RTT线程调度时,获得最高优先级的算法问题

发布于 2013-02-22 23:02:07 浏览：2284 订阅该版

RTT在线程调度时,每次都需要获取当前最高优先级的线程,此时使用了一张位图:
```
const rt_uint8_t rt_lowest_bitmap[] =
{
    /* 00 */ 0, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 10 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 20 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 30 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 40 */ 6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 50 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 60 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 70 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 80 */ 7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* 90 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* A0 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* B0 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* C0 */ 6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* D0 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* E0 */ 5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    /* F0 */ 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};
```
及线程控制块内的一些相关参数:(number,high_mask,number_mask),还一些全局参数(rt_thread_ready_priority_group,rt_thread_ready_table[32]),通过在线程startup函数内操作这些参数,
```//...
    /* calculate priority attribute */
#if RT_THREAD_PRIORITY_MAX > 32
    thread->number      = thread->current_priority >> 3;            /* 5bit */
    thread->number_mask = 1L << thread->number;
    thread->high_mask   = 1L << (thread->current_priority & 0x07);  /* 3bit */
#else
    thread->number_mask = 1L << thread->current_priority;
#endif
//...```
然后在调试函数rt_schedule内计算最高优先级线程:
```
//...
        register rt_ubase_t highest_ready_priority;

#if RT_THREAD_PRIORITY_MAX == 8
        highest_ready_priority = rt_lowest_bitmap[rt_thread_ready_priority_group];
#else
        register rt_ubase_t number;
        /* find out the highest priority task */
        if (rt_thread_ready_priority_group & 0xff)
        {
            number = rt_lowest_bitmap[rt_thread_ready_priority_group & 0xff];
        }
        else if (rt_thread_ready_priority_group & 0xff00)
        {
            number = rt_lowest_bitmap[(rt_thread_ready_priority_group >> 8) & 0xff] + 8;
        }
        else if (rt_thread_ready_priority_group & 0xff0000)
        {
            number = rt_lowest_bitmap[(rt_thread_ready_priority_group >> 16) & 0xff] + 16;
        }
        else
        {
            number = rt_lowest_bitmap[(rt_thread_ready_priority_group >> 24) & 0xff] + 24;
        }

#if RT_THREAD_PRIORITY_MAX > 32
        highest_ready_priority = (number << 3) +
                                 rt_lowest_bitmap[rt_thread_ready_table[number]];
#else
        highest_ready_priority = number;
#endif
#endif
//...
```
这里我不太明白这个过程的原理,还个那个位图又是怎么来的? 为什么是这样的?测试得来的吗?原理没看懂...
麻烦老大解释下...