简单的linux内核时间片轮换

学号<202>
原创作品转载请注明出处https://github.com/mengning/linuxkernel/
sudo apt-get install qemu # install QEMU
sudo ln -s /usr/bin/qemu-system-i386 /usr/bin/qemu
wget --no-check-certificate https://www.kernel.org/pub/linux/kernel/v3.x/linux-    3.9.4.tar.xz # download Linux Kernel 3.9.4 source code
wget --no-check-certificate https://raw.github.com/mengning/mykernel/master/mykernel_for_linux3.9.4sc.patch # download mykernel_for_linux3.9.4sc.patch
xz -d linux-3.9.4.tar.xz
tar -xvf linux-3.9.4.tar
cd linux-3.9.4
patch -p1 < ../mykernel_for_linux3.9.4sc.patch
make allnoconfig
make

其中wget指令后要加--no-check-contificate 否则认证无法通过 

指令qemu -kernel arch/x86/boot/bzImage可以看到如图成果

打开mymain.c文件如图

打开myinterrupt.c文件，如图

会在start_kernel中运行，定长时间跳到Interrupt中去执行一次

二、一个简单的时间片轮转多道程序

obj-y = mymain.o myinterrupt.o
mymain.o:
    cc -c mymain.c mypcb.h
myinterrupt.o:
     cc -c myinterrupt.c mypcb.h

　　修改Makefile ，通过git-clone获取mymain.c 和 interrupt.c

程序运行如图

下面开始分析程序

struct Thread {
    unsigned long        ip;
    unsigned long        sp;
};

typedef struct PCB{
    int pid;
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    char stack[KERNEL_STACK_SIZE];
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long    task_entry;
    struct PCB *next;
}tPCB;

定义了进程和线程的结构体。

pid表示进程编号

state表示进程的状态

stack表示进程的栈

thread表示进程的ip sp的值

task_entry表示进程的入口，第一次执行时候指令所处的位置

next指向下一个进程的地址

 mymain.c

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*/
    );
}

这个整个程序的入口，首先初始化所有进程，只有进程0的运行态置为runnable，将线程定义成一个循环链表，方便之后通过next直接点用下一条进程。

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);
        }
    }
}

这段代码是进程要循行的，init将进程入口设置成了my_process，这个程序通过循环打印输出来表明是哪个进程到了这里面

在jinterrupt.c里面

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;      
}

定时器中断时，此函数调用一次，自增一次time_count，自增到达2000次输出一次

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)
        ); 
     
    }
    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;    
}

切换进程的函数。进程切换就是更改sp,bp,ip三个寄存器的值和pcb的结构，就可以运行在不用的线程下

汇编指令解析

1.ebp入栈，保存栈底

2.esp给prev->thread.sp 将当前esp保存到进程的sp

3.next给thread.sp->esp，切换的进程sp给esp,因为这个进程曾经就是这么获取的esp的值

4.$1f表示段1地址，即将段1地址->prev->thread.ip 当前进程如果再去执行的话要到段1去执行

56.将next->thread.ip给eip

7.栈顶元素pop作为ebp

两次操作ebp，先将进程的ebp入栈，然后执行下一个进程，执行完下一个进程后，esp不变，ebp就能够重新得到ebp，能起到保护ebp的作用，也不需要保存ebp的值在数据结构中