[PATCH v3 4/8] lib/string_kunit: add performance benchmarks for strlen

Feng Jiang
2026-01-20T06:59:16Z

Introduce a benchmarking framework to the string_kunit test suite to
measure the execution efficiency of string functions.

The implementation is inspired by crc_benchmark(), measuring throughput
(MB/s) and latency (ns/call) across a range of string lengths. It
includes a warm-up phase, disables preemption during measurement, and
uses a fixed seed for reproducible results.

This allows for comparing different implementations (e.g., generic C vs.
architecture-optimized assembly) within the KUnit environment.

Initially, provide benchmarks for strlen().

Suggested-by: Andy Shevchenko <andy@kernel.org>
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
---
lib/Kconfig.debug | 11 +++
lib/tests/string_kunit.c | 151 +++++++++++++++++++++++++++++++++++++++
2 files changed, 162 insertions(+)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ba36939fda79..21b058ae815f 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2475,6 +2475,17 @@ config STRING_HELPERS_KUNIT_TEST
depends on KUNIT
default KUNIT_ALL_TESTS

+config STRING_KUNIT_BENCH
+ bool "Benchmark string functions at runtime"
+ depends on STRING_KUNIT_TEST
+ help
+ Enable performance measurement for string functions.
+
+ This measures the execution efficiency of string functions
+ during the KUnit test run.
+
+ If unsure, say N.
+
config FFS_KUNIT_TEST
tristate "KUnit test ffs-family functions at runtime" if !KUNIT_ALL_TESTS
depends on KUNIT
diff --git a/lib/tests/string_kunit.c b/lib/tests/string_kunit.c
index 8f836847a80e..e20e924d1c67 100644
--- a/lib/tests/string_kunit.c
+++ b/lib/tests/string_kunit.c
@@ -6,7 +6,9 @@
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <kunit/test.h>
+#include <linux/math64.h>
#include <linux/module.h>
+#include <linux/prandom.h>
#include <linux/printk.h>
#include <linux/slab.h>
#include <linux/string.h>
@@ -20,6 +22,9 @@
#define STRING_TEST_MAX_LEN 128
#define STRING_TEST_MAX_OFFSET 16

+#define STRING_BENCH_SEED 888
+#define STRING_BENCH_WORKLOAD 1000000UL
+
static void string_test_memset16(struct kunit *test)
{
unsigned i, j, k;
@@ -700,6 +705,151 @@ static void string_test_strends(struct kunit *test)
KUNIT_EXPECT_TRUE(test, strends("", ""));
}

