[PATCH 6.18.y] mm/damon/ops-common: use nr_accesses moving sum for quota score

[PATCH 6.18.y] mm/damon/ops-common: use nr_accesses moving sum for quota score

[SJ Park]

Since commit 42f994b71404 ("mm/damon/core: implement scheme-specific apply
interval"), DAMOS scheme can be applied at any time.  At that time,
nr_accesses may not be fully aggregated.  But the quota prioritization
score is calculated using the not fully aggregated count.  As a result,
the performance of DAMOS could be degraded.  Fix by using
damon_nr_accesses_mvsum() instead.

The user impact of the issue is suboptimum DAMOS performance under certain
setups.  Nonetheless, the bug was there from the beginning of the setup
availability.  In other words, the suboptimum performance is the baseline
of the setup and hence it didn't cause regression.  Also the extent of the
suboptimality was not big enough to be found from users and testers.
Still, this is a clear bug that is better to be fixed, and can be easily
fixed.

Link: https://lore.kernel.org/20260719161136.90191-1-sj@kernel.org
Fixes: 42f994b71404 ("mm/damon/core: implement scheme-specific apply interval")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.7.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 6c25083f7ae7e2660d766169e5b4d3e96010503f)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/ops-common.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index ceaa2a6e1c9a3..929aab600c745 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -101,8 +101,9 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 	unsigned int age_weight = s->quota.weight_age;
 	int hotness;
 
-	freq_subscore = r->nr_accesses * DAMON_MAX_SUBSCORE /
-		damon_max_nr_accesses(&c->attrs);
+	freq_subscore = mult_frac(r->nr_accesses_bp / 10000,
+			DAMON_MAX_SUBSCORE,
+			damon_max_nr_accesses(&c->attrs));
 
 	age_in_sec = (unsigned long)r->age * c->attrs.aggr_interval / 1000000;
 	for (age_in_log = 0; age_in_log < DAMON_MAX_AGE_IN_LOG && age_in_sec;
-- 
2.47.3

Re: [PATCH 6.18.y] mm/damon/ops-common: use nr_accesses moving sum for quota score

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The commit message claims to fix the issue by using a new helper function `damon_nr_accesses_mvsum()`, but this function is never defined or called.
--

commit 3c964119e352fe243aaf217b238b33be4fa99148
Author: SJ Park <sj@kernel.org>

mm/damon/ops-common: use nr_accesses moving sum for quota score

Since commit 42f994b71404 ("mm/damon/core: implement scheme-specific apply
interval"), DAMOS scheme can be applied at any time.  At that time,
nr_accesses may not be fully aggregated.  But the quota prioritization
score is calculated using the not fully aggregated count.  As a result,
the performance of DAMOS could be degraded.  Fix by using
damon_nr_accesses_mvsum() instead.

Link: https://lore.kernel.org/20260719161136.90191-1-sj@kernel.org

> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index ceaa2a6e1c9a3..929aab600c745 100644
> --- a/mm/damon/ops-common.c
> +++ b/mm/damon/ops-common.c
> @@ -101,8 +101,9 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
>  	unsigned int age_weight = s->quota.weight_age;
>  	int hotness;
>  
> -	freq_subscore = r->nr_accesses * DAMON_MAX_SUBSCORE /
> -		damon_max_nr_accesses(&c->attrs);
> +	freq_subscore = mult_frac(r->nr_accesses_bp / 10000,
> +			DAMON_MAX_SUBSCORE,
> +			damon_max_nr_accesses(&c->attrs));

[Severity: Medium]
The commit message claims to fix the issue by using a new helper function
damon_nr_accesses_mvsum(), but this function is never defined or called.

Instead, the implementation in damon_hot_score() manually inlines the moving
sum calculation (r->nr_accesses_bp / 10000).

Is there a missing patch that adds damon_nr_accesses_mvsum(), or should the
commit message be updated to match the inline calculation?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909050521.225965-1-sj@kernel.org?part=1