Re: [RFC PATCH] mm/damon/core: add short circuit for temporal-goal schemes in damos_adjust_quota()

[SJ Park <sj@kernel.org>]

On Sat, 12 Sep 2026 20:43:12 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> When using the temporal auto-tuning algorithm, the effective size quota
> becomes zero once the goal is [over-]achieved.
> 
> In this situation, damos_adjust_quota() still calculates
> quota->min_score for it.  However, this min_score will not be used in
> this window, because in damon_do_apply_schemes(), damos_quota_is_full()
> will always returns true, preventing the scheme from being applied to
> any region.
> 
> Therefore, add a short circuit for temporal-goal algorithm schemes to
> early return from damos_adjust_quota() before calculating min_score.

Nice catch.  Makes sense to me.

> 
> Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
> ---
> 
> I tested with virtme-ng + perf on a Proactive Memory Reclaim workload;
> within measurement noise (~3%), no measurable difference was observed.
> 
> The purpose of this patch is to reduce unnecessary operations
> (calculating min_score).  But, I'd like to know if this patch needs to
> demonstrate that it provides better performance before it's merged.

This function is supposed to be not performance critical.  I expect performance
difference would be shown only in some setups that I didn't imagine.

But the change is small and makes sense.  I wouldn't mind having no performance
measurement for this small change.

> 
> ---
>  mm/damon/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index ce8c6f99106e..fdcea989f0e0 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3320,6 +3320,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
>  
>  	if (!c->ops.get_scheme_score)
>  		return;
> +	if (quota->esz == 0)
> +		return;

Direct zero esz comparison looks redundant and incomplete.  It doesn't catch
the case that <min_region_sz remaining quota case.  Let's use
damos_quota_is_full() instead.  Also, let's do the check before the
get_scheme_score check.  If quota is already full, get_scheme_score check also
makes no sense.

>  
>  	/* Fill up the score histogram */
>  	memset(c->regions_score_histogram, 0,
> -- 
> 2.55.0


Thanks,
SJ