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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+================================================
+Multi-Queue Block IO Queueing Mechanism (blk-mq)
+================================================
+
+The Multi-Queue Block IO Queueing Mechanism is an API to enable fast storage
+devices to achieve a huge number of input/output operations per second (IOPS)
+through queueing and submitting IO requests to block devices simultaneously,
+benefiting from the parallelism offered by modern storage devices.
+
+Introduction
+============
+
+Background
+----------
+
+Magnetic hard disks have been the de facto standard from the beginning of the
+development of the kernel. The Block IO subsystem aimed to achieve the best
+performance possible for those devices with a high penalty when doing random
+access, and the bottleneck was the mechanical moving parts, a lot slower than
+any layer on the storage stack. One example of such optimization technique
+involves ordering read/write requests according to the current position of the
+hard disk head.
+
+However, with the development of Solid State Drives and Non-Volatile Memories
+without mechanical parts nor random access penalty and capable of performing
+high parallel access, the bottleneck of the stack had moved from the storage
+device to the operating system. In order to take advantage of the parallelism
+in those devices' design, the multi-queue mechanism was introduced.
+
+The former design had a single queue to store block IO requests with a single
+lock. That did not scale well in SMP systems due to dirty data in cache and the
+bottleneck of having a single lock for multiple processors. This setup also
+suffered with congestion when different processes (or the same process, moving
+to different CPUs) wanted to perform block IO. Instead of this, the blk-mq API
+spawns multiple queues with individual entry points local to the CPU, removing
+the need for a lock. A deeper explanation on how this works is covered in the
+following section (`Operation`_).
+
+Operation
+---------
+
+When the userspace performs IO to a block device (reading or writing a file,
+for instance), blk-mq takes action: it will store and manage IO requests to
+the block device, acting as middleware between the userspace (and a file
+system, if present) and the block device driver.
+
+blk-mq has two group of queues: software staging queues and hardware dispatch
+queues. When the request arrives at the block layer, it will try the shortest
+path possible: send it directly to the hardware queue. However, there are two
+cases that it might not do that: if there's an IO scheduler attached at the
+layer or if we want to try to merge requests. In both cases, requests will be
+sent to the software queue.
+
+Then, after the requests are processed by software queues, they will be placed
+at the hardware queue, a second stage queue were the hardware has direct access
+to process those requests. However, if the hardware does not have enough
+resources to accept more requests, blk-mq will places requests on a temporary
+queue, to be sent in the future, when the hardware is able.
+
+Software staging queues
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The block IO subsystem adds requests  in the software staging queues
+(represented by struct blk_mq_ctx) in case that they weren't sent
+directly to the driver. A request is one or more BIOs. They arrived at the
+block layer through the data structure struct bio. The block layer
+will then build a new structure from it, the struct request that will
+be used to communicate with the device driver. Each queue has its own lock and
+the number of queues is defined by a per-CPU or per-node basis.
+
+The staging queue can be used to merge requests for adjacent sectors. For
+instance, requests for sector 3-6, 6-7, 7-9 can become one request for 3-9.
+Even if random access to SSDs and NVMs have the same time of response compared
+to sequential access, grouped requests for sequential access decreases the
+number of individual requests. This technique of merging requests is called
+plugging.
+
+Along with that, the requests can be reordered to ensure fairness of system
+resources (e.g. to ensure that no application suffers from starvation) and/or to
+improve IO performance, by an IO scheduler.
+
+IO Schedulers
+^^^^^^^^^^^^^
+
+There are several schedulers implemented by the block layer, each one following
+a heuristic to improve the IO performance. They are "pluggable" (as in plug
+and play), in the sense of they can be selected at run time using sysfs. You
+can read more about Linux's IO schedulers `here
+<https://www.kernel.org/doc/html/latest/block/index.html>`_. The scheduling
+happens only between requests in the same queue, so it is not possible to merge
+requests from different queues, otherwise there would be cache trashing and a
+need to have a lock for each queue. After the scheduling, the requests are
+eligible to be sent to the hardware. One of the possible schedulers to be
+selected is the NONE scheduler, the most straightforward one. It will just
+place requests on whatever software queue the process is running on, without
+any reordering. When the device starts processing requests in the hardware
+queue (a.k.a. run the hardware queue), the software queues mapped to that
+hardware queue will be drained in sequence according to their mapping.
+
+Hardware dispatch queues
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The hardware queue (represented by struct blk_mq_hw_ctx) is a struct
+used by device drivers to map the device submission queues (or device DMA ring
+buffer), and are the last step of the block layer submission code before the
+low level device driver taking ownership of the request. To run this queue, the
+block layer removes requests from the associated software queues and tries to
+dispatch to the hardware.
+
+If it's not possible to send the requests directly to hardware, they will be
+added to a linked list (``hctx->dispatch``) of requests. Then,
+next time the block layer runs a queue, it will send the requests laying at the
+``dispatch`` list first, to ensure a fairness dispatch with those
+requests that were ready to be sent first. The number of hardware queues
+depends on the number of hardware contexts supported by the hardware and its
+device driver, but it will not be more than the number of cores of the system.
+There is no reordering at this stage, and each software queue has a set of
+hardware queues to send requests for.
+
+.. note::
+
+        Neither the block layer nor the device protocols guarantee
+        the order of completion of requests. This must be handled by
+        higher layers, like the filesystem.
+
+Tag-based completion
+~~~~~~~~~~~~~~~~~~~~
+
+In order to indicate which request has been completed, every request is
+identified by an integer, ranging from 0 to the dispatch queue size. This tag
+is generated by the block layer and later reused by the device driver, removing
+the need to create a redundant identifier. When a request is completed in the
+drive, the tag is sent back to the block layer to notify it of the finalization.
+This removes the need to do a linear search to find out which IO has been
+completed.
+
+Further reading
+---------------
+
+- `Linux Block IO: Introducing Multi-queue SSD Access on Multi-core Systems <http://kernel.dk/blk-mq.pdf>`_
+
+- `NOOP scheduler <https://en.wikipedia.org/wiki/Noop_scheduler>`_
+
+- `Null block device driver <https://www.kernel.org/doc/html/latest/block/null_blk.html>`_
+
+Source code documentation
+=========================
+
+.. kernel-doc:: include/linux/blk-mq.h
+
+.. kernel-doc:: block/blk-mq.c