Re: [RFC] libubd: library for ubd(userspace block driver based on io_uring passthrough)

Ziyang Zhang
2022-06-30T07:16:30Z

Hi, Ming

On 2022/6/29 19:33, Ming Lei wrote:
> On Wed, Jun 29, 2022 at 11:22:23AM +0800, Ziyang Zhang wrote:
>> Hi Ming,
>>
>> On 2022/6/27 23:29, Ming Lei wrote:
>>> Hi Ziyang,
>>>
>>> On Mon, Jun 27, 2022 at 04:20:55PM +0800, Ziyang Zhang wrote:
>>>> Hi Ming,
>>>>
>>>> We are learning your ubd code and developing a library: libubd for ubd.
>>>> This article explains why we need libubd and how we design it.
>>>>
>>>> Related threads:
>>>> (1) https://lore.kernel.org/all/Yk%2Fn7UtGK1vVGFX0@T590/
>>>> (2) https://lore.kernel.org/all/YnDhorlKgOKiWkiz@T590/
>>>> (3) https://lore.kernel.org/all/20220509092312.254354-1-ming.lei@redhat.com/
>>>> (4) https://lore.kernel.org/all/20220517055358.3164431-1-ming.lei@redhat.com/
>>>>
>>>>
>>>> Userspace block driver(ubd)[1], based on io_uring passthrough,
>>>> allows users to define their own backend storage in userspace
>>>> and provides block devices such as /dev/ubdbX.
>>>> Ming Lei has provided kernel driver code: ubd_drv.c[2]
>>>> and userspace code: ubdsrv[3].
>>>>
>>>> ubd_drv.c simply passes all blk-mq IO requests
>>>> to ubdsrv through io_uring sqes/cqes. We think the kernel code
>>>> is pretty well-designed.
>>>>
>>>> ubdsrv is implemented by a single daemon
>>>> and target(backend) IO handling(null_tgt and loop_tgt)
>>>> is embedded in the daemon.
>>>> While trying ubdsrv, we find ubdsrv is hard to be used
>>>> by our backend.
>>>
>>> ubd is supposed to provide one generic framework for user space block
>>> driver, and it can be used for doing lots of fun/useful thing.
>>>
>>> If I understand correctly, this isn't same with your use case:
>>>
>>> 1) your user space block driver isn't generic, and should be dedicated
>>> for Alibaba's uses
>>>
>>> 2) your case has been there for long time, and you want to switch from other
>>> approach(maybe tcmu) to ubd given ubd has better performance.
>>>
>>
>> Yes, you are correct :)
>> The idea of design libubd is actually from libtcmu.
>>
>> We do have some userspace storage system as the IO handling backend,
>> and we need ubd to provide block drivers such as /dev/ubdbX for up layer client apps.
>>
>>
>> I think your motivation is that provides a complete user block driver to users
>> and they DO NOT change any code.
>> Users DO change their code using libubd for embedding libubd into the backend.
>>
>>
>>>> First is description of our backend:
>>>>
>>>> (1) a distributing system sends/receives IO requests
>>>> through network.
>>>>
>>>> (2) The system use RPC calls among hundreds of
>>>> storage servers and RPC calls are associated with data buffers
>>>> allocated from a memory pool.
>>>>
>>>> (3) On each server for each device(/dev/vdX), our backend runs
>>>> many threads to handle IO requests and manage the device.
>>>>
>>>> Second are reasons why ubdsrv is hard to use for us:
>>>>
>>>> (1) ubdsrv requires the target(backend) issues IO requests
>>>> to the io_uring provided by ubdsrv but our backend
>>>> uses something like RPC and does not support io_uring.
>>>
>>> As one generic framework, the io command has to be io_uring
>>> passthrough, and the io doesn't have to be handled by io_uring.
>>
>> Yes, our backend define its own communicating method.
>>
>>>
>>> But IMO io_uring is much more efficient, so I'd try to make async io
>>> (io uring) as the 1st citizen in the framework, especially for new
>>> driver.
>>>
>>> But it can support other way really, such as use io_uring with eventfd,
>>> the other userspace context can handle io, then wake up io_uring context
>>> via eventfd. You may not use io_uring for handling io, but you still
>>> need to communicate with the context for handling io_uring passthrough
>>> command, and one mechanism(such as eventfd) has to be there for the
>>> communication.
>>
>> Ok, eventfd may be helpful.
>> If you read my API, you may find ubdlib_complete_io_request().
>> I think the backend io worker thread can call this function to tell the
>> ubd queue thread(the io_uring context in it) to commit the IO.
>
> The ubdlib_complete_io_request() has to be called in the same pthread
> context, that looks not flexible. When you handle IO via non-io_uring in the same
> context, the cpu utilization in submission/completion side should be
> higher than io_uring. And this way should be worse than the usage in
> ubd/loop, that is why I suggest to use one io_uring for handling both
> io command and io request if possible.

ubdlib_complete_io_request() can be called in the io worker thread,
not in the ubdsrv queue thread(with the io_uring context for handling uring_cmd).

