rt-thread线程切换过程分析

发布于 2010-09-19 23:13:06 浏览：5953 订阅该版

```rt-thread线程切换过程分析（rwen）

发这里希望各位大侠帮忙看看理解有没问题，非常感谢。

基于s3c2410，其他arm系列应该类似的

1、线程创建
线程由struct rt_thread表示。

rt_thread_t rt_thread_create (const char* name,
	void (*entry)(void* parameter), void* parameter,
	rt_uint32_t stack_size,
	rt_uint8_t priority,
	rt_uint32_t tick)
{
	struct rt_thread* thread;
	void* stack_start;

//分配一个线程结构
	thread = (struct rt_thread*) rt_object_allocate(RT_Object_Class_Thread, name);
	if (thread == RT_NULL) return RT_NULL;

//分配线程栈，线程的切换从根本上来说就是栈的切换
	stack_start = (void*)rt_malloc(stack_size);

//初始化
	_rt_thread_init(thread, name, entry, parameter,
		stack_start, stack_size,
		priority, tick);

return thread;
}

static rt_err_t _rt_thread_init(struct rt_thread* thread,
	const char* name,
	void (*entry)(void* parameter), void* parameter,
	void* stack_start, rt_uint32_t stack_size,
	rt_uint8_t priority, rt_uint32_t tick)
{
	/* set thread id and init thread list */
	thread->tid = thread;
	rt_list_init(&(thread->tlist));  //线程队列指针
	
	//线程人口函数及参数
	thread->entry = (void*)entry;  
	thread->parameter = parameter;

/* stack init */
	thread->stack_addr = stack_start;
	thread->stack_size = stack_size;

/* init thread stack */
	rt_memset(thread->stack_addr, '#', thread->stack_size);
	thread->sp = (void*)rt_hw_stack_init(thread->entry, thread->parameter,
		(void *) ((char *)thread->stack_addr + thread->stack_size - 4),
		(void*)rt_thread_exit);

/* priority init */
	RT_ASSERT(priority < RT_THREAD_PRIORITY_MAX);
	thread->init_priority = priority;
	thread->current_priority = priority;

/* tick init */
	thread->init_tick = tick; //线程可运行的tick计数
	thread->remaining_tick = tick;

/* init user data */
	thread->user_data = 0;

//线程定时器，用于唤醒睡眠时的唤醒
	/* init thread timer */
	rt_timer_init(&(thread->thread_timer),
		thread->name,
		rt_thread_timeout,
		thread,
		0,
		RT_TIMER_FLAG_ONE_SHOT);

return RT_EOK;
}

//栈的初始化
rt_uint8_t *rt_hw_stack_init(void *tentry, void *parameter,
	rt_uint8_t *stack_addr, void *texit)
{
	rt_uint32_t *stk;

stk 	 = (rt_uint32_t*)stack_addr;  //栈地址，返回给thread->sp
	*(stk) 	 = (rt_uint32_t)tentry;			/* entry point */ //入口函数
	
	//线程退出时执行的函数，lr是arm的子例程的返回地址，所以在entry退出时，该函数会执行
	*(--stk) = (rt_uint32_t)texit;			/* lr */ 
	*(--stk) = 0;							/* r12 */ 
	*(--stk) = 0;							/* r11 */
	*(--stk) = 0;							/* r10 */
	*(--stk) = 0;							/* r9 */
	*(--stk) = 0;							/* r8 */
	*(--stk) = 0;							/* r7 */
	*(--stk) = 0;							/* r6 */
	*(--stk) = 0;							/* r5 */
	*(--stk) = 0;							/* r4 */
	*(--stk) = 0;							/* r3 */
	*(--stk) = 0;							/* r2 */
	*(--stk) = 0;							/* r1 */
	*(--stk) = (rt_uint32_t)parameter;		/* r0 : argument */ //r0 entry传入参数
	*(--stk) = SVCMODE;						/* cpsr */
	*(--stk) = SVCMODE;						/* spsr */

/* return task's current stack address */
	return (rt_uint8_t *)stk; //栈地址，返回给thread->sp
}

//线程退出，回收资源
static void rt_thread_exit()
{
	struct rt_thread* thread;
    register rt_base_t temp;

/* disable interrupt */
