RTT线程yield出错

#define THREAD_STACK_SIZE   1024
#define THREAD_PRIORITY     10
#define THREAD_TIMESLICE    5
/* 线程入口 */
static void thread_entry(void *parameter)
{
    rt_uint32_t value;
    rt_uint32_t count = 0;
    value = (rt_uint32_t)parameter;
    while (1)
    {
        rt_kprintf("[thread %d count = %d]\n", value, count++);
            if (count > 20) return;
        rt_thread_yield();
    }
}
int timeslice_sample(void)
{
    rt_thread_t tid = RT_NULL;
    /* 创建线程1*/
    tid = rt_thread_create("thread1",
                           thread_entry, (void *)1,
                           THREAD_STACK_SIZE,
                           THREAD_PRIORITY, THREAD_TIMESLICE);
    if (tid != RT_NULL)
        rt_thread_startup(tid);
    /* 创建线程2*/
    tid = rt_thread_create("thread2",
                           thread_entry, (void *)2,
                           THREAD_STACK_SIZE,
                           THREAD_PRIORITY, THREAD_TIMESLICE);
    if (tid != RT_NULL)
        rt_thread_startup(tid);
    return 0;
}
/* 导出到 msh 命令列表中 */
MSH_CMD_EXPORT(timeslice_sample, timeslice sample);

msh />timeslice_sample
[thread 1 count = 0]
[thread 1 count = 1]
[thread 1 count = 2]
[thread 1 count = 3]
[thread 1 count = 4]
[thread 1 count = 5]
[thread 1 count = 6]
[thread 1 count = 7]
[thread 1 count = 8]
[thread 1 count = 9]
[thread 1 count = 10]
[thread 1 count = 11]
[thread 1 count = 12]
[thread 1 count = 13]
[thread 1 count = 14]
[thread 1 count = 15]
[thread 1 count = 16]
[thread 1 count = 17]
[thread 1 count = 18]
[thread 1 count = 19]
[thread 1 count = 20]
[thread 2 count = 0]
[thread 2 count = 1]
[thread 2 count = 2]
[thread 2 count = 3]
[thread 2 count = 4]
[thread 2 count = 5]
[thread 2 count = 6]
[thread 2 count = 7]
[thread 2 count = 8]
[thread 2 count = 9]
[thread 2 count = 10]
[thread 2 count = 11]
[thread 2 count = 12]
[thread 2 count = 13]
[thread 2 count = 14]
[thread 2 count = 15]
[thread 2 count = 16]
[thread 2 count = 17]
[thread 2 count = 18]
[thread 2 count = 19]
[thread 2 count = 20]
msh />