None of the blk-mq files have an explanatory comment at the top
for what that particular file does. Add that and add appropriate
copyright notices as well.
Signed-off-by: NJens Axboe <axboe@fb.com>

Prepare this for the next patch which adds more smarts in the
plugging logic, so that we can save some memory.
Signed-off-by: NJens Axboe <axboe@fb.com>

[  365.164040] BUG: sleeping function called from invalid context at kernel/rtmutex.c:674
[  365.164041] in_atomic(): 1, irqs_disabled(): 1, pid: 26, name: migration/1
[  365.164043] no locks held by migration/1/26.
[  365.164044] irq event stamp: 6648
[  365.164056] hardirqs last  enabled at (6647): [<ffffffff8153d377>] restore_args+0x0/0x30
[  365.164062] hardirqs last disabled at (6648): [<ffffffff810ed98d>] multi_cpu_stop+0x9d/0x120
[  365.164070] softirqs last  enabled at (0): [<ffffffff810543bc>] copy_process.part.28+0x6fc/0x1920
[  365.164072] softirqs last disabled at (0): [<          (null)>]           (null)
[  365.164076] CPU: 1 PID: 26 Comm: migration/1 Tainted: GF           N  3.12.12-rt19-0.gcb6c4a2-rt #3
[  365.164078] Hardware name: QCI QSSC-S4R/QSSC-S4R, BIOS QSSC-S4R.QCI.01.00.S013.032920111005 03/29/2011
[  365.164091]  0000000000000001 ffff880a42ea7c30 ffffffff815367e6 ffffffff81a086c0
[  365.164099]  ffff880a42ea7c40 ffffffff8108919c ffff880a42ea7c60 ffffffff8153c24f
[  365.164107]  ffff880a42ea91f0 00000000ffffffe1 ffff880a42ea7c88 ffffffff81297ec0
[  365.164108] Call Trace:
[  365.164119]  [<ffffffff810060b1>] try_stack_unwind+0x191/0x1a0
[  365.164127]  [<ffffffff81004872>] dump_trace+0x92/0x360
[  365.164133]  [<ffffffff81006108>] show_trace_log_lvl+0x48/0x60
[  365.164138]  [<ffffffff81004c18>] show_stack_log_lvl+0xd8/0x1d0
[  365.164143]  [<ffffffff81006160>] show_stack+0x20/0x50
[  365.164153]  [<ffffffff815367e6>] dump_stack+0x54/0x9a
[  365.164163]  [<ffffffff8108919c>] __might_sleep+0xfc/0x140
[  365.164173]  [<ffffffff8153c24f>] rt_spin_lock+0x1f/0x70
[  365.164182]  [<ffffffff81297ec0>] blk_mq_main_cpu_notify+0x20/0x70
[  365.164191]  [<ffffffff81540a1c>] notifier_call_chain+0x4c/0x70
[  365.164201]  [<ffffffff81083499>] __raw_notifier_call_chain+0x9/0x10
[  365.164207]  [<ffffffff810567be>] cpu_notify+0x1e/0x40
[  365.164217]  [<ffffffff81525da2>] take_cpu_down+0x22/0x40
[  365.164223]  [<ffffffff810ed9c6>] multi_cpu_stop+0xd6/0x120
[  365.164229]  [<ffffffff810edd97>] cpu_stopper_thread+0xd7/0x1e0
[  365.164235]  [<ffffffff810863a3>] smpboot_thread_fn+0x203/0x380
[  365.164241]  [<ffffffff8107cbf8>] kthread+0xc8/0xd0
[  365.164250]  [<ffffffff8154440c>] ret_from_fork+0x7c/0xb0
[  365.164429] smpboot: CPU 1 is now offline
Signed-off-by: NMike Galbraith <bitbucket@online.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