+/* Target string lengths for benchmarking */
+static const size_t bench_lens[] = {
+ 0, 1, 7, 8, 16, 31, 64, 127, 512, 1024, 3173, 4096
+};
+
+/**
+ * alloc_max_bench_buffer() - Allocate buffer for the max test case.
+ * @test: KUnit context for managed allocation.
+ * @lens: Array of lengths used in the benchmark cases.
+ * @count: Number of elements in the @lens array.
+ * @buf_len: [out] Pointer to store the actually allocated buffer
+ * size (including null).
+ *
+ * Return: Pointer to the allocated memory, or NULL on failure.
+ */
+static void *alloc_max_bench_buffer(struct kunit *test,
+ const size_t *lens, size_t count, size_t *buf_len)
+{
+ void *buf;
+ size_t i, max_len = 0;
+
+ for (i = 0; i < count; i++) {
+ if (max_len < lens[i])
+ max_len = lens[i];
+ }
+
+ /* Add space for NUL terminator */
+ max_len += 1;
+
+ buf = kunit_kzalloc(test, max_len, GFP_KERNEL);
+ if (buf && buf_len)
+ *buf_len = max_len;
+
+ return buf;
+}
+
+/**
+ * fill_random_string() - Fill buffer with random non-null bytes.
+ * @buf: Buffer to fill.
+ * @len: Number of bytes to fill.
+ */
+static void fill_random_string(char *buf, size_t len)
+{
+ size_t i;
+ struct rnd_state state;
+
+ if (!buf || !len)
+ return;
+
+ /* Use a fixed seed to ensure deterministic benchmark results */
+ prandom_seed_state(&state, 888);
+ prandom_bytes_state(&state, buf, len);
+
+ /* Replace null bytes to avoid early string termination */
+ for (i = 0; i < len; i++) {
+ if (buf[i] == '\0')
+ buf[i] = 0x01;
+ }
+
+ buf[len - 1] = '\0';
+}
+
+/**
+ * STRING_BENCH() - Benchmark string functions.
+ * @iters: Number of iterations to run.
+ * @func: Function to benchmark.
+ * @...: Variable arguments passed to @func.
+ *
+ * Disables preemption and measures the total time in nanoseconds to execute
+ * @func(@__VA_ARGS__) for @iters times, including a small warm-up phase.
+ *
+ * Context: Disables preemption during measurement.
+ * Return: Total execution time in nanoseconds (u64).
+ */
+#define STRING_BENCH(iters, func, ...) \
+({ \
+ u64 __bn_t; \
+ size_t __bn_i; \
+ size_t __bn_iters = (iters); \
+ size_t __bn_warm_iters = max_t(size_t, __bn_iters / 10, 50U); \
+ /* Volatile function pointer prevents dead code elimination */ \
+ typeof(func) (* volatile __func) = (func); \
+ \
+ for (__bn_i = 0; __bn_i < __bn_warm_iters; __bn_i++) \
+ (void)__func(__VA_ARGS__); \
+ \
+ preempt_disable(); \
+ __bn_t = ktime_get_ns(); \
+ for (__bn_i = 0; __bn_i < __bn_iters; __bn_i++) \
+ (void)__func(__VA_ARGS__); \
+ __bn_t = ktime_get_ns() - __bn_t; \
+ preempt_enable(); \
+ __bn_t; \
+})
+
+/**
+ * STRING_BENCH_BUF() - Benchmark harness for single-buffer functions.
+ * @test: KUnit context.
+ * @buf_name: Local char * variable name to be defined.
+ * @buf_size: Local size_t variable name to be defined.
+ * @func: Function to benchmark.
+ * @...: Extra arguments for @func.
+ *
+ * Prepares a randomized, null-terminated buffer and iterates through lengths
+ * in bench_lens, defining @buf_name and @buf_size in each loop.
+ */
+#define STRING_BENCH_BUF(test, buf_name, buf_size, func, ...) \
+do { \
+ char *buf_name, *_bn_buf; \
+ size_t buf_size, _bn_i, _bn_iters, _bn_size = 0; \
+ u64 _bn_t, _bn_mbps = 0, _bn_lat = 0; \
+ \
+ if (!IS_ENABLED(CONFIG_STRING_KUNIT_BENCH)) \
+ kunit_skip(test, "not enabled"); \
+ \
+ _bn_buf = alloc_max_bench_buffer(test, bench_lens, \
+ ARRAY_SIZE(bench_lens), &_bn_size); \
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, _bn_buf); \
+ \
+ fill_random_string(_bn_buf, _bn_size); \
+ _bn_buf[_bn_size - 1] = '\0'; \
+ \
+ for (_bn_i = 0; _bn_i < ARRAY_SIZE(bench_lens); _bn_i++) { \
+ buf_size = bench_lens[_bn_i]; \
+ buf_name = _bn_buf + _bn_size - buf_size - 1; \
+ _bn_iters = STRING_BENCH_WORKLOAD / \
+ max_t(size_t, buf_size, 1U); \
+ \
+ _bn_t = STRING_BENCH(_bn_iters, func, ##__VA_ARGS__); \
+ \
+ if (_bn_t > 0) { \
+ _bn_mbps = (u64)(buf_size) * _bn_iters * 1000; \
+ _bn_mbps = div64_u64(_bn_mbps, _bn_t); \
+ _bn_lat = div64_u64(_bn_t, _bn_iters); \
+ } \
+ kunit_info(test, "len=%zu: %llu MB/s (%llu ns/call)\n", \
+ buf_size, _bn_mbps, _bn_lat); \
+ } \
+} while (0)
+
+static void string_bench_strlen(struct kunit *test)
+{
+ STRING_BENCH_BUF(test, buf, len, strlen, buf);
+}
+
static struct kunit_case string_test_cases[] = {
KUNIT_CASE(string_test_memset16),
KUNIT_CASE(string_test_memset32),
@@ -725,6 +875,7 @@ static struct kunit_case string_test_cases[] = {
KUNIT_CASE(string_test_strtomem),
KUNIT_CASE(string_test_memtostr),
KUNIT_CASE(string_test_strends),
+ KUNIT_CASE(string_bench_strlen),
{}
};

--
2.25.1