1 wget https://raw.github.com/mengning/mykernel/master/mykernel-2.0_for_linux-5.4.34.patch
 2 sudo apt install axel
 3 axel -n 20 https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.4.34.tar.xz
 4 xz -d linux-5.4.34.tar.xz
 5 tar -xvf linux-5.4.34.tar
 6 cd linux-5.4.34
 7 patch -p1 < ../mykernel-2.0_for_linux-5.4.34.patch
 8 sudo apt install build-essential libncurses-dev bison flex libssl-dev libelf-dev
 9 make defconfig
10 make -j$(nproc)
11 sudo apt install qemu # install QEMU
12 qemu-system-x86_64 -kernel arch/x86/boot/bzImage

#define MAX_TASK_NUM        4
// 最大进程数设为4
#define KERNEL_STACK_SIZE   2048
// 每个进程分配堆栈大小设为2048
/* CPU-specific state of this task */
struct Thread {
    unsigned long        ip;
    unsigned long        sp;
};

typedef struct PCB{
    int pid;
    volatile long state;    /*进程运行状态： -1 unrunnable, 0 runnable, >0 stopped */
    unsigned long stack[KERNEL_STACK_SIZE]; // 给进程分配的堆栈，为long类型数组
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long    task_entry;
    struct PCB *next; // 指向下一个进程PCB的指针
}tPCB;

void my_schedule(void);
2. mymain.c
#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]; // 进程列表
tPCB * my_current_task = NULL; // 指向当前进程PCB
volatile int my_need_sched = 0;

void my_process(void);

/*系统最先开始执行此处*/
void __init my_start_kernel(void)
{
    int pid = 0;
    int i;
    /*初始化进程0*/
    task[pid].pid = pid;
    task[pid].state = 0; // 设置进程运行状态为正在运行
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process; // 每个进程入口都指向my_process的起始地址
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1]; // 栈顶指针为数组的最高地址，栈向下增长
    task[pid].next = &task[pid]; // 指向了当前进程0，当前只有一个进程
    /*初始化其他进程*/
    for(i=1;i<MAX_TASK_NUM;i++)
    {
        memcpy(&task[i],&task[0],sizeof(tPCB)); // 给其余进程PCB分配空间
        task[i].pid = i;
        task[i].thread.sp = (unsigned long)(&task[i].stack[KERNEL_STACK_SIZE-1]); // 每个进程分配各自的栈空间
        task[i].next = task[i-1].next;
        task[i-1].next = &task[i];
    }
    /* 启动进程0 */
    pid = 0;
    my_current_task = &task[pid]; // 当前进程指向进程0
    asm volatile(
        "movq %1,%%rsp\n\t"     /* 把进程0栈顶地址赋给rsp寄存器 */
        "pushq %1\n\t"             /* 相当于push rbp */
        "pushq %0\n\t"             /* 将ip入栈 */
        "ret\n\t"                 /* 将ip出栈至rip寄存器，这两步操作是因为程序员无法直接操作rip寄存器*/
        : 
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)    /* input c or d mean %ecx/%edx*/
    );
} 

int i = 0;

void my_process(void)
{    
    while(1)
    {
        i++;
        if(i%10000000 == 0)
        {
            if(my_need_sched == 1) // 运行10000000次循环后判断中断标志是否置1
            {
                printk(KERN_NOTICE "process %d is running!\n",my_current_task->pid);
                my_need_sched = 0;
                my_schedule(); // 进行进程切换工作
                printk(KERN_NOTICE "having changed to process %d!\n",my_current_task->pid);
            }
        }     
    }
}            

#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(time_count%1000 == 0 && my_need_sched != 1)
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<\n");
        my_need_sched = 1; // 时钟计数1000次，发生中断
    } 
    time_count ++ ;  
    return;      
}

void my_schedule(void)
{
    tPCB * next;
    tPCB * prev;

    if(my_current_task == NULL 
        || my_current_task->next == NULL) // 当前没有进程或没有下一个进程时不进行进程切换
    {
        return;
    }
    printk(KERN_NOTICE ">>>process is changing...<<<\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; 
        /* switch to next process */
        asm volatile(    
            "pushq %%rbp\n\t"         /* save rbp of prev */
            "movq %%rsp,%0\n\t"     /* save rsp of prev */
            "movq %2,%%rsp\n\t"     /* restore  rsp of next，切换至next进程的堆栈 */
            "movq $1f,%1\n\t"       /* save rip of prev */    
            "pushq %3\n\t" 
            "ret\n\t"                 /* restore  rip of next */
            "1:\t"                  /* next process start here */
            "popq %%rbp\n\t"  /*恢复next栈底指针至rbp寄存器*/
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        ); 
    }  
    return;    
}