linux时间片轮转小程序

王涵宇 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.

一个简化了的时间片轮转程序，先运行实验楼linux自带的时间片轮转程序，结果如下：



2.代码

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"

tPCB task[MAX_TASK_NUM];	//pcb数组
tPCB * my_current_task = NULL;//指向pcb的指针
volatile int my_need_sched = 0;//是否需要调度

void my_process(void);

//进程的层次结构PCB->thread->ip,sp,每一个线程有自己的栈，程序入口my_process
//task数组记录了N个线程结构体，入口是myprocess

void __init my_start_kernel(void)
{
//0号进程的初始化
int pid = 0;
int i;
/* Initialize process 0*/
task[pid].pid = pid;
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;//起点是进程函数
task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1];//栈顶是定义的stack
task[pid].next = &task[pid];//指针指向自己，因为启动时候没有其他进程
/*fork more process */	//创建更多的进程
for(i=1;i<MAX_TASK_NUM;i++)
{
memcpy(&task[i],&task[0],sizeof(tPCB));	//把0号进程的状态赋值N个
task[i].pid = i;
task[i].state = -1;//进程不运行
task[i].thread.sp = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-1];//每个进程有自己的堆栈
task[i].next = task[i-1].next;
task[i-1].next = &task[i];//新fork的进程加到列表尾部
}
/* start process 0 by task[0] */
//0号进程开始执行
pid = 0;
my_current_task = &task[pid];
//保存上一个eip,到新的eip执行
asm volatile(
"movl %1,%%esp\n\t" 	/* set task[pid].thread.sp to esp */
"pushl %1\n\t" 	        /* push ebp */	//由于esp=ebp的原因，相当于push ebp
"pushl %0\n\t" 	        /* push task[pid].thread.ip */ //把当前eip压栈
"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)//函数
{
int i = 0;
while(1)
{
i++;
if(i%10000000 == 0)//一千万次打印一次
{
printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid);
if(my_need_sched == 1)//判定条件符合，执行my_schedule
{
my_need_sched = 0;

a19a
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.
* 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;

//有问题
if(my_current_task == NULL
|| my_current_task->next == NULL)
{
return;
}
printk(KERN_NOTICE ">>>my_schedule<<<\n");
/* schedule */
next = my_current_task->next;//当前进程的下一个进程
prev = my_current_task;
if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */	//当前进程正在执行，切换到下一个进程
{
/* switch to next process */	//进程切换关键代码
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;//此进程作为当前正在执行的进程
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" 			//保存程序入口
"ret\n\t" 	            /* restore  eip */
: "=m" (prev->thread.sp),"=m" (prev->thread.ip)
: "m" (next->thread.sp),"m" (next->thread.ip)
);
}
return;
}

3.分析



4.总结：

通过代码能够得到时间片轮转的成功原因:

a.通过栈保存上下文信息;

b.ret指令弹出栈存储的ip（程序入口）到eip寄存器，使得程序能顺利拿到下一条指令

c.得益于pcb结构体通过指针相连

d.state记录状态，-1表示不可运行，0表示可以运行