linux内核分析-简单的操作系统内核源码解读

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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;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];    /*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。