基于mykernel的一个简单的时间片轮转多道程序内核代码分析

学号023作品

本实验资源来源： https://github.com/mengning/linuxkernel/ 

一、观察简易操作系统

此处使用实验楼的虚拟机打开终端

cd LinuxKernel/linux-3.9.4

rm -rf mykernel

patch -p1 < ../mykernel_for_linux3.9.4sc.patch

make allnoconfig

make ＃编译内核请耐心等待

qemu -kernel arch/x86/boot/bzImage

在QEMU窗口中我们可以看到一个运行中的简易操作系统，并输出字符串：

>>>>>my_timer_handler here<<<<<

和my_start_kernel here

运行截图如下：

在此处分析一下mymain.c和myinterrupt.c

mymain.c

在my_start_kernel函数中有一个循环，不停地输出my_start_kernel here

通过时钟中断周期函数调用my_timer_handler函数，可输出>>>>>my_timer_handler here<<<<<的字符串

 

总而言之，在mykernel启动之后，系统调用my_start_kernel函数，并利用时钟中断周期函数周期性调用my_timer_handler函数。

 

二、一个简单的时间片轮转多道程序分析

1、从https://github.com/mengning/mykernel/tree/master/mykernel-1.1上获取mymain.c、myinterrupt.c、pcb.c三个文件

2、在mykernel文件下覆盖mymain.c、myinterrupt.c，并新建pcb.c文件

3、cd linux-3.9.4 执行下列命令，重新编译内核

make allnoconfig

make

qemu -kernel arch/x86/boot/bzImage

运行截图如下：

可见系统进程在切换

 

以下进行代码分析

mypcb.h ： 进程控制块PCB结构体定义。

mymain.c： 初始化各个进程并启动0号进程。

myinterrupt.c：时钟中断处理和进程调度算法。

mypcb.h：

/*    mykernel--Simple simulation of the linux OS  process schedule
 *
 *  linux/mykernel/mypcb.h
 *
 *  Kernel internal my_timer_handler
 *
 *  Copyright (C) 2013  Mengning
 *  
 *  Modified 2014 Yunquan Zhang  <[e-mail]>
 * 
 * 
 *  You can redistribute or modify this program under the terms
 *  of the GNU General Public License as published by
 *  the Free Software Foundation.
 *
 * You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#define MAX_TASK_NUM 10 // max num of task in system
#define KERNEL_STACK_SIZE 1024*8
#define PRIORITY_MAX 30 //priority range from 0 to 30

/* CPU-specific state of this task */
struct Thread {
    unsigned long    ip;//point to cpu run address
    unsigned long    sp;//point to the thread stack's top address
    //todo add other attrubte of system thread
};
//PCB Struct
typedef struct PCB{
    int pid; // pcb id 
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];// each pcb stack size is 1024*8
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long    task_entry;//the task execute entry memory address
    struct PCB *next;//pcb is a circular linked list
    unsigned long priority;// task priority ////////
    //todo add other attrubte of process control block
}tPCB;

//void my_schedule(int pid);
void my_schedule(void);

在这个文件里，定义了 Thread 结构体，用于存储当前进程中正在执行的线程的ip和sp，PCB结构体中的各个字段含义如下

pid：进程号

state：进程状态，在模拟系统中，所有进程控制块信息都会被创建出来，其初始化值就是-1，如果被调度运行起来，其值就会变成0

stack：进程使用的堆栈

thread：当前正在执行的线程信息

task_entry：进程入口函数

next：指向下一个PCB，模拟系统中所有的PCB是以链表的形式组织起来的。

my_schedule声明，在myinterrupt.c中实现，在mymain.c中的各个进程函数会根据一个全局变量的状态来决定是否调用它，从而实现主动调度。

 

mymain.c

    /*
     *  linux/mykernel/mymain.c
     *
     *  Kernel internal my_start_kernel
     *
     *  Copyright (C) 2013  Mengning
     *
     */
    #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);
            }     
        }
    }

my_start_kernel 是系统启动后最先调用的函数，在这个函数里完成了0号进程的初始化和启动，并创建了其它的进程PCB，以方便后面的调度。在模拟系统里，每个进程的函数代码都是一样的，即 my_process 函数，my_process 在执行的时候，会打印出当前进程的 id，从而使得我们能够看到当前哪个进程正在执行。

 

myinterrupt.c

/*
 *  linux/mykernel/myinterrupt.c
 *
 *  Kernel internal my_timer_handler
 *
 *  Copyright (C) 2013  Mengning
 *
 */
#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;    
}

my_time_handler()函数，是个时间片轮转，周期性地发出中断信号，也就是my_need_sched。my_start_kernel()完成每个进程初始化，每个进程的任务都是my_process(),由于这个函数中有个无限循环，任务永远不会结束；并且启动了0号进程。任务需要调度时根据任务链表顺序进行调度。

实验总结

通过本次实验，对于进程调度和中断有了更加深刻的了解。总的来说，操作系统的工作需要进行进程的启动与切换，切换时需要保存上下文，切换的程序完成后需要恢复上下文，进程切换通过时钟中断进行控制。