Hardware integer division is slow. The function damon_max_nr_accesses(),
which is called very frequently (e.g., once per region per sample
interval inside damon_update_region_access_rate), performs an integer
division: attrs->aggr_interval / attrs->sample_interval.

However, the struct damon_attrs already caches this exact ratio in the
internal field aggr_samples (since earlier commits). We can eliminate
the hardware division in the hot path by simply returning aggr_samples.

This significantly reduces the CPU cycle overhead of updating the access
rates for thousands of regions.

Signed-off-by: Josh Law <objecting@objecting.org>
---
 include/linux/damon.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 6bd71546f7b2..fffdb08326a2 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -960,8 +960,7 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx)
 static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs)
 {
 	/* {aggr,sample}_interval are unsigned long, hence could overflow */
-	return min(attrs->aggr_interval / attrs->sample_interval,
-			(unsigned long)UINT_MAX);
+	return min(attrs->aggr_samples, (unsigned long)UINT_MAX);
 }
 
 
-- 
2.34.1

On Sun, 22 Mar 2026 18:46:41 +0000 Josh Law <objecting@objecting.org> wrote:

> Hardware integer division is slow. The function damon_max_nr_accesses(),
> which is called very frequently (e.g., once per region per sample
> interval inside damon_update_region_access_rate), performs an integer
> division: attrs->aggr_interval / attrs->sample_interval.
> 
> However, the struct damon_attrs already caches this exact ratio in the
> internal field aggr_samples (since earlier commits). We can eliminate
> the hardware division in the hot path by simply returning aggr_samples.
> 
> This significantly reduces the CPU cycle overhead of updating the access
> rates for thousands of regions.
> 
> Signed-off-by: Josh Law <objecting@objecting.org>
> ---
>  include/linux/damon.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index 6bd71546f7b2..fffdb08326a2 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -960,8 +960,7 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx)
>  static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs)
>  {
>  	/* {aggr,sample}_interval are unsigned long, hence could overflow */
> -	return min(attrs->aggr_interval / attrs->sample_interval,
> -			(unsigned long)UINT_MAX);
> +	return min(attrs->aggr_samples, (unsigned long)UINT_MAX);

checkpatch gives below warning:

WARNING: min() should probably be min_t(unsigned long, attrs->aggr_samples, UINT_MAX)
#39: FILE: include/linux/damon.h:963:
+       return min(attrs->aggr_samples, (unsigned long)UINT_MAX);

Can we use min_t() as suggested?

Otherwise, this patch looks good to me.


Thanks,
SJ

[...]

On 22 March 2026 21:30:15 GMT, SeongJae Park <sj@kernel.org> wrote:
>On Sun, 22 Mar 2026 18:46:41 +0000 Josh Law <objecting@objecting.org> wrote:
>
>> Hardware integer division is slow. The function damon_max_nr_accesses(),
>> which is called very frequently (e.g., once per region per sample
>> interval inside damon_update_region_access_rate), performs an integer
>> division: attrs->aggr_interval / attrs->sample_interval.
>> 
>> However, the struct damon_attrs already caches this exact ratio in the
>> internal field aggr_samples (since earlier commits). We can eliminate
>> the hardware division in the hot path by simply returning aggr_samples.
>> 
>> This significantly reduces the CPU cycle overhead of updating the access
>> rates for thousands of regions.
>> 
>> Signed-off-by: Josh Law <objecting@objecting.org>
>> ---
>>  include/linux/damon.h | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>> 
>> diff --git a/include/linux/damon.h b/include/linux/damon.h
>> index 6bd71546f7b2..fffdb08326a2 100644
>> --- a/include/linux/damon.h
>> +++ b/include/linux/damon.h
>> @@ -960,8 +960,7 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx)
>>  static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs)
>>  {
>>  	/* {aggr,sample}_interval are unsigned long, hence could overflow */
>> -	return min(attrs->aggr_interval / attrs->sample_interval,
>> -			(unsigned long)UINT_MAX);
>> +	return min(attrs->aggr_samples, (unsigned long)UINT_MAX);
>
>checkpatch gives below warning:
>
>WARNING: min() should probably be min_t(unsigned long, attrs->aggr_samples, UINT_MAX)
>#39: FILE: include/linux/damon.h:963:
>+       return min(attrs->aggr_samples, (unsigned long)UINT_MAX);
>
>Can we use min_t() as suggested?
>
>Otherwise, this patch looks good to me.
>
>
>Thanks,
>SJ
>
>[...]


On it!

V/R

Josh Law