丁春阳--原创作品转载请注明出处 + https://github.com/mengning/linuxkernel/
一、实验步骤
使用实验楼在线的环境，用其虚拟机打开shell，输入如下指令：
cd LinuxKernel/linux-3.9.4
rm -rf mykernel
patch -p1 < ../mykernel_for_linux3.9.4sc.patch
make allnoconfig
make
qemu -kernel arch/x86/boot/bzImage
启动内核，如下图所示:
修改/shiyanlou/LinuxKernel/linux-3.9.4/mykernel中的mymain.c；myinterrupt.c中的代码，并将mypcb.h的代码也放入其中，输入make命令，重新编译内核,输入qemu -kernel arch/x86/boot/bzImage 命令，启动内核，如下图所示：
二、重点代码分析
1.mypcb.h
/*
* linux/mykernel/mypcb.h
*
* Kernel internal PCB types
*
* Copyright (C) 2013 Mengning
*
*/
#define MAX_TASK_NUM 4//最大进程数
#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);//进程调度 进程控制块结构体中的pid指进程id，为进程的唯一标识，stack[KERNEL_STACK_SIZE]为进程对应的堆栈，thread是指线程信息，task_entry为可执行程序的入口,next为指向下一个进程控制块节点的指针。
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"
tPCB task[MAX_TASK_NUM];
tPCB * my_current_task = NULL;
volatile int my_need_sched = 0;//是否需要调度的标志
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].stack[KERNEL_STACK_SIZE-1] - 1) = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-1];
task[i].thread.sp = (unsigned long)(&task[i].stack[KERNEL_STACK_SIZE-1]);
task[i].next = task[i-1].next;//将第i个进程的进程控制块节点插入进程控制块链表中
task[i-1].next = &task[i];
}
/* start process 0 by task[0] */
pid = 0;
my_current_task = &task[pid];///当前进程指针指向0进程
asm volatile(//嵌入汇编代码，启动进程0
"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 */
:
: "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) /* input c or d mean %ecx/%edx*/
);
}
int i = 0;
void my_process(void)
{
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;//将进程是否需要调度标志重置为0。
my_schedule();
}
printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
}
}
}进程启动：用嵌入式汇编代码实现，先将原堆栈栈顶的地址存入ESP寄存器，将当前EBP寄存器的值入栈，将当前进程的EIP
的值入栈，ret命令将入栈进程的EIP存到EIP寄存器中。
进程执行函数：my_process()中存在一个死循环,循环1000万次才有一次机会判断进程是否需要调度，若需要调度则调用 my_schedule()函数，为主动调度。
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 current running process,
* so it use kernel stack of current running process
*/
void my_timer_handler(void)//被Linux内核周期性的调用
{
#if 1
if(time_count%1000 == 0 && my_need_sched != 1)//时间片的大小为1000
{
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)//异常处理
{
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;
}第一种情况，进程已经执行过，先保存ebp，再保存当前esp到当前进程pcb中，将next进程的堆栈栈顶的值存到esp中，保存当前进程的eip的值到pcb中，将next进程继续执行的代码位置压栈，然后出栈到eip。
第二种情况，进程尚未执行过（next->state != 0），则进行如下动作进行进程切换，先保存ebp到堆栈，再保存当前esp到当前进程pcb中，将next进程的堆栈栈顶的值存到esp中，将next进程的堆栈基地址的值存到ebp中，保存当前进程的eip的值到pcb中，将next进程继续执行的代码位置压栈，然后出栈到eip。
三、总结
计算机有三大法宝：存储程序计算机，函数调用堆栈，中断。操作系统有两大宝剑：即中断上下文和进程上下文。中断上下文的切换是保存现场和恢复现场；进程在执行过程中，当时间片用完切换时需要先保存当前的进程执行环境，下次进程被调度时，需要恢复进程的上下文环境，这样才能实现多道程序的并发执行。