上次对函数调用进行了一定的剖析，这次课程带领我了解了linux进程切换的一些操作，做如下笔记：
“张涛 + 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 ”（课程要求）
/*
* 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;<span style="white-space:pre"> </span>//线程id
volatile long state; <span style="white-space:pre"> </span>//线程状态
char stack[KERNEL_STACK_SIZE];<span style="white-space:pre"> </span>//线程栈
struct Thread thread;<span style="white-space:pre"> </span>//线程结构，主要存储栈指针和执行的起始指针
unsigned long task_entry;<span style="white-space:pre"> </span>//形成的链表的起始指针（由于为单链表，此指针为必须的）
struct PCB *next;<span style="white-space:pre"> </span>//通过next指针行程一个线程的单链表
}tPCB;
void my_schedule(void);
首先介绍下头文件：头文件主要定义了使用到的线程使用到的一些结构进行了打包。
下面看初始化函数：
extern tPCB task[MAX_TASK_NUM];
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];/*由于栈是想低地址增长的，故初始化sp为数组最后一个元素的地址,栈空间可以在定义中看到为8k*/
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;<span style="white-space:pre"> </span>//使用数组作为存储结构，对栈指针进行适配
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 */<span style="white-space:pre"> </span> //其实就是帧指针进栈，因为此时刚开始，帧栈指针指向的地址相同。
"pushl %0\n\t" /* push task[pid].thread.ip */
"ret\n\t" /* pop task[pid].thread.ip to eip */ //觉得不采用jump的原因是由于长跳转与近跳转的原因
"popl %%ebp\n\t"
:
: "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) /* input c or d mean %ecx/%edx*/
);
}
下面就是线程的执行函数，即task_entry，这里设置每个线程函数均为my_process
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(); //进行线程调度
}
printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
}
}
}
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; //触发<span style="font-family: Arial, Helvetica, sans-serif;">my_process进行线程调度</span>
}
time_count ++ ;
#endif
return;
}
最核心的为线程调度代码：
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; //获取当前和即将运行的线程结构
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 */ //通过ret进指令跳转，因为之前已经将需要跳转的ip压栈，不通过跳转指令，应该是考虑到长跳转之类的
"1:\t" /* next process start here */ <span style="color:#ff0000;">//这里添加了下一条执行指令的标号，但运行未运行过的线程中没有这个标号不知道为什么？</span>
"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); <span style="color:#ff0000;">//举得这里其实应该是从next跳转回prev的过程？因为此时次线程已经运行并退出了,受讨论的启发，觉得他说的很有道理</span>
}
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 */ <span style="color:#ff0000;">//这里也有两点存疑：1.如上面说的，这里未定义标号1；2.push了ebp却未pop？</span>
"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;
}
总之，通过此次课的学习，对1.进程的切换时的保存现场；2.程序指针的跳转均有了一定了了解，收获颇丰。
有点遗憾，还没有付上实验楼的截图，因为本地网络比较差，本地搭建环境qemu一直无法获取；实验楼编译后也有些问题，网络好些进行补上。