    temp = rt_hw_interrupt_disable();

/* get current thread */
	thread = rt_current_thread;

//从运行队列中删除
	/* remove from schedule */
	rt_schedule_remove_thread(thread);

/* change stat */
	thread->stat = RT_THREAD_CLOSE; //状态

/* release thread timer */
	rt_timer_detach(&(thread->thread_timer));

/* enable interrupt */
    rt_hw_interrupt_enable(temp);

if (rt_object_is_systemobject((rt_object_t)thread) == RT_EOK)
	{
		rt_object_detach((rt_object_t)thread);
	}

//切换到下一个线程
	/* switch to next task */
	rt_schedule();
}

2.1、线程切换

void rt_schedule()
{
    register rt_uint8_t number;
    register rt_base_t level;
    register rt_uint8_t highest_ready_priority;

struct rt_thread *to_thread;
    struct rt_thread *from_thread;

/* disable interrupt */
    level = rt_hw_interrupt_disable();

/* check the scheduler is enabled or not */
    if (rt_scheduler_lock_nest == 0)
    {
    
       //在位图数组中找出有可运行线程的最高优先级队列
        /* 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
        //取出最高优先级队列中的第一个线程，O(1)复杂度
        /* get switch to thread */
        to_thread = rt_list_entry(rt_thread_priority_table[highest_ready_priority].next,
                                  struct rt_thread, tlist);

/* if the destination thread is not the same as current thread */
        if (to_thread != rt_current_thread)
        {
            rt_current_priority = highest_ready_priority;
            from_thread = rt_current_thread;  //被切换出去的是当前线程
            rt_current_thread = to_thread;

/* switch to new thread */
            if (rt_interrupt_nest == 0)
            {
               //线程切换
                rt_hw_context_switch((rt_uint32_t)&from_thread->sp, (rt_uint32_t)&to_thread->sp);
            }
            else
            {
							  //在中断上下文中切换
                rt_hw_context_switch_interrupt((rt_uint32_t)&from_thread->sp,
                                               (rt_uint32_t)&to_thread->sp);
            }
        }
    }

/* enable interrupt */
    rt_hw_interrupt_enable(level);
}

/*
 * void rt_hw_context_switch(rt_uint32 from, rt_uint32 to);
 * r0 --> from
 * r1 --> to
 */
.globl rt_hw_context_switch
rt_hw_context_switch:
  //保存被切换出来的线程的下一条指令地址，下一次该线程被执行时，将从这里开始
	stmfd	sp!, {lr}		@ push pc (lr should be pushed in place of PC)
	//保存寄存器，再一次保存lr，参考栈初始化函数，栈空间对应的寄存器位置
	stmfd	sp!, {r0-r12, lr}	@ push lr & register file
	mrs	r4, cpsr
	stmfd	sp!, {r4}		@ push cpsr
	mrs	r4, spsr
	stmfd	sp!, {r4}		@ push spsr

//保存当前线程栈指针
	str	sp, [r0]			@ store sp in preempted tasks TCB
	//载入新切换进来的线程的栈指针，参考栈初始化函数rt_hw_stack_init，注意各个栈空间各位置的值
	ldr	sp, [r1]			@ get new task stack pointer

从栈中弹出spsr/cpsr
	ldmfd	sp!, {r4}		@ pop new task spsr
	msr	spsr_cxsf, r4
	ldmfd	sp!, {r4}		@ pop new task cpsr
	msr	cpsr_cxsf, r4

//弹出其他寄存器，注意这里，对于新创建的线程，pc对应于thread->entry，就是线程的切入点，
  // 所以下面ldmfd执行完成后，系统将继续执行新线程的thread->entry
  //lr对应于rt_thread_exit，即是说，该线程退出时，该函数会执行。
  // 对于已经执行过的，再次被换入的线程，将从rt_hw_context_switch的下一条指令开始执行
	ldmfd	sp!, {r0-r12, lr, pc}	@ pop new task r0-r12, lr & pc

2.2 在中断中切换？```