Improve send_signal BPF test stability

Substitute sleep with dummy CPU intensive computation.
Finish aforemention computation as soon as signal was
delivered to the test process. Make the BPF code to
only execute when PID global variable is set

Signed-off-by: Mykola Lysenko <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/send_signal.c

···
 #include <sys/resource.h>
 #include "test_send_signal_kern.skel.h"
 
-int sigusr1_received = 0;
+static int sigusr1_received;
 
 static void sigusr1_handler(int signum)
 {
-	sigusr1_received++;
+	sigusr1_received = 1;
 }
 
 static void test_send_signal_common(struct perf_event_attr *attr,
···
 
 	if (pid == 0) {
 		int old_prio;
+		volatile int j = 0;
 
 		/* install signal handler and notify parent */
-		signal(SIGUSR1, sigusr1_handler);
+		ASSERT_NEQ(signal(SIGUSR1, sigusr1_handler), SIG_ERR, "signal");
 
 		close(pipe_c2p[0]); /* close read */
 		close(pipe_p2c[1]); /* close write */
···
 		ASSERT_EQ(read(pipe_p2c[0], buf, 1), 1, "pipe_read");
 
 		/* wait a little for signal handler */
-		sleep(1);
+		for (int i = 0; i < 100000000 && !sigusr1_received; i++)
+			j /= i + 1;
 
 		buf[0] = sigusr1_received ? '2' : '0';
+		ASSERT_EQ(sigusr1_received, 1, "sigusr1_received");
 		ASSERT_EQ(write(pipe_c2p[1], buf, 1), 1, "pipe_write");
 
 		/* wait for parent notification and exit */
···
 			goto destroy_skel;
 		}
 	} else {
-		pmu_fd = syscall(__NR_perf_event_open, attr, pid, -1,
+		pmu_fd = syscall(__NR_perf_event_open, attr, pid, -1 /* cpu */,
 				 -1 /* group id */, 0 /* flags */);
 		if (!ASSERT_GE(pmu_fd, 0, "perf_event_open")) {
 			err = -1;
···
 	ASSERT_EQ(read(pipe_c2p[0], buf, 1), 1, "pipe_read");
 
 	/* trigger the bpf send_signal */
-	skel->bss->pid = pid;
-	skel->bss->sig = SIGUSR1;
 	skel->bss->signal_thread = signal_thread;
+	skel->bss->sig = SIGUSR1;
+	skel->bss->pid = pid;
 
 	/* notify child that bpf program can send_signal now */
 	ASSERT_EQ(write(pipe_p2c[1], buf, 1), 1, "pipe_write");

tools/testing/selftests/bpf/progs/test_send_signal_kern.c

···
 {
 	int ret;
 
-	if (status != 0 || sig == 0 || pid == 0)
+	if (status != 0 || pid == 0)
 		return 0;
 
 	if ((bpf_get_current_pid_tgid() >> 32) == pid) {