for
the enum, and hugeapge_bp for sysfs input?

> 
> A new module is introduced to demonstrate the use of huge pages
> collapse autotuning. The module launches a kdamond thread for a 
> certain task provided by the user through monitored_pid module argument.

If it is only for demonstration, I think it is more fit to be a sample module
(placed under samples/damon/).

> 
> This module also has a user autotuning knob which allows the user to
> adjust the aggressiveness of page collapsing.
> 
> Benchmarks
> ================
> In progress, will add them in the next RFC series.

Looking forward to!

I may request not that much test results for the quota goal.  If the module is
not just a sample but for a real world use case, I may hope somewhat reasonable
test resulsts.

> 
> TODO
> ================
> 
> - Since DAMOS_COLLAPSE is not in upstream and this is just a
>   RFC, I have used DAMOS_HUGEPAGE instead. This will change
>   to DAMOS_COLLAPSE in future series.

It is now in mm-new and it is recommended to use it as a baseline for DAMON
patches.  So please feel free to use DAMOS_COLLAPSE from the next version,
unless there is a reason to worry if DAMOS_COLLAPSE will be dropped from
mm-new.

> 
> Patches Sequence
> ================
> Patch 1 -> damon_modules_new_vaddr_ctx_target
> Patch 2 -> Introduce DAMOS_QUOTA_HUGEPAGE and autotuning
> Patch 3 -> Module that demonstrates how to use DAMOS_QUOTA_HUGEPAGE
>            and the new VADDR ctx creation
> Patch 4 -> Documentation
> 
> [1] https://lore.kernel.org/e67f05ad-dbb9-45e6-ba30-b167a99ac67d@huawei-partners.com

Adding more context about what this link is would be nice.  I can find the link
marker from the above text, but I was unable to expect what this is without
opening the link.


Thanks,
SJ

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