陈康+原创作品转载请注明出处+本文章为《linux内核分析》（MOOC课程http://mooc.study.163.com/course/USTC-1000029000 ）的实验报告，主要探讨Linux内核是采用怎样的机制实现进程间的轮转，最终完成一个简单的时间片轮转多道程序内核代码，实现进程调度。

1 实验环境

2 实验过程

cd LinuxKernel/linux-3.9.4rm -rf mykernelpatch -p1 < ../mykernel_for_linux3.9.4sc.patchmake allnoconfigmakeqemu -kernel arch/x86/boot/bzImage运行结果如下图所示：可以看到每隔固定的时间进程会切换到下一个，如此反复。
3 源代码及分析
修改的代码在mykernel文件夹下，主要涉及三个文件mypcb.h 用来声明简单的线程和进程PCB结构的文件；mymain.c 用来初始化和启动进程，定义进程代码逻辑的文件；myinterupt.c 用来处理时间中断函数，定义进程切换过程的文件。
3.1 mypcb.h代码分析
mypcb.h代码如下：
#define MAX_TASK_NUM        4#define KERNEL_STACK_SIZE   1024*2 /* CPU-specific state of this task */struct Thread {    unsigned long		ip;    unsigned long		sp;};typedef struct PCB{    int pid;    volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */    unsigned long 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);主要定义了两个结构体Thread和PCB。Thread中的ip代表进行执行的代码位置；sp代表进程执行的堆栈位置。PCB中的pid代表进程号，该进程号独一无二的，代表该进程；state代表进程的状态（未运行、运行、停止）；stack代表进程运行的堆栈空间；thread代表进程中执行运行任务的线程；task_entry代表进程的入口地址；next代表进程指向的下一个进程。
3.2 mymain.c代码分析
mymain.c代码如下：
#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].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);        }         }}
my_start_kernel是入口函数，最先执行该函数。主要作用是初始化各个进程的PCB结构，然后通过嵌入的汇编语句设置堆栈和eip，使内核执行PCB的id为0的进程的入口函数，使pid为0的进程开始执行。
my_process是示例进程的入口函数，进程执行时主要执行该函数。
3.3 myinterrupt.c代码分析
myinterrupt.c代码如下：
#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;    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)/* -1 unrunnable, 0 runnable, >0 stopped */    {            	my_current_task = next;     	printk(KERN_NOTICE ">>>switch %d to %d<<</n",prev->pid,next->pid);      	/* 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)    	);     }      return;	}
my_timer_handler是时钟中断执行函数，这里主要定时设置my_need_sched变量，该进程入口函数的进程调度。
my_schedule函数执行进程的调度，通过内嵌汇编语言，将当前进程的相关寄存器（esp ebp eip）保存起来，更改esp和eip寄存器使之能够执行下一个进程。
4 实验总结
内核主要通过PCB结构体来维护进程的状态及其它必要的参数信息，内核会维护一个进程PCB数组来管理系统中启动的所有进程。同时该实验也说明了进程如何进行切换，怎样保存现场和恢复现场，使切换到该进程时，能够接着上次执行到的位置进行执行，并且恢复到切换前的上下文环境。