完成一个简单的时间片轮转多道程序内核代码

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通过完成简单的时间片轮转多道程序内核代码加深对 Linux 内核线程的理解

先上代码：

  • myPCB.h

    /*
 *  linux/mykernel/mypcb.h
 *
 *  describe PCB
 *
 *  by Yuanhang Luo
 *
 */  

#define MAX_TASK_NUM 4
#define KERNEL_STACK_SIZE 1024*8

struct Thread{
    unsigned long ip; /* save the state of ip */
    unsigned long sp; /* save the state of sp */
};

typedef struct PCB{
    int pid;
    volatile long state;
    char stack[KERNEL_STACK_SIZE];
    struct Thread thread;
    unsigned long task_entry;
    struct PCB *next;
}tPCB;

void my_schedule(void);

  • mymain.c

    /*
 *  linux/mykernel/mymain.c
 *
 *  Kernel internal my_start_kernel
 *
 *  by  Yuanhang Luo
 *
 */

#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);
    }     
    }
}

  • 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.
 */
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 *prev;
    tPCB *next;

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

    printk(KERN_NOTICE ">>>MY 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;    

}

##重要汇编代码分析

asm volatile(
    "movl %1,%%esp\n\t"
       "pushl %1\n\t" 
    "pushl %0\n\t" 
    "ret\n\t" 
    "popl %%ebp\n\t"
    : 
    : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) 
);

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  • 保存恢复进程上下文

    asm volatile(   
    "pushl %%ebp\n\t"    //保存当前 ebp
    "movl %%esp,%0\n\t"     //保存 esp
    "movl %2,%%esp\n\t"        //载入下一个进程的 esp
    "movl $1f,%1\n\t"          //保存 eip
    "pushl %3\n\t"         //
    "ret\n\t"        //载入 eip
    "1:\t"        //下一个进程开始执行
    "popl %%ebp\n\t"    //
    : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
    : "m" (next->thread.sp),"m" (next->thread.ip)
); 

//如果没有正在运行的进程
asm volatile(   
    "pushl %%ebp\n\t"      //保存 ebp
    "movl %%esp,%0\n\t"    //保存 esp
    "movl %2,%%esp\n\t"    //载入 esp
    "movl %2,%%ebp\n\t"    //载入 ebp
    "movl $1f,%1\n\t"      //保存 eip  
    "pushl %3\n\t" 
    "ret\n\t"              //载入上下文
    : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
    : "m" (next->thread.sp),"m" (next->thread.ip)
);

##举例分析：如果有三个进程

  • 从 mymain.c 中的__init my_start_kernel方法开始执行
  • 新建 pid=0的进程，并且将其状态设置为0（runnable），设置进程入口地址、栈地址
  • 从0号进程复制1、2号进程，并且将0的 next 赋值为1号的入口地址，1号赋值为2号的入口地址
  • 将0号进程赋值为当前正在执行进程（my_current_task = &task[0];）
  • 执行汇编代码：保存进程信息，开始执行0号进程
  • 发生中断，需要切换进程
  • 执行汇编代码：保存当前进程的ebp、esp、eip（当前进程上下文）
  • 载入下一个进程（1号）的上下文（esp、eip）
  • 下一个进程（1号）开始执行
  • 再次发生中断，需要切换进程……
  • ……

##总结

  • CPU 和内核代码共同完成保存现场和恢复现场

  • 操作系统『两把剑』

    • 中断上下文切换
    • 进程上下文切换