学步园

struct task_struct *kthread_create(int (*threadfn)(void *data),
                   void *data,
                   const char namefmt[],
                   ...)
{
    struct kthread_create_info create;

    create.threadfn = threadfn;
    create.data = data;
    init_completion(&create.started);
    init_completion(&create.done);

    spin_lock(&kthread_create_lock);
    list_add_tail(&create.list, &kthread_create_list);
    spin_unlock(&kthread_create_lock);

    wake_up_process(kthreadd_task);
    wait_for_completion(&create.done);

    if (!IS_ERR(create.result)) {
        va_list args;
        va_start(args, namefmt);
        vsnprintf(create.result->comm, sizeof(create.result->comm),
              namefmt, args);
        va_end(args);
    }
    return create.result;

将kthread_create_info挂到全局队列，然后换醒进程kthreadd_task

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

    kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
    numa_default_policy();
    pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
    kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
    unlock_kernel();

    /*
     * The boot idle thread must execute schedule()
     * at least once to get things moving:
     */
    init_idle_bootup_task(current);
    preempt_enable_no_resched();
    schedule();
    preempt_disable();

    /* Call into cpu_idle with preempt disabled */
    cpu_idle();
}

从上面可以看出就是kthreadd,这个是干嘛？

int kthreadd(void *unused)
{
    struct task_struct *tsk = current;

    /* Setup a clean context for our children to inherit. */
    set_task_comm(tsk, "kthreadd");
    ignore_signals(tsk);
    set_user_nice(tsk, KTHREAD_NICE_LEVEL);
    set_cpus_allowed_ptr(tsk, CPU_MASK_ALL_PTR);

    current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;

    for (;;) {
        set_current_state(TASK_INTERRUPTIBLE);
        if (list_empty(&kthread_create_list))
            schedule();
        __set_current_state(TASK_RUNNING);

        spin_lock(&kthread_create_lock);
        while (!list_empty(&kthread_create_list)) {
            struct kthread_create_info *create;

            create = list_entry(kthread_create_list.next,
                        struct kthread_create_info, list);
            list_del_init(&create->list);
            spin_unlock(&kthread_create_lock);

            create_kthread(create);

            spin_lock(&kthread_create_lock);
        }
        spin_unlock(&kthread_create_lock);
    }

    return 0;
}
从这个函数里面 create_kthread(create);如果这个kthread_create_list为空，那么这个内核线程一直睡。

static void create_kthread(struct kthread_create_info *create)
{
    int pid;

    /* We want our own signal handler (we take no signals by default). */
    pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
    if (pid < 0) {
        create->result = ERR_PTR(pid);
    } else {
        struct sched_param param = { .sched_priority = 0 };
        wait_for_completion(&create->started);
        read_lock(&tasklist_lock);
        create->result = find_task_by_pid_ns(pid, &init_pid_ns);
        read_unlock(&tasklist_lock);
        /*
         * root may have changed our (kthreadd's) priority or CPU mask.
         * The kernel thread should not inherit these properties.
         */
        sched_setscheduler(create->result, SCHED_NORMAL, &param);
        set_user_nice(create->result, KTHREAD_NICE_LEVEL);
        set_cpus_allowed_ptr(create->result, CPU_MASK_ALL_PTR);
    }
    complete(&create->done);
}
可以看到kernel_thread的故事。