[kernel] add real time flag for threads

Threads marked as real time do not have the preemption timer run against
them and will continue to run until they voluntarily yield or are
preempted by a higher priority thread from irq context.

app/tests/thread_tests.c

@@ -450,11 +450,7 @@ static volatile int preempt_count;
 
 static int preempt_tester(void *arg)
 {
-#define COUNT (8*1024*1024)
-
-	int i;
-	for (i = 0; i < COUNT; i++)
-		__asm__ volatile("nop");
+	spin(1000000);
 
 	printf("exiting ts %lld\n", current_time_hires());
 
@@ -465,12 +461,14 @@ static int preempt_tester(void *arg)
 
 static void preempt_test(void)
 {
+	/* create 5 threads, let them run. If the system is properly timer preempting,
+	 * the threads should interleave each other at a fine enough granularity so
+	 * that they complete at roughly the same time. */
 	printf("testing preemption\n");
 
 	preempt_count = 5;
 
-	int i;
-	for (i = 0; i < preempt_count; i++)
+	for (int i = 0; i < preempt_count; i++)
 		thread_detach_and_resume(thread_create("preempt tester", &preempt_tester, NULL, LOW_PRIORITY, DEFAULT_STACK_SIZE));
 
 	while (preempt_count > 0) {
@@ -478,6 +476,25 @@ static void preempt_test(void)
 	}
 
 	printf("done with preempt test, above time stamps should be very close\n");
+
+	/* do the same as above, but mark the threads as real time, which should
+	 * effectively disable timer based preemption for them. They should
+	 * complete in order, about a second apart. */
+	printf("testing real time preemption\n");
+
+	preempt_count = 5;
+
+	for (int i = 0; i < preempt_count; i++) {
+		thread_t *t = thread_create("preempt tester", &preempt_tester, NULL, LOW_PRIORITY, DEFAULT_STACK_SIZE);
+		thread_set_real_time(t);
+		thread_detach_and_resume(t);
+	}
+
+	while (preempt_count > 0) {
+		thread_sleep(1000);
+	}
+
+	printf("done with real-time preempt test, above time stamps should be 1 second apart\n");
 }
 
 int thread_tests(void)
@@ -495,3 +512,5 @@ int thread_tests(void)
 
 	return 0;
 }
+
+/* vim: set ts=4 sw=4 noexpandtab: */