魏昊卿——《Linux内核分析》第二周作业：了解操作系统是怎样工作的
一、实验部分
使用实验楼的虚拟机打开shell
cd LinuxKernel/linux-3.9.4 qemu -kernel arch/x86/boot/bzImage
然后cd mykernel 您可以看到qemu窗口输出的内容的代码mymain.c和myinterrupt.c
使用自己的Linux系统环境搭建过程参见mykernel,其中也可以找到一个简单的时间片轮转多道程序内核代码
mymain.c
myinterrupt.c
二、分析进程的启动和进程的切换机制
- 往往系统都有很多进程比较复杂，我们假定当前系统只有两个进程0和1，第一次调度是从0切换到1，也就是prev=0，next=1，第二次调度正好相反。
- 这时再看myinterrupt.c的汇编代码，保存prev的进程（0）上下文，下次调度是next进程就是0了，反之进程1是next那它肯定之前作为prev被调度出去过。理解进程上下文的保存和恢复极为关键。
- $1f就是指标号1:的代码在内存中存储的地址
- 再来看特殊一点代码切换到一个新的进程，也就是next没有被保存过进程上下文，它从没有被执行过，这时稍特殊一点即else部分的汇编代码
源代码：
mypcb.h
1 /*
2 * linux/mykernel/mypcb.h
3 *
4 * Kernel internal PCB types
5 *
6 * Copyright (C) 2013 Mengning
7 *
8 */
9
10 #define MAX_TASK_NUM 4
11 #define KERNEL_STACK_SIZE 1024*8
12
13 /* CPU-specific state of this task */
14 struct Thread {
15 unsigned long ip;
16 unsigned long sp;
17 };
18
19 typedef struct PCB{
20 int pid;
21 volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
22 char stack[KERNEL_STACK_SIZE];
23 /* CPU-specific state of this task */
24 struct Thread thread;
25 unsigned long task_entry;
26 struct PCB *next;
27 }tPCB;
28
29 void my_schedule(void);
详细解析：
1.
struct Thread {
unsigned long ip;
unsigned long sp;
};
用来存储ip和sp
2.
typedef struct PCB{
int pid; 进程id
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */ 进程状态
char stack[KERNEL_STACK_SIZE]; 进程堆栈
/* CPU-specific state of this task */
struct Thread thread;
unsigned long task_entry; 进程入口
struct PCB *next; 进程链表
}tPCB;
定义进程管理相关的数据结构，具体内容如上
3.
void my_schedule(void);
调度器
mymain.c
1 /*
2 * linux/mykernel/mymain.c
3 *
4 * Kernel internal my_start_kernel
5 *
6 * Copyright (C) 2013 Mengning
7 *
8 */
9 #include <linux/types.h>
10 #include <linux/string.h>
11 #include <linux/ctype.h>
12 #include <linux/tty.h>
13 #include <linux/vmalloc.h>
14
15
16 #include "mypcb.h"
17
18 tPCB task[MAX_TASK_NUM];
19 tPCB * my_current_task = NULL;
20 volatile int my_need_sched = 0; 是否需要调度
21
22 void my_process(void);
23
24
25 void __init my_start_kernel(void)
26 {
27 int pid = 0;
28 int i;
29 /* Initialize process 0*/初始化0号进程
30 task[pid].pid = pid; 进程id
31 task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped */进程状态
32 task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process; 进程入口
33 task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1]; 栈顶
34 task[pid].next = &task[pid];
35 /*fork more process */ 生成其他进程
36 for(i=1;i<MAX_TASK_NUM;i++) 新进程的相关信息
37 {
38 memcpy(&task[i],&task[0],sizeof(tPCB));
39 task[i].pid = i;
40 task[i].state = -1;
41 task[i].thread.sp = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-1];
42 task[i].next = task[i-1].next;
43 task[i-1].next = &task[i];
44 }
45 /* start process 0 by task[0] */ 启动0号进程
46 pid = 0;
47 my_current_task = &task[pid];
48 asm volatile(
49 "movl %1,%%esp\n\t" /* set task[pid].thread.sp to esp */
50 "pushl %1\n\t" /* push ebp */
51 "pushl %0\n\t" /* push task[pid].thread.ip */
52 "ret\n\t" /* pop task[pid].thread.ip to eip */
53 "popl %%ebp\n\t"
54 :
55 : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) /* input c or d mean %ecx/%edx*/
56 );
57 }
58 void my_process(void)
59 {
60 int i = 0;
61 while(1)
62 {
63 i++;
64 if(i%10000000 == 0)
65 {
66 printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid);
67 if(my_need_sched == 1)
68 {
69 my_need_sched = 0;
70 my_schedule();
71 }
72 printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
73 }
74 }
75 }
详细解析：
1.分析void __init my_start_kernel(void)
/* Initialize process 0*/初始化0号进程
task[pid].pid = pid; 进程id
task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped */进程状态
task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process; 进程入口
task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1]; 栈顶
task[pid].next = &task[pid]; 指向下一个进程的指针
/*fork more process */ 生成其他进程
for(i=1;i<MAX_TASK_NUM;i++) 新进程的相关信息，复制0号进程
{
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;
task[i-1].next = &task[i];
}
/* start process 0 by task[0] */ 启动0号进程，采用嵌入式汇编，构建了CPU的运行环境
pid = 0;
my_current_task = &task[pid];
asm volatile(
"movl %1,%%esp\n\t" /* set task[pid].thread.sp to esp */ 将sp的值放到esp中
"pushl %1\n\t" /* push ebp */ ebp等于esp，将ebp压栈
"pushl %0\n\t" /* push task[pid].thread.ip */ eip压栈
"ret\n\t" /* pop task[pid].thread.ip to eip */ 返回process函数的头部
"popl %%ebp\n\t" ebp出栈
:
: "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) /* input c or d mean %ecx/%edx*/
);
2.void my_process(void) 所有进程的代码相同，主动调度机制
void my_process(void)
{
int i = 0;
while(1)
{
i++;
if(i%10000000 == 0)执行1000万次后输出
{
printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid); ，打印
if(my_need_sched == 1)
{
my_need_sched = 0;
my_schedule();
}
printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
}
}
}
myinterrupt.c
1 /*
2 * linux/mykernel/myinterrupt.c
3 *
4 * Kernel internal my_timer_handler
5 *
6 * Copyright (C) 2013 Mengning
7 *
8 */
9 #include <linux/types.h>
10 #include <linux/string.h>
11 #include <linux/ctype.h>
12 #include <linux/tty.h>
13 #include <linux/vmalloc.h>
14
15 #include "mypcb.h"
16
17 extern tPCB task[MAX_TASK_NUM];
18 extern tPCB * my_current_task;
19 extern volatile int my_need_sched;
20 volatile int time_count = 0;
21
22 /*
23 * Called by timer interrupt.
