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

[Joe Simmons-Talbott <joest@redhat.com>]

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