基于mykernel的时间片轮转调度

时间:2021-10-25 19:54:51

学号: 363

原创作品,转载请注明出处。
本实验资源来源: https://github.com/mengning/linuxkernel/

 

一、 实验环境配置

本次实验在实验楼完成:

在实验楼的终端下输入下面命令:

cd LinuxKernel/linux-3.9.4
rm -rf mykernel
patch -p1 < ../mykernel_for_linux3.9.4sc.patch
make allnoconfig
make 
qemu -kernel arch/x86/boot/bzImage

可查看运行结果

基于mykernel的时间片轮转调度

 关闭qemu窗口,进入mykernel文件夹,可以查看mymain.c和myinterrupt.c文件。

基于mykernel的时间片轮转调度

mymain.c的代码不断循环的去执行,周期性的产生时钟中断信号,去执行myinterrupt.c的代码。

 

二、实现时间片轮转多道程序

将mymain.c,myinterrupt.c,mypcb.h三个文件复制替换到mykernel文件夹下。

运行如下:

基于mykernel的时间片轮转调度

可以看到进程1切换到了进程2。

三、时间片轮转多道程序的代码分析

mypcb.h

/*
 *  linux/mykernel/mypcb.h
 *
 *  Kernel internal PCB types
 *
 *  Copyright (C) 2013  Mengning
 *
 */

#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*2 # unsigned long
/* CPU-specific state of this task */
struct Thread {
    unsigned long        ip;
    unsigned long        sp;
};

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

void my_schedule(void);

可以看到最大进程数定义为四个,程序控制块PCB中定义了pid,状态statue,线程thread,进程入口函数task_entry等.

 

mymain.c文件

/*
 *  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].stack[KERNEL_STACK_SIZE-1] - 1) = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-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 */
        : 
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)    /* input c or d mean %ecx/%edx*/
    );
} 

int i = 0;

void my_process(void)
{    
    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);
        }     
    }
}

 

在这个文件中void __init my_start_kernel(void)这个函数fork了4个新进程,把新fork的进程加入到进程链表在这个文件中。
汇编过程如下:
(1)将0号进程的esp的值赋给ESP寄存器
(2)将0号进程的esp的值压栈(此时堆栈状态为进程0的堆栈)
(3)将0号进程的eip的值压栈
(4)通过ret指令,让栈顶的eip的值出栈到EIP寄存器中(间接改变EIP寄存器的值),完成进程0的启动

 

myinterupt.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 */
    {        
        my_current_task = next; 
        printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);  
        /* 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)
        ); 
    }  
    return;    
}

通过my_time_handler()函数定时地不断向cpu发出中断,从而实现了时间片轮转。每调用1000次,就去将全局变量my_need_sched的值修改为1,通知正在执行的进程执行调度程序my_schedule。从而在my_schedule函数中完成进程的不断切换。

 

四、总结


(1)进程和中断在操作系统是是非常重要的两个部分,需要熟练掌握。
(2)EIP寄存器储存着当前执行的代码,可以通过更改EIP寄存器的值来更改当前执行的代码,从而实现进程切换。出于安全考虑,EIP寄存器的值不能被直接改变,但可以通过压栈+ret指令来间接改变。
(3)进程在执行过程中,当时间片用完之后需要进程切换时,需要保存当前的执行上下文环境,下次被调度的时候,需要回复进程的上下文环境。