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

Feng Jiang
2026-01-21T05:45:20Z

On 2026/1/20 15:46, Andy Shevchenko wrote:
> On Tue, Jan 20, 2026 at 02:58:48PM +0800, Feng Jiang wrote:
>> 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().
>
> ...
>
>> +#define STRING_BENCH_SEED 888
>> +#define STRING_BENCH_WORKLOAD 1000000UL
>
> Can also be (1 * MEGA) from units.h.

Fixed.

> ...
>
>> +static const size_t bench_lens[] = {
>> + 0, 1, 7, 8, 16, 31, 64, 127, 512, 1024, 3173, 4096
>
> Leave trailing comma.

Fixed.

> ...
>
>> +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;
>
> if (!buf)
> return NULL;
>
> *buf_len ...
> return buf;
>

Fixed.

> ...
>
>> +static void fill_random_string(char *buf, size_t len)
>> +{
>> + size_t i;
>> + struct rnd_state state;
>
> Reversed xmas tree ordering?

Fixed.

>> + 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';
>> +}
>
> ...
>
>> +#define STRING_BENCH(iters, func, ...) \
>
> Is this same / similar code to crc_benchmark()? Perhaps we need to have KUnit
> provided macro / environment to perform such tests... Have you talked to KUnit
> people about all this?
>

I haven't reached out to the KUnit maintainers yet. This implementation is currently
a lightweight adaptation specifically for string benchmarks. However, I agree that
a generic KUnit benchmarking harness would be beneficial for the kernel. For now,
I'll refine this version based on your feedback.

>> +({ \
>> + 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); \
>
> Try to avoid max_t() as much as possible. Wouldn't max() suffice?
>

Will do.

>> + /* 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'; \
>
> You have already this there in the function, no?
>

Indeed, that's redundant. I'll remove it.

>> + 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); \
>
> max()

Fixed.

>> + _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)
>

Thanks for the catch. I will incorporate all your suggestions into v4.

--
With Best Regards,
Feng Jiang