kselftests/sched: cleanup the child processes

Previously, 'make -C sched run_tests' will block forever when it occurs
something wrong where the *selftests framework* is waiting for its child
processes to exit.

[root@iaas-rpma sched]# ./cs_prctl_test

 ## Create a thread/process/process group hiearchy
Not a core sched system
tid=74985, / tgid=74985 / pgid=74985: ffffffffffffffff
Not a core sched system
    tid=74986, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74988, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74989, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74990, / tgid=74986 / pgid=74985: ffffffffffffffff
Not a core sched system
    tid=74987, / tgid=74987 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74991, / tgid=74987 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74992, / tgid=74987 / pgid=74985: ffffffffffffffff
Not a core sched system
        tid=74993, / tgid=74987 / pgid=74985: ffffffffffffffff

Not a core sched system
(268) FAILED: get_cs_cookie(0) == 0

 ## Set a cookie on entire process group
-1 = prctl(62, 1, 0, 2, 0)
core_sched create failed -- PGID: Invalid argument
(cs_prctl_test.c:272) -
[root@iaas-rpma sched]# ps
    PID TTY          TIME CMD
   4605 pts/2    00:00:00 bash
  74986 pts/2    00:00:00 cs_prctl_test
  74987 pts/2    00:00:00 cs_prctl_test
  74999 pts/2    00:00:00 ps
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chris Hyser <chris.hyser@oracle.com>
Link: https://lore.kernel.org/r/20210902024333.75983-1-lizhijian@cn.fujitsu.com

tools/testing/selftests/sched/cs_prctl_test.c

@@ -62,6 +62,17 @@ enum pid_type {PIDTYPE_PID = 0, PIDTYPE_TGID, PIDTYPE_PGID};
 
 const int THREAD_CLONE_FLAGS = CLONE_THREAD | CLONE_SIGHAND | CLONE_FS | CLONE_VM | CLONE_FILES;
 
+struct child_args {
+	int num_threads;
+	int pfd[2];
+	int cpid;
+	int thr_tids[MAX_THREADS];
+};
+
+static struct child_args procs[MAX_PROCESSES];
+static int num_processes = 2;
+static int need_cleanup = 0;
+
 static int _prctl(int option, unsigned long arg2, unsigned long arg3, unsigned long arg4,
 		  unsigned long arg5)
 {
@@ -78,8 +89,14 @@ static int _prctl(int option, unsigned long arg2, unsigned long arg3, unsigned l
 #define handle_error(msg) __handle_error(__FILE__, __LINE__, msg)
 static void __handle_error(char *fn, int ln, char *msg)
 {
+	int pidx;
 	printf("(%s:%d) - ", fn, ln);
 	perror(msg);
+	if (need_cleanup) {
+		for (pidx = 0; pidx < num_processes; ++pidx)
+			kill(procs[pidx].cpid, 15);
+		need_cleanup = 0;
+	}
 	exit(EXIT_FAILURE);
 }
 
@@ -106,13 +123,6 @@ static unsigned long get_cs_cookie(int pid)
 	return cookie;
 }
 
-struct child_args {
-	int num_threads;
-	int pfd[2];
-	int cpid;
-	int thr_tids[MAX_THREADS];
-};
-
 static int child_func_thread(void __attribute__((unused))*arg)
 {
 	while (1)
@@ -212,10 +222,7 @@ void _validate(int line, int val, char *msg)
 
 int main(int argc, char *argv[])
 {
-	struct child_args procs[MAX_PROCESSES];
-
 	int keypress = 0;
-	int num_processes = 2;
 	int num_threads = 3;
 	int delay = 0;
 	int res = 0;
@@ -262,6 +269,7 @@ int main(int argc, char *argv[])
 
 	printf("\n## Create a thread/process/process group hiearchy\n");
 	create_processes(num_processes, num_threads, procs);
+	need_cleanup = 1;
 	disp_processes(num_processes, procs);
 	validate(get_cs_cookie(0) == 0);