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

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

[Joe Simmons-Talbott]

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. Increase the test duration to accommodate
for lower HZ values.

Link: https://lore.kernel.org/lkml/20260623194239.GA899029@oak/
Signed-off-by: Joe Simmons-Talbott <joest@redhat.com>
---
v2 -> v3:
- Instead of changing cpu.max quota extend the test duration based on
  the HZ value.
- don't call pclose() if popen() fails.
- check return value of fscanf().

v1 -> v2:
- 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 | 44 ++++++++++++++++++++---
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_cpu.c b/tools/testing/selftests/cgroup/test_cpu.c
index 7a40d76b9548..feb7eb6a875c 100644
--- a/tools/testing/selftests/cgroup/test_cpu.c
+++ b/tools/testing/selftests/cgroup/test_cpu.c
@@ -639,6 +639,30 @@ 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)
+		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;
+
 	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
 
 	cpucg = cg_name(root, "cpucg_test");
@@ -710,15 +740,21 @@ static int test_cpucg_max(const char *root)
 static int test_cpucg_max_nested(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 *parent, *child;
 	char quota_buf[32];
 
+	if (hz == -1)
+		hz = 1000;
+	duration_seconds = 1000 / hz;
+	duration_usec = duration_seconds * USEC_PER_SEC;
+
 	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
 
 	parent = cg_name(root, "cpucg_parent");
-- 
2.54.0

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

[Michal Koutný]

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Hi.

On Wed, Jun 24, 2026 at 12:03:57PM -0400, Joe Simmons-Talbott <joest@redhat.com> 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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Re: [PATCH v3] selftests/cgroup: Adjust cpu test duration based on HZ

[Joe Simmons-Talbott]

On Thu, Jun 25, 2026 at 10:23:22AM +0200, Michal Koutný wrote:
> Hi.
> 
> On Wed, Jun 24, 2026 at 12:03:57PM -0400, Joe Simmons-Talbott <joest@redhat.com> 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).

Hi Michal,

Thanks for your feedback.  I'll make the changes you have suggested in v4.

Thanks,
Joe