Linux内核分析 操作系统是如何工作的
Linux内核分析 操作系统是如何工作的
20122137 沙雨济
原创作品转载请注明出处
Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000
1、 内容说明
内嵌汇编语法如下：
具体代码如下：
(1)mypcb.h 头文件
/*
* linux/mykernel/mypcb.h
* Kernel internal PCB types
* Copyright (C) 2013 Mengning
*/
#define MAX_TASK_NUM 4
#define KERNEL_STACK_SIZE 1024*8
/* CPU-specific state of this task */
struct Thread {
unsigned long ip;
unsigned long sp;
};
typedef struct PCB{
int pid;
volatile long state; /* -1unrunnable, 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;
void my_schedule(void);
（2）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"
tPCBtask[MAX_TASK_NUM];
tPCB *my_current_task = NULL;
volatile intmy_need_sched = 0;
voidmy_process(void);
void __initmy_start_kernel(void)
{
int pid = 0;
int i;
/* Initialize process 0*/
task[pid].pid = pid;
task[pid].state = 0;/* -1 unrunnable, 0runnable, >0 stopped */
task[pid].task_entry = task[pid].thread.ip= (unsigned long)my_process;
task[pid].thread.sp = (unsignedlong)&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 = (unsignedlong)&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" /* settask[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*/
);
}
voidmy_process(void)
{
int i = 0;
while(1)
{
i++;
if(i%10000000 == 0)
{
printk(KERN_NOTICE "this isprocess %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);
}
}
}
（3）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 currentrunning process,
* so it use kernel stack ofcurrent 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)/* -1unrunnable, 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;
}
2. 代码分析
在此我只做重点分析，比如较难理解的进程初始化、切换的几段汇编代码。
第一个进程的初始化环境设置：
asm volatile(
"movl %1,%%esp\n\t" /*将进程的堆栈值存入系统堆栈 */
"pushl %1\n\t" /* 将当前ebp寄存器值入栈 */
"pushl %0\n\t" /* 将当前进程的eip入栈*/
"ret\n\t" /*ret命令正好可以让入栈的进程eip保存到eip寄存器中 */
"popl %%ebp\n\t"
:
: "c"(task[pid].thread.ip),"d" (task[pid].thread.sp)
);
进程调度代码：
if(next->state == 0)/*next->state == 0对应进程next对应进程曾经执行过*/
{ //行进程调度关键代码
asm volatile(
"pushl%%ebp\n\t" /* 保存当前ebp到堆栈中 */
"movl %%esp,%0\n\t" /* 保存当前进程堆栈指针到当前进程tcb中*/
"movl %2,%%esp\n\t"/*将下一进程的esp值存到esp寄存器 */
"movl $1f,%1\n\t" /*保存当前进程的eip值，下次恢复进程后将在1：开始执行*/
"pushl %3\n\t" /*将新的eip存到栈中*/
"ret\n\t" /*保存eip到eip寄存器*/
"1:\t" /* 下一进程执行位置*/
"popl %%ebp\n\t" /* 恢复ebp的值*/
: "=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 /*表明next该进程第一次被执行*/
{
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" /* 保存当前进程ebp */
"movl%%esp,%0\n\t" /* 保存当前进程esp */
"movl%2,%%esp\n\t" /* 重新载入esp*/
"movl%2,%%ebp\n\t" /* 重新载入ebp */
"movl$1f,%1\n\t" /* 保存当前eip寄存器值 */
"pushl%3\n\t" /* 把即将执行的进程的eip入栈 */
"ret\n\t" /* 重新载入eip*/
: "=m"(prev->thread.sp),"=m" (prev->thread.ip)
: "m"(next->thread.sp),"m" (next->thread.ip)
);
}
3、 试验截图
4、总结
本次试实验要求实现多道进程的实行，重点在于进程的切换，进程执行过程中，当时间片用完需要进行进程切换时，需要先将当前的进程执行环境进行保存，下次进程被调度时，需要恢复进程的执行环境。
