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

[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>
---
Changes from v1:
- 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 a218d9922234..ac604fb7b409 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)
-- 
2.53.0