24 * it runs in the name of current running process,
25 * so it use kernel stack of current running process
26 */
27 void my_timer_handler(void)
28 {
29 #if 1
30 if(time_count%1000 == 0 && my_need_sched != 1)
31 {
32 printk(KERN_NOTICE ">>>my_timer_handler here<<<\n");
33 my_need_sched = 1;
34 }
35 time_count ++ ;
36 #endif
37 return;
38 }
39
40 void my_schedule(void)
41 {
42 tPCB * next;
43 tPCB * prev;
44
45 if(my_current_task == NULL
46 || my_current_task->next == NULL)
47 {
48 return;
49 }
50 printk(KERN_NOTICE ">>>my_schedule<<<\n");
51 /* schedule */
52 next = my_current_task->next;
53 prev = my_current_task;
54 if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */
55 {
56 /* switch to next process */
57 asm volatile(
58 "pushl %%ebp\n\t" /* save ebp */
59 "movl %%esp,%0\n\t" /* save esp */
60 "movl %2,%%esp\n\t" /* restore esp */
61 "movl $1f,%1\n\t" /* save eip */
62 "pushl %3\n\t"
63 "ret\n\t" /* restore eip */
64 "1:\t" /* next process start here */
65 "popl %%ebp\n\t"
66 : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
67 : "m" (next->thread.sp),"m" (next->thread.ip)
68 );
69 my_current_task = next;
70 printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);
71 }
72 else
73 {
74 next->state = 0;
75 my_current_task = next;
76 printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);
77 /* switch to new process */
78 asm volatile(
79 "pushl %%ebp\n\t" /* save ebp */ 保存当前进程ebp
80 "movl %%esp,%0\n\t" /* save esp */ 保存eso
81 "movl %2,%%esp\n\t" /* restore esp */ 下一进程的esp放入esp中
82 "movl %2,%%ebp\n\t" /* restore ebp */
83 "movl $1f,%1\n\t" /* save eip */ 保存eip
84 "pushl %3\n\t" 将下一个进程的eip保存在堆栈中
85 "ret\n\t" /* restore eip */
86 : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
87 : "m" (next->thread.sp),"m" (next->thread.ip)
88 );
89 }
90 return;
91 }
详细分析：
1.void my_timer_handler(void)设置时间片的大小，时间片用完时设置一下调度标志
2.void my_schedule(void)void my_schedule(void)
{
tPCB * next;
tPCB * prev; 当前进程
if(my_current_task == NULL
|| my_current_task->next == NULL)
{
return;
}
printk(KERN_NOTICE ">>>my_schedule<<<\n");
/* schedule */
next = my_current_task->next; 当前进程的下一进程给next
prev = my_current_task;
if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */如果下一个进程状态为0，则切换进程
{
/* switch to next process */两个运行的进程之间做进程上下文切换
asm volatile(
"pushl %%ebp\n\t" /* save ebp */ 保存当前进程的ebp
"movl %%esp,%0\n\t" /* save esp */ 保存esp
"movl %2,%%esp\n\t" /* restore esp */ 将下一进程的esp放入esp寄存器中
"movl $1f,%1\n\t" /* save eip */
"pushl %3\n\t" 保存eip
"ret\n\t" /* restore eip */ 将下一进程的eip保存在堆栈中
"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; 切换到一个新的进程，该进程设为运行
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 */ 保存ebp
"movl %%esp,%0\n\t" /* save esp */ 保存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" 保存当前进程的入口
"ret\n\t" /* restore eip */
: "=m" (prev->thread.sp),"=m" (prev->thread.ip)
: "m" (next->thread.sp),"m" (next->thread.ip)
);
}
return;
}
三、总结这个星期的学习让我了解了linux操作系统的相关知识，并且老师通过对代码的讲解，着重让我知道了进程切换机制。在时间片轮转或者其他方式下，进程之间形成一个链表，一个进程运行到一定时间或者达到一定条件的时候，就切换成下一个程序运行，进程切换伴随着堆栈切换。 魏昊卿 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
时间： 2024-10-26 12:53:52