Re: [PATCH v2] docs: block: Create blk-mq documentation

[Dongli Zhang]

Hi AndrÃ,

On 6/5/20 10:55 AM, Andrà Almeida wrote:
> Create a documentation providing a background and explanation around the
> operation of the Multi-Queue Block IO Queueing Mechanism (blk-mq).
> 
> The reference for writing this documentation was the source code and
> "Linux Block IO: Introducing Multi-queue SSD Access on Multi-core
> Systems", by Axboe et al.
> 
> Signed-off-by: Andrà Almeida <andrealmeid@xxxxxxxxxxxxx>
> ---
> Changes from v1:
> - Fixed typos
> - Reworked blk_mq_hw_ctx
> 
> Hello,
> 
> This commit was tested using "make htmldocs" and the HTML output has
> been verified.
> 
> Thanks,
> 	AndrÃ
> ---
>  Documentation/block/blk-mq.rst | 154 +++++++++++++++++++++++++++++++++
>  Documentation/block/index.rst  |   1 +
>  2 files changed, 155 insertions(+)
>  create mode 100644 Documentation/block/blk-mq.rst
> 
> diff --git a/Documentation/block/blk-mq.rst b/Documentation/block/blk-mq.rst
> new file mode 100644
> index 000000000000..1f702adbc577
> --- /dev/null
> +++ b/Documentation/block/blk-mq.rst
> @@ -0,0 +1,154 @@
> +.. 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 more slower
> +than any layer on the storage stack. One example of such optimization technique
> +involves ordering read/write requests accordingly 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 (represented by struct
> +:c:type:`blk_mq_ctx`) in the software staging queues in case that they weren't
> +sent directly to the driver. A request is a collection of BIOs. They arrived at
> +the block layer through the data structure struct :c:type:`bio`. The block
> +layer will then build a new structure from it, the struct :c:type:`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 NOOP scheduler, the most straightforward one, that implements a
> +simple FIFO, without performing any reordering. This is useful in the following
> +scenarios: when scheduling will be performed in a next step somewhere in the
> +stack, like block device controllers; the actual sector position of blocks are
> +transparent for the host, meaning it hasn't enough information to take a proper
> +decision; or the overhead of reordering is higher than the handicap of
> +non-sequential accesses.
> +
> +Hardware dispatch queues
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The hardware queues (represented by struct :c:type:`blk_mq_hw_ctx`) have a 1:1
> +correspondence to the device driver's submission queues, and are the last step

I am not clear with the definition of "submission queues". Is it the device
queue with DMA ring buffer?

If it is the DMA ring buffer, multiple blk_mq_hw_ctx would map to the same DMA
ring buffer, e.g., multiple nvme namespaces would share the same tagset. This is
not 1:1 any longer.

> +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 (:c:type:`hctx->dispatch`) of requests. Then,
> +next time the block layer runs a queue, it will send the requests laying at the
> +:c:type:`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.

Assume I am a beginner and does not know about blk-mq well. What I expect is to
expand this sections to explain the usage of sbitmap to manage tags, e.g., like
the comments in block/blk-mq-tag.c or block/blk-mq-tag.h.

In addition, I would be interested in that percpu-refcount is used to track the
lifecycle of requests.

I have no idea how much detail is required for a kernel doc. The is just the
feedback from me by assuming the audience is beginner :)

Thank you very much!

Dongli Zhang