block: Adding ROW scheduling algorithm

In order to decrease the latency of a prioritized request (such as READ requests) the device driver might decide to stop the transmission of a current "low priority" request in order to handle the "high priority" one. The urgency of the request is decided by the block layer I/O scheduler. When the block layer notifies the underlying device driver (eMMC for example) of an urgent request, the device driver might decide to stop the current request transmission. The remainder of the stopped request will be re-inserted back to the scheduler, to be re-scheduled after handling the urgent request.

The above is implemented in the block layer by 2 callbacks of the queue:
- urgent_request_fn() - This callback is registered by the underlying device driver and is called instead of the existing requst_fn() callbacks to handle urgent requests.
- elevator_is_urgent_fn() - This callback is registered by the current I/O scheduler. If present it's used by the block layer to ping the scheduler of an urgent request presence.
NOTE: If one of the above callbacks is not registered, these code pass will never be activated.

The ROW I/O scheduler implements and urgent request notification mechanism.

The policy of ROW I/O scheduler is to prioritize READ requests over WRITE as much as possible without starving the WRITE requests.

The ROW I/O scheduler implements I/O priorities (CLASS_RT, CLASS_BE, CLASS_IDLE). Dissimilar to CFQ, CLASS_BE requests won’t be starved by CLASS_RT, and CLASS_IDLE requests won’t be starved by CLASS_RT/CLASS_BE requests. A tolerance limit is managed to prevent starvation.

The ROW I/O scheduler with the URGENT request support by the device driver improves the READ throughput by ~80% and the READ worst case latency is decreased by ~66%. All measured for READ/WRITE coalition scenarios.

Tanya Brokhman (3):
block: Add support for reinsert a dispatched req
block: Add API for urgent request handling
block: Adding ROW scheduling algorithm

 Documentation/block/row-iosched.txt |  134 +++++
 block/Kconfig.iosched               |   21 +
 block/Makefile                      |    1 +
 block/blk-core.c                    |   99 +++-
 block/blk-settings.c                |   12 +
 block/elevator.c                    |   40 ++
 block/row-iosched.c                 | 1089 +++++++++++++++++++++++++++++++++++
 include/linux/blk_types.h           |    4 +-
 include/linux/blkdev.h              |    6 +
 include/linux/elevator.h            |    7 +
 10 files changed, 1403 insertions(+), 10 deletions(-)
 create mode 100644 Documentation/block/row-iosched.txt
 create mode 100644 block/row-iosched.c

1.7.6

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