Re: [RFC PATCH] mm/damon/ops-common: optimize damon_hot_score() using fls()

[SeongJae Park <sj@kernel.org>]

Hello Liew,

On Fri, 20 Mar 2026 15:24:31 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> The current implementation of damon_hot_score() uses a manual for-loop
> to calculate the value of 'age_in_log'. This can be efficiently replaced
> by the fls().

This makes sense.  But, it seems ilog2() looks more same to what the current
code is trying to do.  How about using ilog2() instead of fls()?

> 
> In a simulated performance test with 10,000,000 iterations, this
> optimization showed a significant reduction in latency:
> - Average Latency: Reduced from ~9ns to ~1ns.
> - P99 Latency: Reduced from ~60ns to ~41ns.
> - Throughput: The loop-based version mostly fell into the 40-50ns range,
>   while the fls-based version shifted significantly towards the 20-39ns
>   range in the test environment.
> 
> Although these results are based on a simulated kernel module test
> environment [1], they indicate a clear instruction-level optimization.
> 
> [1] https://github.com/aethernet65535/damon-hot-score-fls-optimize/blob/master/test-kernel-module/fls.c

Makes sense!

> 
> Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
> ---
> Note on testing methodology:
> I attempted to measure the performance directly within the kernel using
> bpftrace, perf, and ktime inside damon_hot_score(). However, the results
> were highly unstable (ktime), and in some cases (perf/bpftrace) the
> function was difficult to trace reliably (likely due to my own tracing
> limitations).
> 
> Despite the instability of in-kernel ktime measurements, one thing
> remained consistent: the fls-based version significantly improves the
> "long tail" latency compared to the for-loop.
> 
> Test results from the simulated module:
> - fls-based:
>     DAMON Perf Test: Starting 10000000 iterations
>     =============================================
>      Total Iterations : 10000000
>      Average Latency  : 1 ns
>      P95 Latency      : 40 ns
>      P99 Latency      : 41 ns
>     ---------------------------------------------
>      Range (ns)      | Count    | Percent
>     ---------------------------------------------
>      20-39           | 3522000  |   35%
>      40-59           | 6478000  |   64%
>      60-79           | 0        |    0%
>     =============================================
> 
> - for-loop:
>     DAMON Perf Test: Starting 10000000 iterations
>     =============================================
>      Total Iterations : 10000000
>      Average Latency  : 9 ns
>      P95 Latency      : 51 ns
>      P99 Latency      : 60 ns
>     ---------------------------------------------
>      Range (ns)      | Count    | Percent
>     ---------------------------------------------
>      20-39           | 0        |    0%
>      40-59           | 9894000  |   98%
>      60-79           | 98000    |    0%
>     =============================================
> 
> Full raw benchmark results can be found at [2].
> 
> If anyone could suggest a more robust way to profile this specific
> function within live DAMON context, I would greatly appreciate the
> guidance.
> 
> [2] https://github.com/aethernet65535/damon-hot-score-fls-optimize/tree/master/result-raw

Nice test results!  I think this deserves to be in the git history.  Could you
please add this on the commit message area, rather than this commentary area in
the next version?

> 
>  mm/damon/ops-common.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
> index 8c6d613425c1..0294de61a23a 100644
> --- a/mm/damon/ops-common.c
> +++ b/mm/damon/ops-common.c
> @@ -117,9 +117,7 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
>  		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;
> -			age_in_log++, age_in_sec >>= 1)
> -		;
> +	age_in_log = min_t(int, fls(age_in_sec), DAMON_MAX_AGE_IN_LOG);
>  
>  	/* If frequency is 0, higher age means it's colder */
>  	if (freq_subscore == 0)
> -- 
> 2.53.0


Thanks,
SJ