Re: [PATCH v2] selftests/cgroup: Adjust cpu.max quota based on HZ

[Tao Cui <cui.tao@linux.dev>]

Hi Joe,

One comment on the fallback:

  quota_usec = hz != -1 ? USEC_PER_SEC / hz : 1000;

When HZ can't be determined (no CONFIG_IKCONFIG_PROC, or zcat missing),
the fallback to 1000 is the exact value that fails at low HZ — so this
doesn't actually fix such kernels. A larger fallback (e.g. 10000, the
HZ=100 equivalent) would make the tests robust regardless of whether the
config is exposed.

在 2026/6/23 03:43, Joe Simmons-Talbott 写道:
> For lower HZ values a quota of 1000us is much lower than the amount
> of microseconds per tick which makes the tests test_cpucg_max and
> test_cpugc_max_nested fail. Use the amount of microseconds per tick
> as the quota value.
> 
> Signed-off-by: Joe Simmons-Talbott <joest@redhat.com>
> ---
> changes since v1:
> - Try checking /proc/config.gz to get the actual kernel HZ value and
>   fallback to 1000 if the value cannot be determined.
> 
>  tools/testing/selftests/cgroup/test_cpu.c | 33 +++++++++++++++++++++--
>  1 file changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/cgroup/test_cpu.c b/tools/testing/selftests/cgroup/test_cpu.c
> index 7a40d76b9548..65e09555309f 100644
> --- a/tools/testing/selftests/cgroup/test_cpu.c
> +++ b/tools/testing/selftests/cgroup/test_cpu.c
> @@ -639,6 +639,29 @@ test_cpucg_nested_weight_underprovisioned(const char *root)
>  	return run_cpucg_nested_weight_test(root, false);
>  }
>  
> +/*
> + * 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)
> +{
> +	long hz = -1;
> +	FILE *f;
> +	char cmd[256] = "zcat /proc/config.gz 2>/dev/null | grep '^CONFIG_HZ='";
> +
> +	f = popen(cmd, "r");
> +
> +	if (!f)
> +		goto out;
> +
> +	fscanf(f, "CONFIG_HZ=%ld", &hz);
> +
> +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.
> @@ -646,7 +669,8 @@ test_cpucg_nested_weight_underprovisioned(const char *root)
>  static int test_cpucg_max(const char *root)
>  {
>  	int ret = KSFT_FAIL;
> -	long quota_usec = 1000;
> +	long hz = _get_config_hz();
> +	long quota_usec;
>  	long default_period_usec = 100000; /* cpu.max's default period */
>  	long duration_seconds = 1;
>  
> @@ -655,6 +679,8 @@ static int test_cpucg_max(const char *root)
>  	char *cpucg;
>  	char quota_buf[32];
>  
> +	quota_usec = hz != -1 ? USEC_PER_SEC / hz : 1000;
> +
>  	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
>  
>  	cpucg = cg_name(root, "cpucg_test");
> @@ -710,7 +736,8 @@ static int test_cpucg_max(const char *root)
>  static int test_cpucg_max_nested(const char *root)
>  {
>  	int ret = KSFT_FAIL;
> -	long quota_usec = 1000;
> +	long quota_usec;
> +	long hz = _get_config_hz();
>  	long default_period_usec = 100000; /* cpu.max's default period */
>  	long duration_seconds = 1;
>  
> @@ -719,6 +746,8 @@ static int test_cpucg_max_nested(const char *root)
>  	char *parent, *child;
>  	char quota_buf[32];
>  
> +	quota_usec = hz != -1 ? USEC_PER_SEC / hz : 1000;
> +
>  	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
>  
>  	parent = cg_name(root, "cpucg_parent");