簡單的linux內核時間片輪換

學號<202>
原創作品轉載請注明出處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 否則認證無法通過 

指令qemu -kernel arch/x86/boot/bzImage可以看到如圖成果

打開mymain.c文件如圖

打開myinterrupt.c文件，如圖

會在start_kernel中運行，定長時間跳到Interrupt中去執行一次

二、一個簡單的時間片輪轉多道程序

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 ，通過git-clone獲取mymain.c 和 interrupt.c

程序運行如圖

下面開始分析程序

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;

定義了進程和線程的結構體。

pid表示進程編號

state表示進程的狀態

stack表示進程的棧

thread表示進程的ip sp的值

task_entry表示進程的入口，第一次執行時候指令所處的位置

next指向下一個進程的地址

 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*/
    );
}

這個整個程序的入口，首先初始化所有進程，只有進程0的運行態置為runnable，將線程定義成一個循環鏈表，方便之后通過next直接點用下一條進程。

 

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);
        }
    }
}

這段代碼是進程要循行的，init將進程入口設置成了my_process，這個程序通過循環打印輸出來表明是哪個進程到了這里面

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;      
}

定時器中斷時，此函數調用一次，自增一次time_count，自增到達2000次輸出一次

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;    
}

切換進程的函數。進程切換就是更改sp,bp,ip三個寄存器的值和pcb的結構，就可以運行在不用的線程下

匯編指令解析

1.ebp入棧，保存棧底

2.esp給prev->thread.sp 將當前esp保存到進程的sp

3.next給thread.sp->esp，切換的進程sp給esp,因為這個進程曾經就是這么獲取的esp的值

4.$1f表示段1地址，即將段1地址->prev->thread.ip 當前進程如果再去執行的話要到段1去執行

56.將next->thread.ip給eip

7.棧頂元素pop作為ebp

兩次操作ebp，先將進程的ebp入棧，然后執行下一個進程，執行完下一個進程后，esp不變，ebp就能夠重新得到ebp，能起到保護ebp的作用，也不需要保存ebp的值在數據結構中