linux-hardening - Re: [PATCH v2 08/14] lib/string_kunit: add performance benchmark for strlen()

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Message-ID: <aWYGWLEekWh3jJVf@smile.fi.intel.com>
Date: Tue, 13 Jan 2026 10:46:16 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Feng Jiang <jiangfeng@...inos.cn>
Cc: pjw@...nel.org, palmer@...belt.com, aou@...s.berkeley.edu,
	alex@...ti.fr, kees@...nel.org, andy@...nel.org,
	akpm@...ux-foundation.org, ebiggers@...nel.org,
	martin.petersen@...cle.com, ardb@...nel.org,
	ajones@...tanamicro.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 v2 08/14] lib/string_kunit: add performance benchmark for
 strlen()

On Tue, Jan 13, 2026 at 04:27:42PM +0800, Feng Jiang wrote:
> Introduce a benchmark to compare the architecture-optimized strlen()
> implementation against the generic C version (__generic_strlen).
> 
> The benchmark uses a table-driven approach to evaluate performance
> across different string lengths (short, medium, and long). It employs
> ktime_get() for timing and get_random_bytes() followed by null-byte
> filtering to generate test data that prevents early termination.
> 
> This helps in quantifying the performance gains of architecture-specific
> optimizations on various platforms.

...

> +static void string_test_strlen_bench(struct kunit *test)
> +{
> +	char *buf;
> +	size_t buf_len, iters;
> +	ktime_t start, end;
> +	u64 time_arch, time_generic;
> +
> +	buf_len = get_max_bench_len(bench_cases, ARRAY_SIZE(bench_cases)) + 1;
> +
> +	buf = kunit_kzalloc(test, buf_len, GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buf);
> +
> +	for (size_t i = 0; i < ARRAY_SIZE(bench_cases); i++) {
> +		get_random_nonzero_bytes(buf, bench_cases[i].len);
> +		buf[bench_cases[i].len] = '\0';
> +
> +		iters = bench_cases[i].iterations;
> +
> +		/* 1. Benchmark the architecture-optimized version */
> +		start = ktime_get();
> +		for (unsigned int j = 0; j < iters; j++) {
> +			OPTIMIZER_HIDE_VAR(buf);
> +			(void)strlen(buf);

First Q: Are you sure the compiler doesn't replace this with __builtin_strlen() ?

> +		}
> +		end = ktime_get();
> +		time_arch = ktime_to_ns(ktime_sub(end, start));
> +
> +		/* 2. Benchmark the generic C version */
> +		start = ktime_get();
> +		for (unsigned int j = 0; j < iters; j++) {
> +			OPTIMIZER_HIDE_VAR(buf);
> +			(void)__generic_strlen(buf);
> +		}

Are you sure the warmed up caches do not affect the benchmark? I think you need
to flush / make caches dirty or so on each iteration.

> +		end = ktime_get();
> +		time_generic = ktime_to_ns(ktime_sub(end, start));
> +
> +		string_bench_report(test, "strlen", &bench_cases[i],
> +				time_arch, time_generic);
> +	}
> +}


-- 
With Best Regards,
Andy Shevchenko