cgroup: Add test_cpucg_max_nested() testcase

The cgroup cpu controller selftests have a test_cpucg_max() testcase
that validates the behavior of the cpu.max knob. Let's also add a
testcase that verifies that the behavior works correctly when set on a
nested cgroup.
Signed-off-by: NDavid Vernet <void@manifault.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

tools/testing/selftests/cgroup/test_cpu.c

@@ -617,6 +617,68 @@ static int test_cpucg_max(const char *root)
 	return ret;
 }
 
+/*
+ * This test verifies that a process inside of a nested cgroup whose parent
+ * group has a cpu.max value set, is properly throttled.
+ */
+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;
+	char *parent, *child;
+
+	parent = cg_name(root, "cpucg_parent");
+	child = cg_name(parent, "cpucg_child");
+	if (!parent || !child)
+		goto cleanup;
+
+	if (cg_create(parent))
+		goto cleanup;
+
+	if (cg_write(parent, "cgroup.subtree_control", "+cpu"))
+		goto cleanup;
+
+	if (cg_create(child))
+		goto cleanup;
+
+	if (cg_write(parent, "cpu.max", "1000"))
+		goto cleanup;
+
+	struct cpu_hog_func_param param = {
+		.nprocs = 1,
+		.ts = {
+			.tv_sec = usage_seconds,
+			.tv_nsec = 0,
+		},
+		.clock_type = CPU_HOG_CLOCK_WALL,
+	};
+	if (cg_run(child, hog_cpus_timed, (void *)&param))
+		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)
+		goto cleanup;
+
+	if (user_usec >= expected_usage_usec)
+		goto cleanup;
+
+	if (values_close(usage_usec, expected_usage_usec, 95))
+		goto cleanup;
+
+	ret = KSFT_PASS;
+
+cleanup:
+	cg_destroy(child);
+	free(child);
+	cg_destroy(parent);
+	free(parent);
+
+	return ret;
+}
+
 #define T(x) { x, #x }
 struct cpucg_test {
 	int (*fn)(const char *root);
@@ -629,6 +691,7 @@ struct cpucg_test {
 	T(test_cpucg_nested_weight_overprovisioned),
 	T(test_cpucg_nested_weight_underprovisioned),
 	T(test_cpucg_max),
+	T(test_cpucg_max_nested),
 };
 #undef T