When run_task function starts to run in element

Hi all, 


I starts to learn click router recently. I have a question that make me confused. Does anybody give me some advices. I will be appreciated. 
In each element, there is a function named run_task. I can not understand when this run_task function will be executed. 
If my following understanding is not correct, please help to point them out.
>From the document, it seems that there is a kernel thread. Whether this thread will manage a queue, in this queue, the tasks in this queue are the run_tasks tasks. When I learn the code, I want to find where the run_task function will be called, however I can not find the places. There are many places that call the function _task.fast_reschedule(). What is the function and aim for this function _task.fast_reschedule()? Whether does it have some relation with the task queue?  Where is the code for task scheduling? I do not know where they are.


I am looking forward to anybody's reply.