Core: Refine yield for high performance timer.

sched.c

@@ -67,6 +67,7 @@ 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;
+unsigned long long _st_stat_thread_yield2 = 0;
 #endif
 
 
@@ -559,18 +560,22 @@ void st_thread_yield()
 {
     _st_thread_t *me = _ST_CURRENT_THREAD();
 
+    #ifdef DEBUG
+    ++_st_stat_thread_yield;
+    #endif
+
+    /* Check sleep queue for expired threads */
+    _st_vp_check_clock();
+
     // 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;
+    ++_st_stat_thread_yield2;
     #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);