一个简单的时间片轮转Linux内核分析

Linux内核 操作系统

by 干宇航
原创作品转载请注明出处 《Linux内核分析》MOOC课程

实验环境：windows10 pro + Virtual box 5.14 下 Ubuntu 15.10 64位

采用孟宁老师提供的mykernel环境

环境截图

mypcb.h 代码

/*  mykernel--Simple simulation of the linux OS  process schedule
 *
 *  linux/mykernel/mypcb.h
 *
 *  Kernel internal my_timer_handler
 *
 *  Copyright (C) 2013  Mengning
 *  
 *  Modified 2014 Yunquan Zhang  <zhangyunquan@hotmail.com>
 * 
 * 
 *  You can redistribute or modify this program under the terms
 *  of the GNU General Public License as published by
 *  the Free Software Foundation.
 *
 * You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#define MAX_TASK_NUM 10 // max num of task in system
#define KERNEL_STACK_SIZE 1024*8
#define PRIORITY_MAX 30 //priority range from 0 to 30
/* CPU-specific state of this task */
struct Thread {
    unsigned long   ip;//point to cpu run address
    unsigned long   sp;//point to the thread stack's top address
    //todo add other attrubte of system thread
};
//PCB Struct
typedef struct PCB{
    int pid; // pcb id 
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];// each pcb stack size is 1024*8
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long   task_entry;//the task execute entry memory address
    struct PCB *next;//pcb is a circular linked list
    unsigned long priority;// task priority ////////
    //todo add other attrubte of process control block
}tPCB;
//void my_schedule(int pid);
void my_schedule(void);

首先来看mypcb.h里面有什么东西
thread 里面 ip,sp分别是eip和esp
PCB

• pid 进程id
• state 进程状态 -1 未就绪 0就绪
• stack 定义了一个内核堆栈
• thread
• task_entry
• 本身类型的指针

然后定义了一个调度器
my_schedule

mymain.c代码

/*
 *  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
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].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] */
    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);
        }     
    }
}

先把mypcb.h包含进来
先声明一个task的数组
当前的task的指针
声明一个标志 my_need_sched

my_start_kernel

零号进程初始化
状态就绪
起点、入口是my_process
即函数执行的是my_process
堆栈的栈顶，定义了一个stack
指针指向它自己，因为系统只有它自己。

将零号进程的状态，复制过来

每个进程有它自己的堆栈
新fork的进程添加到这个task链表的尾部

开始执行零号进程
有一段汇编代码，比较关键

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*/
    );

ret之后0号进程启动起来

下面我们来看一下my_process的代码
执行一千万次，之后主动调度

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%100 == 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 */
    {
        /* 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)
    	); 
    	my_current_task = next; 
    	printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);   	
    }
    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 */  
            "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; 
}

再来看myinterrupt的代码

全局变量声明三个变量
定义时间片 time_count

my_timer_handler中
当调度过一次之后，把my_need_sched赋为1

然后来看my_schedule的函数

前面链表相关的操作和检查

下面if 检查是否就绪
然后进行上下文切换。