一个简单的时间片轮转多道程序内核代码

陈可 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

嵌入式汇编基本格式：

asm(“汇编语句”
输出寄存器

输入寄存器

会被修改的寄存器 );

其中，“汇编语句”是你写汇编指令的地方；“输出寄存器”表示当这段嵌入汇编执行完之后，哪些寄存器用于存放输出数据。此地，这些寄存器会分别对应一C 语言表达式或一个内存地址；“输入寄存器”表示在开始执行汇编代码时，这里指定的一些寄存器中应存放的输入值，它们也分别对应着一C变量或常数值。

mykernel平台

mykernel可以看作一个逻辑上的硬件平台，从my_start_kernel开始执行，并提供了时钟中断机制周期性执行my_time_handler中断处理程序，执行完后中断返回总是可以回到my_start_kernel中断的位置继续执行。当然中断保存现场恢复现场的细节都处理好了。我们的工作就是编写my_start_kernel和my_time_handler着两个函数。

运行平台mykernel

课程一开始已经提供了my_start_kernel和my_time_handler这两个函数的简单实现来帮助我们熟悉这个平台。

所以先构建这个平台，然后运行一下看下效果：

cd LinuxKernel/linux-3.9.4
rm -rf mykernel
patch -p1 < ../mykernel_for_linux3.9.4sc.patch
make allnoconfig
make ＃编译内核请耐心等待
qemu -kernel arch/x86/boot/bzImage

分析代码：

mymain.c:

/*
*  linux/mykernel/mymain.c
*
*  Kernel internal my_start_kernel
*
*  Copyright (C) 2013  Mengning
*
*/
#include <linux/types.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/stackprotector.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/delay.h>
#include <linux/ioport.h>
#include <linux/init.h>
#include <linux/initrd.h>
#include <linux/bootmem.h>
#include <linux/acpi.h>
#include <linux/tty.h>
#include <linux/percpu.h>
#include <linux/kmod.h>
#include <linux/vmalloc.h>
#include <linux/kernel_stat.h>
#include <linux/start_kernel.h>
#include <linux/security.h>
#include <linux/smp.h>
#include <linux/profile.h>
#include <linux/rcupdate.h>
#include <linux/moduleparam.h>
#include <linux/kallsyms.h>
#include <linux/writeback.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/cgroup.h>
#include <linux/efi.h>
#include <linux/tick.h>
#include <linux/interrupt.h>
#include <linux/taskstats_kern.h>
#include <linux/delayacct.h>
#include <linux/unistd.h>
#include <linux/rmap.h>
#include <linux/mempolicy.h>
#include <linux/key.h>
#include <linux/buffer_head.h>
#include <linux/page_cgroup.h>
#include <linux/debug_locks.h>
#include <linux/debugobjects.h>
#include <linux/lockdep.h>
#include <linux/kmemleak.h>
#include <linux/pid_namespace.h>
#include <linux/device.h>
#include <linux/kthread.h>
#include <linux/sched.h>
#include <linux/signal.h>
#include <linux/idr.h>
#include <linux/kgdb.h>
#include <linux/ftrace.h>
#include <linux/async.h>
#include <linux/kmemcheck.h>
#include <linux/sfi.h>
#include <linux/shmem_fs.h>
#include <linux/slab.h>
#include <linux/perf_event.h>
#include <linux/file.h>
#include <linux/ptrace.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>

#include <asm/io.h>
#include <asm/bugs.h>
#include <asm/setup.h>
#include <asm/sections.h>
#include <asm/cacheflush.h>

#ifdef CONFIG_X86_LOCAL_APIC
#include <asm/smp.h>
#endif

void __init my_start_kernel(void)
{
int i = 0;
while(1)
{
i++;
if(i%100000 == 0)
printk(KERN_NOTICE "my_start_kernel here  %d \n",i);

}
}

myinterrupt.c:

/*
*  linux/mykernel/myinterrupt.c
*
*  Kernel internal my_timer_handler
*
*  Copyright (C) 2013  Mengning
*
*/
#include <linux/kernel_stat.h>
#include <linux/export.h>
#include <linux/interrupt.h>
#include <linux/percpu.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/swap.h>
#include <linux/pid_namespace.h>
#include <linux/notifier.h>
#include <linux/thread_info.h>
#include <linux/time.h>
#include <linux/jiffies.h>
#include <linux/posix-timers.h>
#include <linux/cpu.h>
#include <linux/syscalls.h>
#include <linux/delay.h>
#include <linux/tick.h>
#include <linux/kallsyms.h>
#include <linux/irq_work.h>
#include <linux/sched.h>
#include <linux/sched/sysctl.h>
#include <linux/slab.h>

#include <asm/uaccess.h>
#include <asm/unistd.h>
#include <asm/div64.h>
#include <asm/timex.h>
#include <asm/io.h>

#define CREATE_TRACE_POINTS
#include <trace/events/timer.h>

/*
* Called by timer interrupt.
*/
void my_timer_handler(void)
{
printk(KERN_NOTICE "\n>>>>>>>>>>>>>>>>>my_timer_handler here<<<<<<<<<<<<<<<<<<\n\n");
}

分析

这两段代码很简单就是实现了在时钟中断到来时输出>>>>>>>>>>>>>>>>>my_timer_handler here<<<<<<<<<<<<<<<<<<

，其余时间输出my_start_kernel here+i的效果

实现一个简单的时间片轮转多道程序内核代码

接下来通过修改my_start_kernel和my_time_handler这两个函数来实现一个简单的时间片轮转多道程序内核代码。

分析老师提供的两个三个文件（mymain.c,myinterrupt.c和mypcb.h）

mypcb.h：

/*
*  linux/mykernel/mypcb.h
*
*  Kernel internal PCB types
*
*  Copyright (C) 2013  Mengning
*
*/

#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*8

/* CPU-specific state of this task */
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;

void my_schedule(void);

mypcb.h主要就是定义了一个进程控制块（PCB）的数据结构

mymain.c

/*
*  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;

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);
}
}
}

嵌入式汇编代码中%0，%1，%2..代表输出输入部分的第0，1，2..个寄存器init my_start_kernel函数先MAX_TASK_NUM个进程，这些进程都“指向”void my_process(void)，然后将这些进程的next都初始化，方便之后进行进程调度。在void my_process(void)中，程序会先每循环10000000次输出this is process+进程编号并判断是否需要调度了，若需要调度了，则自己主动调用调度算法my_schedule()。

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%1000 == 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 */
{
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;
}

my_timer_handler会先每执行1000次（也就是时钟中断到来1000次）就将是否需要进程调度flag置为1。

my_schedule()进程调度算法会先判断next进程是否已经执行过，若执行过，先保存prev进程的上下文，然后恢复next的上下文，修改eip。若没有执行过则将next进程的state置为已执行过，然后保存prev进程的上下文，修改eip。

运行效果

总结：

每个进程都有它自己的堆栈，进程调度时调度算法会保存prev进程进程上下文（堆栈（用ebp,esp维护），寄存器），next进程会恢复进程上下文。