linux-kernel - Re: [PATCH v5 4/8] lib/string_kunit: add performance benchmark for strlen()

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Message-ID: <dcdb82e8-691a-4cc1-88d5-755039ed0313@kylinos.cn>
Date: Tue, 27 Jan 2026 17:33:10 +0800
From: Feng Jiang <jiangfeng@...inos.cn>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: pjw@...nel.org, palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr,
 akpm@...ux-foundation.org, kees@...nel.org, andy@...nel.org,
 ebiggers@...nel.org, martin.petersen@...cle.com, sohil.mehta@...el.com,
 charlie@...osinc.com, conor.dooley@...rochip.com, samuel.holland@...ive.com,
 linus.walleij@...aro.org, nathan@...nel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH v5 4/8] lib/string_kunit: add performance benchmark for
 strlen()

On 2026/1/27 16:57, Andy Shevchenko wrote:
> On Tue, Jan 27, 2026 at 09:25:54AM +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 framework allows for comparing different implementations (e.g.,
>> generic C vs. architecture-optimized assembly) within the KUnit
>> environment.
> 
> Acked-by: Andy Shevchenko <andy@...nel.org>
> A few nit-picks below.
> 

Thanks for the Acked-by and the detailed review.

> ...
> 
>> +static void *alloc_max_bench_buffer(struct kunit *test,
>> +		const size_t *lens, size_t count, size_t *buf_len)
>> +{
>> +	size_t max_len = 0;
>> +	void *buf;
>> +
>> +	for (size_t i = 0; i < count; i++)
>> +		max_len = max(lens[i], max_len);
> 
> You also need minmax.h.

Will add.

> ...
> 
>> +#define STRING_BENCH(iters, func, ...)					\
>> +({									\
>> +	/* Volatile function pointer prevents dead code elimination */	\
>> +	typeof(func) (* volatile __func) = (func);			\
>> +	size_t __bn_iters = (iters);					\
>> +	size_t __bn_warm_iters;						\
>> +	u64 __bn_t;							\
> 
> 
> Perhaps a short comment here
> 
> 	/* Use 10% of the given iterations (maximum 50) to warm up */

Will add this comment.

>> +	__bn_warm_iters = max(__bn_iters / 10, 50U);			\
>> +									\
>> +	for (size_t __bn_i = 0; __bn_i < __bn_warm_iters; __bn_i++)	\
>> +		(void)__func(__VA_ARGS__);				\
>> +									\
>> +	preempt_disable();						\
>> +	__bn_t = ktime_get_ns();					\
>> +	for (size_t __bn_i = 0; __bn_i < __bn_iters; __bn_i++)		\
>> +		(void)__func(__VA_ARGS__);				\
>> +	__bn_t = ktime_get_ns() - __bn_t;				\
>> +	preempt_enable();						\
>> +	__bn_t;								\
>> +})
> 
> ...
> 
>> +#define STRING_BENCH_BUF(test, buf_name, buf_size, func, ...)		\
>> +do {									\
>> +	size_t buf_size, _bn_i, _bn_iters, _bn_size = 0;		\
>> +	u64 _bn_t, _bn_mbps = 0, _bn_lat = 0;				\
>> +	char *buf_name, *_bn_buf;					\
>> +									\
>> +	_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);				\
>> +									\
>> +	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(buf_size, 1U);	\
>> +									\
>> +		_bn_t = STRING_BENCH(_bn_iters, func, ##__VA_ARGS__);	\
> 
>> +									\
> 
> Remove unneeded blank line.

Will fix.

>> +		if (_bn_t > 0) {					\
>> +			_bn_mbps = (u64)(buf_size) * _bn_iters		\
> 
> Why buf_size in the parentheses here and not anywhere else (above)?

It was a bit inconsistent. I'll remove the unneeded parentheses for buf_size.

> I assume it's just an external temporary variable? But why do we need to have
> it in the parameters to the macro?

This is necessary because buf_size often needs to be passed as an argument
to the function under test. For instance, when benchmarking strnlen, the
caller must pass the current length as an argument:
STRING_BENCH_BUF(test, buf, len, strnlen, buf, len);

>> +					* (NSEC_PER_SEC / MEGA);	\
> > Leave '*' on the previous line.

Will fix.

>> +			_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)
> 

Thank you again for your patience and for helping me improve the quality of
this patch.

-- 
With Best Regards,
Feng Jiang