Linux内核分析之二——时间片轮转多道程序上下文切换机制之堆栈分析

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作者：姚开健

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

在计算机中，能够保证计算机程序运行的一些重要机制包括：存储程序，函数调用堆栈机制，中断机制以及多道程序上下文切换机制。本文要介绍的就是在时间片轮转的情况下，多道程序上下文切换的机制是如何工作的。以一个小小的kernel，运行着几个程序，然后在时间片消耗完之后如何来切换程序，来模拟操作系统的进程切换机制。

首先由三个程序文件：

1、mypcb.h

/* *  linux/mykernel/mypcb.h *  Kernel internal PCB types

*  Copyright (C) 2013  Mengning * */#define MAX_TASK_NUM        4#define KERNEL_STACK_SIZE   1024*8/* CPU-specific state of this task */struct Thread {    unsigned longip;   unsigned longsp;};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 longtask_entry;    struct PCB *next;}tPCB;void my_schedule(void);

2，mymain.c

#include <linux/types.h>#include <linux/string.h>#include <linux/ctype.h>#include <linux/tty.h>#include <linux/vmalloc.h>#include "mypcb.h"tPCB task[MAX_TASK_NUM];tPCB * my_current_task = NULL;volatile int my_need_sched = 0;void my_process(void);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);        }         }}

3、myinterrupt.c

#include <linux/types.h>#include <linux/string.h>#include <linux/ctype.h>#include <linux/tty.h>#include <linux/vmalloc.h>#include "mypcb.h"extern tPCB task[MAX_TASK_NUM];extern tPCB * my_current_task;extern volatile int my_need_sched;volatile int time_count = 0;/* * Called by timer interrupt. * it runs in the name of current running process, * so it use kernel stack of current running process */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;}

程序运行过程如：

从上面的文件可以看到，我们定义一个PCB，里面有进程的标号，堆栈指针，堆栈，eip，下一进程指针等等，程序首先在void __init my_start_kernel(void)函数执行，先初始化0号进程，即task[0]，

 /* 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];

接着初始化1,2,3号进程，

  /*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];    }

接着运行0号进程，这时需要详细说明进程如何开始运行：

 /* 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*/);}

0号进程首先运行上面的汇编代码，分别是1，把进程PCB初始化时的栈顶指针送到CPU的esp，2，ebp压栈，因为是0号进程，ebp与esp相同，所以可以用pushl %esp，3，初始化的eip压栈再出栈，此时程序eip为函数void my_process(void)的地址，程序跳转到此函数运行。如果系统的时钟中断函数my_timer_handler时间片运行完，则把可以切换程序标志置为true，所以当my_process函数运行足够时间后，便会访问切换标志，如果为true，则进行程序切换，访问my_schedule(void)函数。

当从0号进程切换至1号进程时，执行

 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)    );              }

这是进程切换的关键步骤，首先1，pushl %ebp，pushl %esp，保存刚才运行的进程的ebp和esp，2，接着把新进程原来初始化的esp（包括ebp）送到CPU的esp,ebp，接着保存旧进程的eip（$1f,1的意思是标号1的代码段的地址，f意思是forward，即标号1下面的地址）3，接着把新进程的eip压栈再出栈，程序跳转至my_process函数执行，接着以此类推至3号进程切换至0号进程，执行：

   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);       }

这时的进程切换也是与之前的切换大同小异，也是先保存旧进程的ebp,esp，接着把新进程保存的esp恢复，然后eip压栈再出栈，跳转至1：处执行，ebp出栈。

通过此程序我们知道，进程的运行开始时需要把栈顶地址（即栈数组的末尾地址）送至CPU的esp，并初始化ebp，接着把进程需要运行的函数地址压栈，再ret（即popl %eip），程序就能进入函数运行，进程启动完毕。

在程序进行切换时，需要保持旧进程的ebp,esp，然后再按照新进程的运行开始过程（如上面所说）一样初始化新进程的相关数据。

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