Delivery Status Notification

Hi! anybody knows that where is set_user_nice() function called in the kernel? i couldn't find that. i just found that in forking operation do_fork()->copy_process()->dup_task_struct() called then set_user_nice() called but i couldn't find that where. Also how/where do_fork() function called in kernel i couldn't find. Thanks in advance... -- Enes Albay