linux系统内核流转浅析

SJTUBEAR  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

我们通过简单地内核来模拟一下linux的系统调度，代码如下：

/*
 *  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 = ;

void my_process(void);

void __init my_start_kernel(void)
{
    int pid = ;
    int i;
    /* Initialize process 0*/
    task[pid].pid = pid;
    task[pid].state = ;/* -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-];
    task[pid].next = &task[pid];
    /*fork more process */
    for(i=;i<MAX_TASK_NUM;i++)
    {
        memcpy(&task[i],&task[],sizeof(tPCB));
        task[i].pid = i;
        task[i].state = -;
        task[i].thread.sp = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-];
        task[i].next = task[i-].next;
        task[i-].next = &task[i];
    }
    /* start process 0 by task[0] */
    pid = ;
    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 = ;
    while()
    {
        i++;
        if(i% == )
        {
            printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid);
            if(my_need_sched == )
            {
                my_need_sched = ;
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
        }
    }
}

这是我们模拟kernel的主程序mymain.c/

linux启动后，系统初始化完成，mymain.c开始运行。

通过将my_process的地址传入结构体中的ip，再将ip压栈后ret，将地址pop给eip，将系统引导至myprocess开始运行。

其中while1,保证kernel死循环。

每执行10000000次检查全局变量my_need_sche，通过my_schedule()来进行进程的切换。

而对于my_need_sche的修改和my_schedule()函数具体的实现是在时钟中断中模拟进行的。

 contributor
RawBlameHistory    lines ( sloc)  2.452 kb
/*
 *  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 = ;

/*
 * 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% ==  && my_need_sched != )
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<\n");
        my_need_sched = ;
    }
    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 == )/* -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 = ;
        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;
}

我们看到，每一千次中断，就模拟切换一次进程。

通过对当前环境的保存以及建立新的堆栈，来完成对进程的切换。

实验结果：

通过实验结果我们可以得知，就是这样通过使用堆栈，对进程的数据进行封存，并建立新的堆栈开启新的进程的过程，使得进程可以自由的切换。

通过对ret指令，将下一条进程的eip首地址赋eip，完成进程的调度！