Re: [PATCH 1/2] mm/damon/core: optimize kdamond_apply_schemes() by inverting scheme and region loops

[SeongJae Park <sj@kernel.org>]

Hello Josh,

On Sun, 22 Mar 2026 18:46:40 +0000 Josh Law <objecting@objecting.org> wrote:

> Currently, kdamond_apply_schemes() iterates over all targets, then over all
> regions, and finally calls damon_do_apply_schemes() which iterates over
> all schemes. This nested structure causes scheme-level invariants (such as
> time intervals, activation status, and quota limits) to be evaluated inside
> the innermost loop for every single region.
> 
> If a scheme is inactive, has not reached its apply interval, or has already
> fulfilled its quota (quota->charged_sz >= quota->esz), the kernel still
> needlessly iterates through thousands of regions only to repeatedly
> evaluate these same scheme-level conditions and continue.
> 
> This patch inlines damon_do_apply_schemes() into kdamond_apply_schemes()
> and inverts the loop ordering. It now iterates over schemes on the outside,
> and targets/regions on the inside.
> 
> This allows the code to evaluate scheme-level limits once per scheme.
> If a scheme's quota is met or it is inactive, we completely bypass the
> O(Targets * Regions) inner loop for that scheme. This drastically reduces
> unnecessary branching, cache thrashing, and CPU overhead in the kdamond
> hot path.

That makes sense in high level.  But, this will make a kind of behavioral
difference that could be user-visible.  I am failing at finding a clear use
case that really depends on the old behavior.  But, still it feels like not a
small change to me.

So, I'd like to be conservative to this change, unless there are good evidences
showing very clear and impactful real world benefits.  Can you share such
evidences if you have?


Thanks,
SJ

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