[RFC] iosched: make a difference between read/readahead requests

Hi Jens,

This patchset does two jobs:
	1) do io schedule differently on READ/READA requests.
		- to help improve I/O latency and throughput
	2) do notification/action on READA => READ events
		- to make the elevators better informed
		- to prevent the priority inversion problem
		- also brings some CPU overheads*
(*) I'm not able to provide the numbers at the moment.
    But sure for the next time.


The patches come in two groups:

1) explicitly schedule READA requests
Note: currently only the deadline elevator is touched.

[PATCH 1/7] iosched: introduce WRITEA                                                  
[PATCH 2/7] iosched: introduce parameter deadline.reada_expire                         
[PATCH 3/7] iosched: introduce deadline_add_drq_fifo()                                 
[PATCH 4/7] iosched: submit READA requests on possible readahead code path             

2) notify/act on pending reads
Naming issue: how about pending_read/need_page/... for kick_page?

[PATCH 5/7] iosched: introduce elv_kick_page()                                         
[PATCH 6/7] iosched: run elv_kick_page() on sync read                                  
[PATCH 7/7] iosched: introduce deadline_kick_page()                                    

Most overheads should be in functions deadline_kick_page() and
deadline_add_drq_fifo(). I'll explore the details later.

Any comments are welcome, thanks.

Fengguang Wu
--
Dept. Automation                University of Science and Technology of China