[  365.164040] BUG: sleeping function called from invalid context at kernel/rtmutex.c:674
[  365.164041] in_atomic(): 1, irqs_disabled(): 1, pid: 26, name: migration/1
[  365.164043] no locks held by migration/1/26.
[  365.164044] irq event stamp: 6648
[  365.164056] hardirqs last  enabled at (6647): [<ffffffff8153d377>] restore_args+0x0/0x30
[  365.164062] hardirqs last disabled at (6648): [<ffffffff810ed98d>] multi_cpu_stop+0x9d/0x120
[  365.164070] softirqs last  enabled at (0): [<ffffffff810543bc>] copy_process.part.28+0x6fc/0x1920
[  365.164072] softirqs last disabled at (0): [<          (null)>]           (null)
[  365.164076] CPU: 1 PID: 26 Comm: migration/1 Tainted: GF           N  3.12.12-rt19-0.gcb6c4a2-rt #3
[  365.164078] Hardware name: QCI QSSC-S4R/QSSC-S4R, BIOS QSSC-S4R.QCI.01.00.S013.032920111005 03/29/2011
[  365.164091]  0000000000000001 ffff880a42ea7c30 ffffffff815367e6 ffffffff81a086c0
[  365.164099]  ffff880a42ea7c40 ffffffff8108919c ffff880a42ea7c60 ffffffff8153c24f
[  365.164107]  ffff880a42ea91f0 00000000ffffffe1 ffff880a42ea7c88 ffffffff81297ec0
[  365.164108] Call Trace:
[  365.164119]  [<ffffffff810060b1>] try_stack_unwind+0x191/0x1a0
[  365.164127]  [<ffffffff81004872>] dump_trace+0x92/0x360
[  365.164133]  [<ffffffff81006108>] show_trace_log_lvl+0x48/0x60
[  365.164138]  [<ffffffff81004c18>] show_stack_log_lvl+0xd8/0x1d0
[  365.164143]  [<ffffffff81006160>] show_stack+0x20/0x50
[  365.164153]  [<ffffffff815367e6>] dump_stack+0x54/0x9a
[  365.164163]  [<ffffffff8108919c>] __might_sleep+0xfc/0x140
[  365.164173]  [<ffffffff8153c24f>] rt_spin_lock+0x1f/0x70
[  365.164182]  [<ffffffff81297ec0>] blk_mq_main_cpu_notify+0x20/0x70
[  365.164191]  [<ffffffff81540a1c>] notifier_call_chain+0x4c/0x70
[  365.164201]  [<ffffffff81083499>] __raw_notifier_call_chain+0x9/0x10
[  365.164207]  [<ffffffff810567be>] cpu_notify+0x1e/0x40
[  365.164217]  [<ffffffff81525da2>] take_cpu_down+0x22/0x40
[  365.164223]  [<ffffffff810ed9c6>] multi_cpu_stop+0xd6/0x120
[  365.164229]  [<ffffffff810edd97>] cpu_stopper_thread+0xd7/0x1e0
[  365.164235]  [<ffffffff810863a3>] smpboot_thread_fn+0x203/0x380
[  365.164241]  [<ffffffff8107cbf8>] kthread+0xc8/0xd0
[  365.164250]  [<ffffffff8154440c>] ret_from_fork+0x7c/0xb0
[  365.164429] smpboot: CPU 1 is now offline
Signed-off-by: NMike Galbraith <bitbucket@online.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

Cleaner, reduces text size when cpu hotplug is disabled.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

__smp_call_function_single already avoids multiple IPIs by internally
queing up the items, and now also is available for non-SMP builds as
a trivially correct stub, so there is no need to wrap it.  If the
additional lock roundtrip cause problems my patch to convert the
generic IPI code to llists is waiting to get merged will fix it.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

The new blk-mq code added new instances of __cpuinit usage.
We removed this a couple versions ago; we now want to remove
the compat no-op stubs.  Introducing new users is not what
we want to see at this point in time, as it will break once
the stubs are gone.
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Linux currently has two models for block devices:

- The classic request_fn based approach, where drivers use struct
  request units for IO. The block layer provides various helper
  functionalities to let drivers share code, things like tag
  management, timeout handling, queueing, etc.

- The "stacked" approach, where a driver squeezes in between the
  block layer and IO submitter. Since this bypasses the IO stack,
  driver generally have to manage everything themselves.

With drivers being written for new high IOPS devices, the classic
request_fn based driver doesn't work well enough. The design dates
back to when both SMP and high IOPS was rare. It has problems with
scaling to bigger machines, and runs into scaling issues even on
smaller machines when you have IOPS in the hundreds of thousands
per device.

The stacked approach is then most often selected as the model
for the driver. But this means that everybody has to re-invent
everything, and along with that we get all the problems again
that the shared approach solved.

This commit introduces blk-mq, block multi queue support. The
design is centered around per-cpu queues for queueing IO, which
then funnel down into x number of hardware submission queues.
We might have a 1:1 mapping between the two, or it might be
an N:M mapping. That all depends on what the hardware supports.

blk-mq provides various helper functions, which include:

- Scalable support for request tagging. Most devices need to
  be able to uniquely identify a request both in the driver and
  to the hardware. The tagging uses per-cpu caches for freed
  tags, to enable cache hot reuse.

- Timeout handling without tracking request on a per-device
  basis. Basically the driver should be able to get a notification,
  if a request happens to fail.

- Optional support for non 1:1 mappings between issue and
  submission queues. blk-mq can redirect IO completions to the
  desired location.

- Support for per-request payloads. Drivers almost always need
  to associate a request structure with some driver private
  command structure. Drivers can tell blk-mq this at init time,
  and then any request handed to the driver will have the
  required size of memory associated with it.

- Support for merging of IO, and plugging. The stacked model
  gets neither of these. Even for high IOPS devices, merging
  sequential IO reduces per-command overhead and thus
  increases bandwidth.

For now, this is provided as a potential 3rd queueing model, with
the hope being that, as it matures, it can replace both the classic
and stacked model. That would get us back to having just 1 real
model for block devices, leaving the stacked approach to dm/md
devices (as it was originally intended).

Contributions in this patch from the following people:

Shaohua Li <shli@fusionio.com>
Alexander Gordeev <agordeev@redhat.com>
Christoph Hellwig <hch@infradead.org>
Mike Christie <michaelc@cs.wisc.edu>
Matias Bjorling <m@bjorling.me>
Jeff Moyer <jmoyer@redhat.com>
Acked-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>