iosched: prevent aliased requests from starving other I/O

Hi, Jens,

If you recall, I posted an RFC patch for this back in July of last year:
http://lkml.org/lkml/2010/7/13/279

The basic problem is that a process can issue a never-ending stream of
async direct I/Os to the same sector on a device, thus starving out
other I/O in the system (due to the way the alias handling works in both
cfq and deadline).  The solution I proposed back then was to start
dispatching from the fifo after a certain number of aliases had been
dispatched.  Vivek asked why we had to treat aliases differently at all,
and I never had a good answer.  So, I put together a simple patch which
allows aliases to be added to the rb tree (it adds them to the right,
though that doesn't matter as the order isn't guaranteed anyway).  I
think this is the preferred solution, as it doesn't break up time slices
in CFQ or batches in deadline.  I've tested it, and it does solve the
starvation issue.  Let me know what you think.

Cheers,
Jeff

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
block: reorganize queue draining

Reorganize queue draining related code in preparation of queue exit
changes.

* Factor out actual draining from elv_quiesce_start() to
  blk_drain_queue().

* Make elv_quiesce_start/end() responsible for their own locking.

* Replace open-coded ELVSWITCH clearing in elevator_switch() with
  elv_quiesce_end().

This patch doesn't cause any visible functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
block: reorder elevator switch sequence

Elevator switch sequence first attached the new elevator, then tried
registering it (sysfs) and if that failed attached back the old
elevator.  However, sysfs registration doesn't require the elevator to
be attached, so there is no reason to do the "detach, attach new,
register, maybe re-attach old" sequence.  It can just do "register,
detach, attach".

* elevator_init_queue() is updated to set ->elevator_data directly and
  return 0 / -errno.  This allows elevator_exit() on an unattached
  elevator.

* __elv_unregister_queue() which was necessary to unregister
  unattached q is removed in favor of __elv_register_queue() which can
  register unattached q.

* elevator_attach() becomes a single assignment and obscures more then
  it helps.  Dropped.

This will help cleaning up io_context handling across elevator switch.

This patch doesn't introduce visible behavior change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
block: remove elevator_queue->ops

elevator_queue->ops points to the same ops struct ->elevator_type.ops
is pointing to.  The only effect of caching it in elevator_queue is
shorter notation - it doesn't save any indirect derefence.

Relocate elevator_type->list which used only during module init/exit
to the end of the structure, rename elevator_queue->elevator_type to
->type, and replace elevator_queue->ops with elevator_queue->type.ops.

This doesn't introduce any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Conflicts:

	block/blk.h
block: Add support for reinsert a dispatched req

Add support for reinserting a dispatched request back to the
scheduler's internal data structures.
This capability is used by the device driver when it chooses to
interrupt the current request transmission and execute another (more
urgent) pending request. For example: interrupting long write in order
to handle pending read. The device driver re-inserts the
remaining write request back to the scheduler, to be rescheduled
for transmission later on.

Add API for verifying whether the current scheduler
supports reinserting requests mechanism. If reinsert mechanism isn't
supported by the scheduler, this code path will never be activated.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>

Conflicts:
	include/linux/elevator.h

Change-Id: I053120da7d8887d2a9bd4d29b93877c752ba85d9
block: Add API for urgent request handling

This patch add support in block & elevator layers for handling
urgent requests. The decision if a request is urgent or not is taken
by the scheduler. Urgent request notification is passed to the underlying
block device driver (eMMC for example). Block device driver may decide to
interrupt the currently running low priority request to serve the new
urgent request. By doing so READ latency is greatly reduced in read&write
collision scenarios.

Note that if the current scheduler doesn't implement the urgent request
mechanism, this code path is never activated.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>

Conflicts:
	include/linux/elevator.h

Change-Id: I9f726669ba405dd3c01890ad4e7ed3d1cf02ecef
block: update row scheduler V4 from leanKernel-galaxy-nexus

Change-Id: Ibd2017392effe26e2f08959aa05d350dd8d18cc4

Conflicts:
	include/linux/elevator.h
block: row: hardcode values research by the community, you guys know who you are.

Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>

Conflicts:
	block/row-iosched.c

Change-Id: I0d773c812d5b0b17e44486fa1277e63f0d69c1cc

Committer note: cherry-picked from franciscofranco/Tuna_JB_pre1