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

[Andrà Almeida]

On 6/5/20 8:45 PM, Dongli Zhang wrote:
> 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?
> 

Yes, the memory area where hardware can perform DMA and access requests.

> 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.
>

I see, thanks for the feedback. What do you think about this version:

The hardware queues (represented by struct :c:type:`blk_mq_hw_ctx`) is a
struct used by device drivers to map the to 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 (: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 :)
> 

Indeed, it would be good to have this kind of information available. We
can easily include comments in source files given that they are in
kernel-doc format, just using `kernel-doc:: block/blk-mq-tag.c`.
However, as I see, there's some useful information that need to be
converted to kernel-doc before being ready to show up here in the
documentation.

So, while I agree with you, I think this can be a next step for block
documentation: update, format and expand blk-mq-tag comments, then add
them in this file.

> Thank you very much!
> 
> Dongli Zhang
> 

Thanks for the feedback :)

AndrÃ