mm/damon/core-test: add a unit test for the feedback loop algorithm

Implement a simple kunit test for testing the behavior of the feedback
loop algorithm for the aim-oriented feedback-friven DAMOS aggressiveness
auto tuning.

Link: https://lkml.kernel.org/r/20231130023652.50284-6-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/core-test.h

@@ -446,6 +446,37 @@ static void damos_test_filter_out(struct kunit *test)
 	damos_free_filter(f);
 }
 
+static void damon_test_feed_loop_next_input(struct kunit *test)
+{
+	unsigned long last_input = 900000, current_score = 200;
+
+	/*
+	 * If current score is lower than the goal, which is always 10,000
+	 * (read the comment on damon_feed_loop_next_input()'s comment), next
+	 * input should be higher than the last input.
+	 */
+	KUNIT_EXPECT_GT(test,
+			damon_feed_loop_next_input(last_input, current_score),
+			last_input);
+
+	/*
+	 * If current score is higher than the goal, next input should be lower
+	 * than the last input.
+	 */
+	current_score = 250000000;
+	KUNIT_EXPECT_LT(test,
+			damon_feed_loop_next_input(last_input, current_score),
+			last_input);
+
+	/*
+	 * The next input depends on the distance between the current score and
+	 * the goal
+	 */
+	KUNIT_EXPECT_GT(test,
+			damon_feed_loop_next_input(last_input, 200),
+			damon_feed_loop_next_input(last_input, 2000));
+}
+
 static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_target),
 	KUNIT_CASE(damon_test_regions),
@@ -461,6 +492,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_moving_sum),
 	KUNIT_CASE(damos_test_new_filter),
 	KUNIT_CASE(damos_test_filter_out),
+	KUNIT_CASE(damon_test_feed_loop_next_input),
 	{},
 };