Kernel rest_init相关

Linux系统里，有些进程只有kernel部分的代码，即由一个kernel函数进入，在sched的时候，将其与用户进程同等对待。

PID为０的叫swapper或sched进程，对应函数为rest_init

init进程PID为１，

kthreadd进程PID为２，

0号进程，即rest_init主体如下

 static noinline void __init_refok rest_init(void)
 {
     int pid;

     rcu_scheduler_starting();
     smpboot_thread_init();
     /*
      * We need to spawn init first so that it obtains pid 1, however
      * the init task will end up wanting to create kthreads, which, if
      * we schedule it before we create kthreadd, will OOPS.
      */
     kernel_thread(kernel_init, NULL, CLONE_FS);
     numa_default_policy();
     pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
     rcu_read_lock();
     kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
     rcu_read_unlock();
     complete(&kthreadd_done);

     /*
      * The boot idle thread must execute schedule()
      * at least once to get things moving:
      */
     init_idle_bootup_task(current);
     schedule_preempt_disabled();
     /* Call into cpu_idle with preempt disabled */
     cpu_startup_entry(CPUHP_ONLINE);

先是rcu_scheduler_starting函数，这个函数的作用是

再是smpboot_thread_init函数，这个函数注册smpboot时候的notifier，响应CPU动作

 static struct notifier_block smpboot_thread_notifier = {
     .notifier_call = smpboot_thread_call,
     .priority = CPU_PRI_SMPBOOT,
 };

 void __cpuinit smpboot_thread_init(void)
 {
     register_cpu_notifier(&smpboot_thread_notifier);
 }

然后调用kernel_thread函数fork init进程

 /*
  * Create a kernel thread.
  */
 pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 {
     return do_fork(flags|CLONE_VM|CLONE_UNTRACED, (unsigned long)fn,
         (unsigned long)arg, NULL, NULL);
 }

do_fork函数被调用

这里有一点要注意，init进程执行的时候要等kerneladd进程执行后才被complete(&kthreadd_done)唤醒，然后真正执行，不然直接调度到init的话会OOPs

numa_default_policy被调用设置内存numa默认策略

然后调用kernel_thread函数fork kthreadd进程

然后用find_task_by_pid_ns(pid, &init_pid_ns)获取kthreadd的task_struct

最后唤醒complete(&kthreadd_done)进程

最后，0号进程，即空闲进程，要让一切跑起来，调用一次schedule