Re: [PATCH v3] selftests/cgroup: Adjust cpu test duration based on HZ

[Michal Koutný]

Hi.

On Wed, Jun 24, 2026 at 12:03:57PM -0400, Joe Simmons-Talbott <joest@xxxxxxxxxx> wrote:
> +/*
> + * Best effort attempt to get the kernel's HZ value from the config.
> + * Return the HZ value if found otherwise return -1 to indicate failure.
> + */
> +static long
> +_get_config_hz(void)

drop underscore from the static function

> +{
> +	long hz = -1;

use the default 1000 here to simplify the callers

> +	FILE *f;
> +	char cmd[256] = "zcat /proc/config.gz 2>/dev/null | grep '^CONFIG_HZ='";
> +
> +	f = popen(cmd, "r");
> +
> +	if (!f)
> +		return hz;
> +
> +	if (fscanf(f, "CONFIG_HZ=%ld", &hz) == EOF)
> +		goto out;
> +
> +out:
> +	pclose(f);
> +	return hz;
> +}
> +
>  /*
>   * This test creates a cgroup with some maximum value within a period, and
>   * verifies that a process in the cgroup is not overscheduled.
> @@ -646,15 +670,21 @@ test_cpucg_nested_weight_underprovisioned(const char *root)
>  static int test_cpucg_max(const char *root)
>  {
>  	int ret = KSFT_FAIL;
> +	long hz = _get_config_hz();
>  	long quota_usec = 1000;
>  	long default_period_usec = 100000; /* cpu.max's default period */
> -	long duration_seconds = 1;
> +	long duration_seconds;
>  
> -	long duration_usec = duration_seconds * USEC_PER_SEC;
> +	long duration_usec;
>  	long usage_usec, n_periods, remainder_usec, expected_usage_usec;
>  	char *cpucg;
>  	char quota_buf[32];
>  
> +	if (hz == -1)
> +		hz = 1000;
> +	duration_seconds = 1000 / hz;
> +	duration_usec = duration_seconds * USEC_PER_SEC;

I'd do the calculation in usecs

	duration_usec = duration_seconds * USEC_PER_SEC * 1000 / hz;

so that actual duration is more precise (for hz=300 which is the only
that doesn't divide 1000)

All in all, make the adjustments for HZ with less code (since I expect
this will need adjustments for SMPs in future).

Thanks,
Michal

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