操作系统是怎么工作的——mykernel环境的搭建

可以参见：https://github.com/mengning/mykernel

首先感谢：http://www.euryugasaki.com/archives/1014

1.搭建实验环境（实验环境centos6.5）

wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.9.4.tar.xz # download Linux Kernel 3.9.4 source code

wget --no-check-certificate https://raw.github.com/mengning/mykernel/master/mykernel_for_linux3.9.4sc.patch # downloadmykernel_for_linux3.9.4sc.patch

xz -d linux-3.9.4.tar.xz

tar -xvf linux-3.9.4.tar

cd linux-3.9.4

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

make allnoconfig

make

#在进行一下步骤时，当时系统提示没有qemu命令，需要进行相关配置！

qemu -kernel arch/x86/boot/bzImage

ln -s /usr/bin/qemu-system-i386 /usr/bin/qemu

2.代码分析

2.1 mypcb.h

#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*8                 //进程控制块

/* CPU-specific state of this task */
struct Thread {                        //存储ip,sp
    unsigned long        ip;
    unsigned long        sp;
};

typedef struct PCB{
    int pid;                        //进程的id
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];            //内核堆栈
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long    task_entry;            //指定的入口，平时入口为main函数
    struct PCB *next;                    //进程用链表连接
}tPCB;

void my_schedule(void);                    //函数，调度器

2.2 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];               //声明tPCB类型的数组
tPCB * my_current_task = NULL;           //声明当前task的指针
volatile int my_need_sched = 0;        //是否需要调度

void my_process(void);


void __init my_start_kernel(void)
{
    int pid = 0;
    int i;
    /* Initialize process 0*/
    task[pid].pid = pid;         //初始化0号进程
    task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped ，状态正在运行*/
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process;    //入口
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1];//
    task[pid].next = &task[pid];    //指向自己，系统启动只有0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;  //新进程加到进程链表尾部
        task[i-1].next = &task[i];
    }
    /* start process 0 by task[0] */
    pid = 0;
    my_current_task = &task[pid];
    asm volatile(
        "movl %1,%%esp
	"     /* set task[pid].thread.sp（%1） to esp */
        "pushl %1
	"             /* push ebp */
        "pushl %0
	"             /* push task[pid].thread.ip */
        "ret
	"             /* pop task[pid].thread.ip to eip ，ret之后0号进程正式启动*/
        "popl %%ebp
	"
        : 
        : "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)        //循环1000万次判断是否需要调度
        {
            printk(KERN_NOTICE "this is process %d -
",my_current_task->pid);
            if(my_need_sched == 1)
            {
                my_need_sched = 0;
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +
",my_current_task->pid);
        }     
    }
}

2.3 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)   //设置时间片的大小，时间片用完时设置调度的标志
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<
");
        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<<<
");
    /* 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<<<
",prev->pid,next->pid);  
        /* switch to next process */
        asm volatile(    
            "pushl %%ebp
	"     /* save ebp */
            "movl %%esp,%0
	"     /* save esp */
            "movl %2,%%esp
	"     /* restore  esp */
            "movl $1f,%1
	"       /* save eip，%1f指接下来的标号为1的位置 */    
            "pushl %3
	" 
            "ret
	"                 /* restore  eip */
            "1:	"                  /* next process start here */
            "popl %%ebp
	"
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        ); 
     
    }
    else
    {
        next->state = 0;
        my_current_task = next;
        printk(KERN_NOTICE ">>>switch %d to %d<<<
",prev->pid,next->pid);
        /* switch to new process */
        asm volatile(    
            "pushl %%ebp
	"      /* save ebp */
            "movl %%esp,%0
	"     /* save esp */
            "movl %2,%%esp
	"     /* restore  esp */
            "movl %2,%%ebp
	"     /* restore  ebp */
            "movl $1f,%1
	"       /* save eip */    
            "pushl %3
	" 
            "ret
	"                 /* restore  eip */
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        );          
    }   
    return;    
}

2.4总结如下

2.4.1

my_start_kernel()帮助我们创建进程；

my_timer_handler()来记录时间，触发调度；

my_start_kernel()中创建的0号进程的入口地址是my_process()。

2.4.2

my_process()作为每个进程的入口地址，开始逐个执行；

通过到达时间片的轮转时刻，my_process()会调用my_schedule()来保护进程堆栈现场，完成进程间的切换;

在mymain.c中实现内核的启动，通过my_start_kernel()来初始化进程；

2.4.3总体框架（不够完善）

2.4.4具体分析

2.4.4.1 首先看入口函数my_start_kernel()

    int pid = 0;
    int i;
    /* Initialize process 0*/
    task[pid].pid = pid;         //初始化0号进程
    task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped ，状态正在运行*/
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process;    //入口
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1];//
    task[pid].next = &task[pid];    //指向自己，系统启动只有0号进程

