cfs-scheduler/starvation.c: Skip test on realtime kernels

This commit introduces a check in the starvation test case to detect and
skip execution on realtime kernels. The test is designed for use with the
Completely Fair Scheduler and produces meaningless results when run on
realtime kernels.

By skipping the test on realtime kernels, we avoid confusion caused by
misleading results.

Changes include:
- Added a detection mechanism for realtime kernels.
- Logic to skip the test execution if the kernel is identified as
  realtime.

Link: https://lore.kernel.org/ltp/[email protected]/
Reviewed-by: Petr Vorel <[email protected]>
Reviewed-by: Li Wang <[email protected]>
Reviewed-by: Cyril Hrubis <[email protected]>
Signed-off-by: Alessandro Carminati <[email protected]>

include/tst_kernel.h

@@ -58,4 +58,13 @@ int tst_check_builtin_driver(const char *driver);
  */
 int tst_check_driver(const char *driver);
 
+/**
+ * tst_check_preempt_rt() - Check if the running kernel is RT.
+ *
+ * Check support for the kernel module (both built-in and loadable).
+ *
+ * Return: -1 if the kernel is RT, 0 otherwise.
+ */
+int tst_check_preempt_rt(void);
+
 #endif	/* TST_KERNEL_H__ */

lib/tst_kernel.c

@@ -214,3 +214,13 @@ int tst_check_driver(const char *driver)
 
 	return -1;
 }
+
+int tst_check_preempt_rt(void)
+{
+	struct utsname uval;
+
+	uname(&uval);
+	if (strstr(uval.version, "PREEMPT_RT"))
+		return 1;
+	return 0;
+}

testcases/kernel/sched/cfs-scheduler/starvation.c

@@ -80,6 +80,9 @@ static void setup(void)
 	int cpu = 0;
 	long ncpus = tst_ncpus_conf();
 
+	if (tst_check_preempt_rt())
+		tst_brk(TCONF, "This test is not designed for the RT kernel");
+
 	CPU_ZERO(&mask);
 
 	/* Restrict test to a single cpu */