linux内核分析-简单的操作系统内核源码解读

##《Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000 ”学习笔记 ##

源码组成：mypcb.h mymain.c myinterrupt.c

1. mypcb.h

#define MAX_TASK_NUM        4   //进程数组链表的最大值
#define KERNEL_STACK_SIZE   1024*8 //内核堆栈的大小

/* CPU-specific state of this task */
struct Thread {
    unsigned long       ip;/*用于保存eip 指向CPU执行的下一条指令的地址*/
    unsigned long       sp;/*用于保存esp 堆栈指针*/
};

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;/*进程入口 本程序中都设置为my_process*/
    struct PCB *next; /*进程链表中的下一个进程*/
}tPCB;

void my_schedule(void);/*用于实现进程上下文的切换调度*/   

2.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 */
    /*进程入口地址与进程的eip设为my_process*/
    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;itask[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];
    /*构建0号进程的堆栈环境 ，启动0号进程
    c语言中嵌入汇编代码，格式如下：
       asm (
        内容
       : 输出
       ：输入
       );
       加入volatitle表示不让编译器优化*/
    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)
            {
               /*调度标志位，在myinterrupt.c的my_timer_handler()函数中设置*/
                my_need_sched = 0;
                /*主动调用进程列表的下一个进程*/
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
        }     
    }
}

3.myinterrupt.c

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;
    /*进程上下文的切换，与函数调用堆栈类似*/
    if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */
    {
        /* switch to next process */
        asm volatile(
            /*保存当前进程的ebp esp*/  
            "pushl %%ebp\n\t"       /* save ebp */
            "movl %%esp,%0\n\t"     /* save esp */
            /*构建下一个进程的esp*/
            "movl %2,%%esp\n\t"     /* restore  esp */
            /*$1f是指下面标号为1的位置，就是下一个进程启动的位置*/
            "movl $1f,%1\n\t"       /* save eip */ 
            /*下一个进程eip压栈*/
            "pushl %3\n\t" 
            /*恢复现场:eip指向下下个进程的地址ebp恢复为第一条指令压栈保存的ebp*/
            "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;//新的进程设置为正在运行状态
        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 */
            /*新的进程从来没有执行过，所以栈为空esp与ebp指向同一位置*/
            "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; 
}

4.小结
进程切换过程与函数调用堆栈基本类似

• 保存当前进程的堆栈环境(esp,eip,ebp)，首先将当前进程的ebp压栈，并将esp和eip保存在当前进程的结构体中。
• 构建下一进程的堆栈环境。然后下一条进程的thread.sp读出，赋给esp;thread.ip读出赋给eip。若下一条进程是新的进程，处于未运行状态，则新进程堆栈为空，esp与ebp指向同一地址，这种情况与函数调用堆栈更加类似。若下一条进程处于运行态，那么ebp与当前进程的ebp指向同一地址。
• 恢复堆栈环境，为下下个进程做准备。恢复eip,即把执行完成的进程的thread.ip(下一进程的指令地址)赋给eip，上例中所有进程的thread.ip设为my_process。