Re: [RFC PATCH v1.1 03/11] mm/damon/tests/core-kunit: test damon_rand()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The KUnit test uses a non-deterministic seed with tight probability bounds, which will cause sporadic test failures.
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commit bc9f1073820b74c3fbb6ac52d5ca363629f99f3d
Author: SeongJae Park <sj@kernel.org>

mm/damon/tests/core-kunit: test damon_rand()

Commit 9012c4e647df ("mm/damon: replace damon_rand() with a per-ctx
lockless PRNG") optimized DAMON for better performance.  Add a kunit
test for ensuring the pseudo randomness quality.

> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index 1cfb8c176b873..756f3b9e2ed3b 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h

[ ... ]

> @@ -1460,6 +1460,26 @@ static void damon_test_is_last_region(struct kunit *test)
>  	damon_free_target(t);
>  }
>  
> +static void damon_test_rand(struct kunit *test)
> +{
> +	struct damon_ctx ctx;
> +	int counts[10] = {};
> +	int i;
> +
> +	prandom_seed_state(&ctx.rnd_state, get_random_u64());
> +	for (i = 0; i < 10000; i++) {
> +		unsigned long rnd = damon_rand(&ctx, 0, 10);
> +
> +		KUNIT_EXPECT_GE(test, rnd, 0);
> +		KUNIT_EXPECT_LE(test, rnd, 9);
> +		counts[rnd]++;
> +	}
> +	for (i = 0; i < 10; i++) {
> +		KUNIT_EXPECT_GE(test, counts[i], 900);
> +		KUNIT_EXPECT_LE(test, counts[i], 1100);

[Severity: Medium]
The test uses a true random seed to initialize the PRNG state, making the
test output non-deterministic. Is it possible that these tight expectation
bounds will cause sporadic test failures in automated CI environments?

Statistically, a binomial distribution with n=10,000 and p=0.1 has a
standard deviation of 30. The allowed variation of +/- 100 is roughly
+/- 3.33 standard deviations. The probability of any single bin falling
outside this range is around 0.086%.

Over 10 bins, does this mean the test could spuriously fail about 0.86%
of the time (roughly 1 in 117 runs)?

> +	}
> +}
> +
>  static struct kunit_case damon_test_cases[] = {
>  	KUNIT_CASE(damon_test_target),

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Sashiko AI review · https://sashiko.dev/#/patchset/20260625050756.91115-1-sj@kernel.org?part=3