Fix usleep for small values

core/thread.c

@@ -331,6 +331,8 @@ int threadUsleep(unsigned long usec)
     current->state              = SLEEPING;
     current->sleepHaveTimeouted = 0;
     current->jiffiesSleeping    = usecs_to_jiffies(usec);
+    if (!current->jiffiesSleeping) // sleep at least 1 jiffies
+        current->jiffiesSleeping = 1;
     next                        = threadSelectNext();
 
     assert(next != current);

userspace/main_user.c

@@ -124,6 +124,7 @@ int func_munmap()
 static void *print_hello(void *arg) {
         (void)arg;
         printf("Hello World from thread %lu\n", gettid());
+        usleep(100);
 
         return NULL;
 }