linux-kernel - [PATCH v2] selftests/cgroup: improve the accuracy of cpu.max tests

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Message-ID: <20250703120325.2905314-1-shashank.mahadasyam@sony.com>
Date: Thu,  3 Jul 2025 21:03:20 +0900
From: Shashank Balaji <shashank.mahadasyam@...y.com>
To: Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shuah Khan <shuah@...nel.org>
Cc: Shashank Balaji <shashank.mahadasyam@...y.com>,
	cgroups@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Shinya Takumi <shinya.takumi@...y.com>
Subject: [PATCH v2] selftests/cgroup: improve the accuracy of cpu.max tests

Current cpu.max tests (both the normal one and the nested one) are inaccurate.

They setup cpu.max with 1000 us quota and the default period (100,000 us).
A cpu hog is run for a duration of 1s as per wall clock time. This corresponds
to 10 periods, hence an expected usage of 10,000 us. We want the measured
usage (as per cpu.stat) to be close to 10,000 us.

Previously, this approximate equality test was done by
`!values_close(usage_usec, duration_usec, 95)`: if the absolute
difference between usage_usec and duration_usec is greater than 95% of
their sum, then we pass. This is problematic for two reasons:

1. Semantics: When one sees `values_close` they expect the error
   percentage to be some small number, not 95. The intent behind using
`values_close` is lost by using a high error percent such as 95. The
intent it's actually going for is "values far".

2. Bound too wide: The condition translates to the following expression:

	|usage_usec - duration_usec| > (usage_usec + duration_usec)*0.95

  	0.05*duration_usec > 1.95*usage_usec (usage < duration)

	usage_usec < 0.0257*duration_usec = 25,641 us

   So, this condition passes as long as usage_usec is lower than 25,641
us, while all we want is for it to be close to 10,000 us.

Fix this by explicitly calcuating the expected usage duration based on the
configured quota, default period, and the duration, and compare usage_usec
and expected_usage_usec using values_close() with a 10% error margin.

Also, use snprintf to get the quota string to write to cpu.max instead of
hardcoding the quota, ensuring a single source of truth.

Signed-off-by: Shashank Balaji <shashank.mahadasyam@...y.com>

---

Changes in v2:
- Incorporate Michal's suggestions:
	- Merge two patches into one
	- Generate the quota string from the variable instead of hardcoding it
	- Use values_close() instead of labs()
	- Explicitly calculate expected_usage_usec
- v1: https://lore.kernel.org/all/20250701-kselftest-cgroup-fix-cpu-max-v1-0-049507ad6832@sony.com/
---
 tools/testing/selftests/cgroup/test_cpu.c | 63 ++++++++++++++++-------
 1 file changed, 43 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_cpu.c b/tools/testing/selftests/cgroup/test_cpu.c
index a2b50af8e9ee..2a60e6c41940 100644
--- a/tools/testing/selftests/cgroup/test_cpu.c
+++ b/tools/testing/selftests/cgroup/test_cpu.c
@@ -2,6 +2,7 @@
 
 #define _GNU_SOURCE
 #include <linux/limits.h>
+#include <sys/param.h>
 #include <sys/sysinfo.h>
 #include <sys/wait.h>
 #include <errno.h>
@@ -645,10 +646,16 @@ test_cpucg_nested_weight_underprovisioned(const char *root)
 static int test_cpucg_max(const char *root)
 {
 	int ret = KSFT_FAIL;
-	long usage_usec, user_usec;
-	long usage_seconds = 1;
-	long expected_usage_usec = usage_seconds * USEC_PER_SEC;
+	long quota_usec = 1000;
+	long default_period_usec = 100000; /* cpu.max's default period */
+	long duration_seconds = 1;
+
+	long duration_usec = duration_seconds * USEC_PER_SEC;
+	long usage_usec, n_periods, remainder_usec, expected_usage_usec;
 	char *cpucg;
+	char quota_buf[32];
+
+	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
 
 	cpucg = cg_name(root, "cpucg_test");
 	if (!cpucg)
@@ -657,13 +664,13 @@ static int test_cpucg_max(const char *root)
 	if (cg_create(cpucg))
 		goto cleanup;
 
