优先级信号量任务切换

10 不同优先级线程获得信号量后输出不同？

发布于 2021-06-11 15:49:18 浏览：320

新建2个线程，其中，send线程得到信号量，变量自加，释放信号量。代码如下
```c
static void sem_send_thread_entry(void* parameter)
{
	rt_err_t uwRet = RT_ERROR;
	while (1)
	{
		uwRet = rt_sem_take(test_sem,RT_WAITING_FOREVER);
		if(uwRet == RT_EOK)
		{
			rt_kprintf("send得到信号量\n");
		}
		ucValue[0]++;
		ucValue[1]++;
		rt_sem_release(test_sem);
		rt_thread_mdelay(5);
	}
}
```
rev线程得到信号量，判断变量相等，释放信号量，代码如下：
```c
static void sem_rev_thread_entry(void* parameter)
{
	rt_err_t uwRet = RT_ERROR;
	while (1)
	{
		uwRet = rt_sem_take(test_sem,RT_WAITING_FOREVER);
		if(uwRet == RT_EOK)
		{
			rt_kprintf("rev得到信号量\n");
		}
		if(ucValue[0] == ucValue[1])
		{
			rt_kprintf("sem ok %d = %d\n",ucValue[0],ucValue[1]);
		}
		else
		{
			rt_kprintf("fail\n");
		}
		rt_sem_release(test_sem);
//		rt_thread_yield();
		rt_thread_mdelay(5);
	}
}
```
创建信号量和线程的代码如下：
```c
void sem_sample(void)
{
	test_sem = rt_sem_create("test_sem",1,RT_IPC_FLAG_FIFO);
	if(test_sem != RT_NULL)
	{
		rt_kprintf("sem create OK");
	}
	
	sem_send_thread = rt_thread_create("sem_send_thread",sem_send_thread_entry,RT_NULL,512,13,20);
	if(sem_send_thread != RT_NULL)
	{
		rt_thread_startup(sem_send_thread);
	}
	
	sem_rev_thread = rt_thread_create("sem_rev_thread",sem_rev_thread_entry,RT_NULL,512,12,20);
	if(sem_rev_thread != RT_NULL)
	{
		rt_thread_startup(sem_rev_thread);
	}
}
```
问题如下：当send优先级高于rev时，send线程执行多一点，输出出现间隔，如图：
![image.png](https://oss-club.rt-thread.org/uploads/20210611/9c6f4f09d2837322ab69691b522eb31a.png.webp)

我觉得是send优先级高，多执行了一些，按照这样的推理，当rev优先级高于send时，应该rev多执行一些，打印重复数字，但实际上没有，而且是连续输出
![image.png](https://oss-club.rt-thread.org/uploads/20210611/907a762cf0c71aa0da4fc0eae6e0f8df.png.webp)
和推理不一致，问题是为什么只是切换了2个线程的优先级，却造成打印的结果很大差别？谢谢