实验二：基于mykernel实现的时间片轮转调度

原创作品转载请注明出处《Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000

如果我写的不好或者有误的地方请留言

• 题目自拟，内容围绕操作系统是如何工作的进行；

• 博客中需要使用实验截图

• 博客内容中需要仔细分析进程的启动和进程的切换机制

• 总结部分需要阐明自己对“操作系统是如何工作的”理解。

实验报告：

1.首先咱们来分析代码

通过分析下面的代码 我们知道PCB究竟长什么样子

struct Thread {
    unsigned long        ip;
    unsigned long        sp;
};
typedef struct PCB{
    int pid;
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long    task_entry;
    struct PCB *next;
}tPCB;

2.接下来咱们分析一下mymain.c

void __init my_start_kernel(void)
{
    int pid = 0;
    int i;
    /* Initialize process 0*/
    task[pid].pid = pid;
    task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped */
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process;
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1];
    task[pid].next = &task[pid];
    /*fork more process */
    for(i=1;i<MAX_TASK_NUM;i++)
    {
        memcpy(&task[i],&task[0],sizeof(tPCB));
        task[i].pid = i;
        task[i].state = -1;
        task[i].thread.sp = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-1];
        task[i].next = task[i-1].next;
        task[i-1].next = &task[i];
    }
    /* start process 0 by task[0] */
    pid = 0;
    my_current_task = &task[pid];
    asm volatile(
        "movl %1,%%esp\n\t"     /* set task[pid].thread.sp to esp */
        "pushl %1\n\t"             /* push ebp */
        "pushl %0\n\t"             /* push task[pid].thread.ip */
        "ret\n\t"                 /* pop task[pid].thread.ip to eip */
        "popl %%ebp\n\t"
        : 
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)    /* input c or d mean %ecx/%edx*/
    );
}   
void my_process(void)
{
    int i = 0;
    while(1)
    {
        i++;
        if(i%10000000 == 0)
        {
            printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid);
            if(my_need_sched == 1)
            {
                my_need_sched = 0;
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
        }     
    }
}

第一步初始化进程0

第二步另外创建3个进程PCB 其中对stack[]中的内容进行了简写

第三步通过嵌入式汇编代码启动进程0

 

第四步执行my_process()函数

3.接下来咱们分析一下myiterrrupt.c

void my_timer_handler(void)
{
#if 1
    if(time_count%1000 == 0 && my_need_sched != 1)
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<\n");
        my_need_sched = 1;
    } 
    time_count ++ ;  
#endif
    return;      
}

void my_schedule(void)
{
    tPCB * next;
    tPCB * prev;

    if(my_current_task == NULL 
        || my_current_task->next == NULL)
    {
        return;
    }
    printk(KERN_NOTICE ">>>my_schedule<<<\n");
    /* schedule */
    next = my_current_task->next;
    prev = my_current_task;
    if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */
    {
        /* switch to next process */
        asm volatile(    
            "pushl %%ebp\n\t"         /* save ebp */
            "movl %%esp,%0\n\t"     /* save esp */
            "movl %2,%%esp\n\t"     /* restore  esp */
            "movl $1f,%1\n\t"       /* save eip */    
            "pushl %3\n\t" 
            "ret\n\t"                 /* restore  eip */
            "1:\t"                  /* next process start here */
            "popl %%ebp\n\t"
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        ); 
        my_current_task = next; 
        printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);       
    }
    else
    {
        next->state = 0;
        my_current_task = next;
        printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);
        /* switch to new process */
        asm volatile(    
            "pushl %%ebp\n\t"         /* save ebp */
            "movl %%esp,%0\n\t"     /* save esp */
            "movl %2,%%esp\n\t"     /* restore  esp */
            "movl %2,%%ebp\n\t"     /* restore  ebp */
            "movl $1f,%1\n\t"       /* save eip */    
            "pushl %3\n\t" 
            "ret\n\t"                 /* restore  eip */
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        );          
    }   
    return;    
}

第一步分析my_timer_handler()函数

第二步分析my_schedule()函数

当next->state == 0时：

 

 

当next->state ！= 0时：

最后分析一下：

发现自己对函数堆栈理解有错误

1.其实所谓的内核堆栈 只是ESP指针指向内核中的地址

esp指向谁 谁就是堆栈 没有什么好解释的

所以可以做到多个pcb的切换

只要将esp指针指到对应的pcb的stack即可

2.关于对下面这2句话的理解也发生了错误

"movl $1f,%1\n\t" 

"1:\t"   

这里我以为eip指向了next的pcb 然后继续下面的汇编代码 我以为还是pre当前的pcb

其实我写这篇博客理解是有偏差的

进栈出栈

这里要明白pcb在哪里 它是谁的一部分

其实在函数调用中

因此汇编代码ret前后就要一分为二来看

ret前建立堆栈

eip转移到next-pcb

ret后拆除堆栈

继续执行next-pcb的代码

理解这一点是内核栈的关键！