一个简单的时间片轮转内核代码的分析（课程作业）

struct Thread {
     unsigned long        ip;
     unsigned long        sp;
 };

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;
    struct PCB *next;
}tPCB;

#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*8

void my_schedule(void);

#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];           //声明一个task数组
tPCB * my_current_task = NULL;     //声明一个进程控制块指针
volatile int my_need_sched = 0;    //设置调度标识符

void my_process(void);           


void __init my_start_kernel(void)
{
    int pid = 0;  
    int i;
    /* Initialize process 0*/
    task[pid].pid = pid;     //初始化进程pid为0.
    task[pid].state = 0;     //初始化0号进程状态为运行
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process;                //初始化程序入口为，my_process
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1]; //初始化堆栈栈顶，esp指向栈顶。
    task[pid].next = &task[pid];  //初始时系统只有一个进程，所以自己指向自己。
    /*创建更多的进程*/
    for(i=1;i<MAX_TASK_NUM;i++)
    {
        memcpy(&task[i],&task[0],sizeof(tPCB)); //将0号进程的状态copy给i号进程
        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];
    }
    /* 开始执行0号进程*/
    pid = 0;
    my_current_task = &task[pid];
    asm volatile(
        "movl %1,%%esp\n\t"     /* 将task[pid].thread.sp赋给esp */
        "pushl %1\n\t"             /* 因为当前栈为空，所以esp=ebp，因此此步为push ebp*/
        "pushl %0\n\t"             /* 将当前eip压栈*/
        "ret\n\t"                 /* pop task[pid].thread.ip to eip */
        "popl %%ebp\n\t"        //pop ebp
        : 
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)    /* input c or d mean %ecx/%edx*/
    );
}   //这段汇编代码完成了该进程运行的堆栈环境的初始化

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 -\n",my_current_task->pid);
            if(my_need_sched == 1)  
            {
                my_need_sched = 0;  //将进程调度标识符置0
                my_schedule();     //调度下一个进程
            }
            printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
        }     
    }
}

#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;   //时钟计数

/*
 * 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)//时钟终端1000次，且调度标识符不为1时输出提示，且将调度标识符变为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)//当当前进程为空或当前下一个进程为空则返回。
    {
        return;
    }
    printk(KERN_NOTICE ">>>my_schedule<<<\n");
    /* schedule */
    next = my_current_task->next;
    prev = my_current_task;
    if(next->state == 0)//判断下一个进程是否为正在执行状态
    {
        /* switch to next process */
        asm volatile(    
            "pushl %%ebp\n\t"         //保存当前进程的ebp
            "movl %%esp,%0\n\t"     //将当前进程的esp保存在prev->thread.sp中
            "movl %2,%%esp\n\t"     //将下一个进程的esp（next->thread.sp）放在esp中
            "movl $1f,%1\n\t"       //将1:标志处的内存地址保存在prev->thread.ip中
            "pushl %3\n\t"        //将下一个进程的eip放入栈中
            "ret\n\t"                 //释放下一个进程的eip，下一个进程开始执行 
            "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    //对于还没有执行过的进程的处理
    {
        next->state = 0;  //将进程状态转为执行状态
        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 */
            "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"      //保存将当前进程的入口
            "ret\n\t"                 /* restore  eip */
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        );          
    }   
    return;