上述完成了对0号进程的初始化，包括

设置task[pid].state；

0号进程的入口地址为my_process()；

task[0]的进程属性中ip被设置成了my_process()函数的入口地址，sp设置成了堆栈的首地址

/*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;  //新进程加到进程链表
        task[i-1].next = &task[i];
    }

以0号进程为模板复制了MAX_TASK_NUM-1个进程，进程链表如下：

之后：

/* start process 0 by task[0] */
    pid = 0;
    my_current_task = &task[pid];
    asm volatile(
        "movl %1,%%esp
	"     /* set task[pid].thread.sp（%1） to esp */
        "pushl %1
	"             /* push ebp */
        "pushl %0
	"             /* push task[pid].thread.ip */
        "ret
	"             /* pop task[pid].thread.ip to eip ，ret之后0号进程正式启动*/
        "popl %%ebp
	"
        : 
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)    /* input c or d mean %ecx/%edx*/
    );

第一步(set task[pid].thread.sp（%1） to esp)，将task[0].thread.sp拿去修改esp的值，这时候内核堆栈的栈顶被修改到了task[0]的sp位置；

第二步（push ebp），在task[0]的sp位置处压入ebp的值，来保护原来的内核堆栈；

第三步(push task[pid].thread.ip ，pop task[pid].thread.ip to eip)，设置task[0].thread.ip的值给eip，这样就能够保证cpu下一步能够执行0号进程，完成了进入my_process()的过程。

注意：此时eip的值已经被修改，CPU进入my_process()，所以最后一句的popl ebp并不会被立即执行了。

2.4.4.2 再看my_process()与my_timer_handler()

void my_process(void)
{
    int i = 0;
    while(1)
    {
        i++;
        if(i%10000000 == 0)        //循环1000万次判断是否需要调度
        {
            printk(KERN_NOTICE "this is process %d -
",my_current_task->pid);
            if(my_need_sched == 1)
            {
                my_need_sched = 0;
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +
",my_current_task->pid);
        }     
    }
}

void my_timer_handler(void)
{
#if 1
    if(time_count%1000 == 0 && my_need_sched != 1)   //设置时间片的大小，时间片用完时设置调度的标志
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<
");
        my_need_sched = 1;
    } 
    time_count ++ ;  
#endif
    return;      
}

经过前面所述步骤，程序执行转到了my_process()。由于这时候所有的进程只有task[0]才是执行态，my_need_sched == 0，无论如何，都不会触发时间片的轮转从而调度其他的进程。但是有my_timer_handler()函数（该函数会被linux内核自动调用），my_timer_handler()能够得以自动执行，每次执行时，会检查时间计数以及当前进程是否应该被调度，当满足条件后，会修改0号进程的my_need_sched值为1，0号进程就被暂停执行，进而调用my_schedule()函数。

2.4.4.3 最后分析核心函数my_schedule()

if(my_current_task == NULL 
        || my_current_task->next == NULL)
    {
        return;
    }

首先进行一个简单的判断（判断当前的任务和接下来要被执行的任务是否为空）

/* 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<<<
",prev->pid,next->pid);  
        /* switch to next process */
        asm volatile(    
            "pushl %%ebp
	"     /* save ebp */
            "movl %%esp,%0
	"     /* save esp */
            "movl %2,%%esp
	"     /* restore  esp */
            "movl $1f,%1
	"       /* save eip，%1f指接下来的标号为1的位置 */    
            "pushl %3
	" 
            "ret
	"                 /* restore  eip */
            "1:	"                  /* next process start here */
            "popl %%ebp
	"
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        ); 
     
    }

在上面的代码中，next指针指向了当前任务的下一个任务，prev指针指向了当前0号进程。由建立的进程链表知：下一个被调度的进程应该是task[3]，而此时task[3]的状态是-1，会执行else中的部分：

else
    {
        next->state = 0;
        my_current_task = next;
        printk(KERN_NOTICE ">>>switch %d to %d<<<
",prev->pid,next->pid);
        /* switch to new process */
        asm volatile(    
            "pushl %%ebp
	"      /* save ebp */
            "movl %%esp,%0
	"     /* save esp */
            "movl %2,%%esp
	"     /* restore  esp */
            "movl %2,%%ebp
	"     /* restore  ebp */
            "movl $1f,%1
	"       /* save eip */    
            "pushl %3
	" 
            "ret
	"                 /* restore  eip */
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        );          
    }

task[3]的状态被更改为执行态，当前任务被修改为task[3]。

此时便开 始进行0号进程的现场保护工作，以便日后的调度。堆栈会保存ebp的值，同时将esp保存到0号进程的sp中。这是因为，当切换回0号进程的时候，可以通过0号进程内sp的值来寻找要执行的task[3]的进程堆栈。然后将task[3]的sp值设置到esp和ebp中，创建好了task[3]的执行堆栈。将task[3]执行任务的入口地址ip设置给eip，完成对任务的执行入口设置。这时候实际上后面的返回仍然不会被执行，因为在修改eip后，cpu又去执行下一步的my_process()了，因此这时候就会出现各种循环调用，利用时间片的统计，完成对进程之间的切换。