Re: [PATCH 2/2] mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses()

[Josh Law]

On 22 March 2026 21:30:15 GMT, SeongJae Park <sj@xxxxxxxxxx> wrote:
>On Sun, 22 Mar 2026 18:46:41 +0000 Josh Law <objecting@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> ---
>>  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