What is kthreadd_task

Question

In the definition of kthread_create a task is waken up, does any one know what is this task doing ?

struct task_struct *kthread_create

{


    struct kthread_create_info create;

    create.threadfn = threadfn;
    create.data = data;
    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)) {
            struct sched_param param = { .sched_priority = 0 };
            va_list args;

            va_start(args, namefmt);
            vsnprintf(create.result->comm, sizeof(create.result->comm),
                      namefmt, args);
            va_end(args);
            /*
             * root may have changed our (kthreadd's) priority or CPU mask.
             * The kernel thread should not inherit these properties.
             */
            sched_setscheduler_nocheck(create.result, SCHED_NORMAL, &param);
            set_cpus_allowed_ptr(create.result, cpu_all_mask);
    }
    return create.result;
}

Answer 1

kthreadd is a kernel daemon,it is started during kernel boot.

init/main.c ---> kernel_thread(kthreadd,...)

(architecture dependent code for e.g arch/arm/kernel/process.c)

As you can see here, kernel_thread() returns a pid. From pid we determine the task_struct and assign it to kthreadd_task.

So whenever a call for creation of kernel thread i.e., kthread_create(), kthreadd_task is waken up, which inturn calls kthreadd() (defined in kernel/kthread.c).

Answer 2

kthreadd_task is pointer to task_struct of kernel thread runnind kthreadd() function defined at http://lxr.linux.no/#linux+v2.6.36/kernel/kthread.c#L231

kthreadd() is main function (and main loop) of daemon kthreadd which is a kernel thread daemon, the parent of all other kernel threads.

So in the code quoted, there is a creation of request to kthreadd daemon. To fulfill this request kthreadd will read it and start a kernel thread. Then it will change the flag. In the code you have a waiting on this flag. After flag changing your function will check the status of new kthread creation.