Core: Support yield current coroutine for high performance timer.

public.h

@@ -98,6 +98,7 @@ extern st_thread_t st_thread_self(void);
 extern void st_thread_exit(void *retval);
 extern int st_thread_join(st_thread_t thread, void **retvalp);
 extern void st_thread_interrupt(st_thread_t thread);
+extern void st_thread_yield();
 extern st_thread_t st_thread_create(void *(*start)(void *arg), void *arg, int joinable, int stack_size);
 extern int st_randomize_stacks(int on);
 extern int st_set_utime_function(st_utime_t (*func)(void));

sched.c

@@ -66,6 +66,7 @@ unsigned long long _st_stat_sched_s = 0;
 
 unsigned long long _st_stat_thread_run = 0;
 unsigned long long _st_stat_thread_idle = 0;
+unsigned long long _st_stat_thread_yield = 0;
 #endif
 
 
@@ -554,6 +555,31 @@ void _st_vp_check_clock(void)
 }
 
 
+void st_thread_yield()
+{
+    _st_thread_t *me = _ST_CURRENT_THREAD();
+
+    // If not thread in RunQ to yield to, ignore and continue to run.
+    if (_ST_RUNQ.next == &_ST_RUNQ) {
+        return;
+    }
+
+    #ifdef DEBUG
+    ++_st_stat_thread_yield;
+    #endif
+
+    /* Check sleep queue for expired threads */
+    _st_vp_check_clock();
+
+    // Append thread to the tail of RunQ, we will back after all threads executed.
+    me->state = _ST_ST_RUNNABLE;
+    _ST_ADD_RUNQ(me);
+
+    // Yield to other threads in the RunQ.
+    _ST_SWITCH_CONTEXT(me);
+}
+
+
 void st_thread_interrupt(_st_thread_t *thread)
 {
     /* If thread is already dead */