[PATCH v2 1/4] mm/damon/ops-common: optimize damon_hot_score() using ilog2()

SeongJae Park
2026-04-26T23:16:29Z

From: Liew Rui Yan <aethernet65535@gmail.com>

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 ilog2(), which is semantically more appropriate for calculating the
logarithmic value of age.

In a simulated-kernel-module performance test with 10,000,000 iterations,
this optimization showed a significant reduction in latency (average
latency reduced from ~12ns to ~1ns).

Test results from the simulated-kernel-module:
- ilog2:
DAMON Perf Test: Starting 10000000 iterations
=============================================
Total Iterations : 10000000
Average Latency : 1 ns
P95 Latency : 41 ns
P99 Latency : 41 ns
---------------------------------------------
Range (ns) | Count | Percent
---------------------------------------------
0-19 | 0 | 0%
20-39 | 2625000 | 26%
40-59 | 7374000 | 73%
60-79 | 0 | 0%
80-99 | 0 | 0%
100+ | 1000 | 0%
=============================================

- for-loop:
DAMON Perf Test: Starting 10000000 iterations
=============================================
Total Iterations : 10000000
Average Latency : 12 ns
P95 Latency : 51 ns
P99 Latency : 60 ns
---------------------------------------------
Range (ns) | Count | Percent
---------------------------------------------
0-19 | 0 | 0%
20-39 | 0 | 0%
40-59 | 9862000 | 98%
60-79 | 135000 | 1%
80-99 | 1000 | 0%
100+ | 2000 | 0%
=============================================

Full raw benchmark results can be found at [1].

[1] https://github.com/aethernet65535/damon-hot-score-fls-optimize/tree/master/result-raw

Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
---
Changes from v2
(https://lore.kernel.org/20260320192020.33004-1-aethernet65535@gmail.com)
- Rebase to latest mm-new.
Changes from v1 (actually it was RFC)
(https://lore.kernel.org/20260320072431.248235-1-aethernet65535@gmail.com)
- Replace fls() with ilog2() per SeongJae Park's suggestion for better
semantic clarity.
- Move performance benchmark results into the commit message and add
comparison between for-loop and ilog2.

mm/damon/ops-common.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 8c6d613425c13..3a0ddc3ac7196 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -117,9 +117,12 @@ 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)
- ;
+ if (age_in_sec)
+ age_in_log = min_t(int, ilog2(age_in_sec) + 1,
+ DAMON_MAX_AGE_IN_LOG);
+ else
+ age_in_log = 0;
+

/* If frequency is 0, higher age means it's colder */
if (freq_subscore == 0)
--
2.47.3