-	if (cg_write(cpucg, "cpu.max", "1000"))
+	if (cg_write(cpucg, "cpu.max", quota_buf))
 		goto cleanup;
 
 	struct cpu_hog_func_param param = {
 		.nprocs = 1,
 		.ts = {
-			.tv_sec = usage_seconds,
+			.tv_sec = duration_seconds,
 			.tv_nsec = 0,
 		},
 		.clock_type = CPU_HOG_CLOCK_WALL,
@@ -672,14 +679,19 @@ static int test_cpucg_max(const char *root)
 		goto cleanup;
 
 	usage_usec = cg_read_key_long(cpucg, "cpu.stat", "usage_usec");
-	user_usec = cg_read_key_long(cpucg, "cpu.stat", "user_usec");
-	if (user_usec <= 0)
+	if (usage_usec <= 0)
 		goto cleanup;
 
-	if (user_usec >= expected_usage_usec)
-		goto cleanup;
+	/*
+	 * The following calculation applies only since
+	 * the cpu hog is set to run as per wall-clock time
+	 */
+	n_periods = duration_usec / default_period_usec;
+	remainder_usec = duration_usec - n_periods * default_period_usec;
+	expected_usage_usec
+		= n_periods * quota_usec + MIN(remainder_usec, quota_usec);
 
-	if (values_close(usage_usec, expected_usage_usec, 95))
+	if (!values_close(usage_usec, expected_usage_usec, 10))
 		goto cleanup;
 
 	ret = KSFT_PASS;
@@ -698,10 +710,16 @@ static int test_cpucg_max(const char *root)
 static int test_cpucg_max_nested(const char *root)
 {
 	int ret = KSFT_FAIL;
-	long usage_usec, user_usec;
-	long usage_seconds = 1;
-	long expected_usage_usec = usage_seconds * USEC_PER_SEC;
+	long quota_usec = 1000;
+	long default_period_usec = 100000; /* cpu.max's default period */
+	long duration_seconds = 1;
+
+	long duration_usec = duration_seconds * USEC_PER_SEC;
+	long usage_usec, n_periods, remainder_usec, expected_usage_usec;
 	char *parent, *child;
+	char quota_buf[32];
+
+	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
 
 	parent = cg_name(root, "cpucg_parent");
 	child = cg_name(parent, "cpucg_child");
@@ -717,13 +735,13 @@ static int test_cpucg_max_nested(const char *root)
 	if (cg_create(child))
 		goto cleanup;
 
-	if (cg_write(parent, "cpu.max", "1000"))
+	if (cg_write(parent, "cpu.max", quota_buf))
 		goto cleanup;
 
 	struct cpu_hog_func_param param = {
 		.nprocs = 1,
 		.ts = {
-			.tv_sec = usage_seconds,
+			.tv_sec = duration_seconds,
 			.tv_nsec = 0,
 		},
 		.clock_type = CPU_HOG_CLOCK_WALL,
@@ -732,14 +750,19 @@ static int test_cpucg_max_nested(const char *root)
 		goto cleanup;
 
 	usage_usec = cg_read_key_long(child, "cpu.stat", "usage_usec");
-	user_usec = cg_read_key_long(child, "cpu.stat", "user_usec");
-	if (user_usec <= 0)
+	if (usage_usec <= 0)
 		goto cleanup;
 
-	if (user_usec >= expected_usage_usec)
-		goto cleanup;
+	/*
+	 * The following calculation applies only since
+	 * the cpu hog is set to run as per wall-clock time
+	 */
+	n_periods = duration_usec / default_period_usec;
+	remainder_usec = duration_usec - n_periods * default_period_usec;
+	expected_usage_usec
+		= n_periods * quota_usec + MIN(remainder_usec, quota_usec);
 
-	if (values_close(usage_usec, expected_usage_usec, 95))
+	if (!values_close(usage_usec, expected_usage_usec, 10))
 		goto cleanup;
 
 	ret = KSFT_PASS;
-- 
2.43.0