You can find ubd_runner.c in my libubd repo. There are many io worker
threads for each ubdsrv queue to handle IO requests.

Actually this idea comes from tcmu-runner. The data flow is:

1) in ubdsrv queue thread, io_uring_enter(): returns(IO reqs received from blk-mq)

2) in ubdsrv queue thread, ubdsrv_reap_requests(): iterate on each cqe(with an IO req),

for READ/WRITE requests, ubd_aio_queue_io() to enqueue the IO req into a io_queue
(each ubdsrv queue has one io_queue). This IO req's status is IO_HANDLING_ASYNC.

for other simple(can be handled very quickly),
handle it right now and call ubdlib_complete_io_request()

3) in ubdsrv queue thread, ubdsrv_commit_and_fetch(): iterate on all IO slots per ubdsrv queue
and setup sqe if one IO(IO completion) is ready to commit.

Here, some IO slots are still IO_HANDLING_ASYNC so no sqe is generated for them.

4) in ubdsrv queue thread, io_uring_enter(): submit all sqes and wait for cqes
(io_uring_enter() will return after at least one IO req is received from blk-mq)

5) When 3) or 4) happens, at the same time in ubdsrv queue IO worker threads:
each io worker thread try to deque and handle one IO req from io_queue per ubdsrv queue.

After the IO worker handles the IO req(WRITE/READ), it calls ubdlib_complete_io_request()
This function can mark this IO req's status to ready to commit.

IO handling/completion and io_uring_enter() can happen at the same time.

Besides, io_uring_enter can:

1) block and wait for cqes until at least
one blk-mq req comes from queue_rq()

2) submit sqes(with last IO completion and next fetch)

so I have to consider how to notify io_uring about io completion
after io_uring_enter() is slept(block and wait for cqes).

In current version of ubd_runner(an async libubd target), I try to use an "unblock"
io_uring_enter_timeout() and caller can set a timeout value for it.
So IO completions happen after io_uring_enter_timeout() call can be committed
by next io_uring_enter_timeout() call...

But this is a very ugly implementation
because I may waste CPU on useless loops in ubdsrv queue thread if
blk-mq reqs do not income frequently.

You mentioned that eventfd may be helpful and I agree with you. :)
I can register an eventfd in io_uring after ubd_aio_queue_io() and write the eventfd
in ubdlib_complete_io_request().

I will fix my code.

>
>>
>>
>>
>>>
>>>>
>>>> (2) ubdsrv forks a daemon and it takes over everything.
>>>> Users should type "list/stop/del" ctrl-commands to interact with
>>>> the daemon. It is inconvenient for our backend
>>>> because it has threads(from a C++ thread library) running inside.
>>>
>>> No, list/stop/del won't interact with the daemon, and the per-queue
>>> pthread is only handling IO commands(io_uring passthrough) and IO request.
>>>
>>
>>
>> Sorry I made a mistake.
>>
>> I mean from user's view,
>> he has to type list/del/stop from cmdlind to control the daemon.
>> (I know the control flow is cmdline-->ubd_drv.c-->ubdsrv daemon).
>>
>> This is a little weird if we try to make a ubd library.
>> So I actually provides APIs in libubd for users to do these list/del/stop works.
>
> OK, that is fine to export APIs for admin purpose.
>
>>
>>
>>>>
>>>> (3) ubdsrv PRE-allocates internal data buffers for each ubd device.
>>>> The data flow is:
>>>> bio vectors <-1-> ubdsrv data buffer <-2-> backend buffer(our RPC buffer).
>>>> Since ubdsrv does not export its internal data buffer to backend,
>>>> the second copy is unavoidable.
>>>> PRE-allocating data buffer may not be a good idea for wasting memory
>>>> if there are hundreds of ubd devices(/dev/ubdbX).
>>>
>>> The preallocation is just virtual memory, which is cheap and not pinned, but
>>> ubdsrv does support buffer provided by io command, see:
>>>
>>> https://github.com/ming1/linux/commit/0a964a1700e11ba50227b6d633edf233bdd8a07d
>>
>> Actually I discussed on the design of pre-allocation in your RFC patch for ubd_drv
>> but you did not reply :)
>>
>> I paste it here:
>>
>> "I am worried about the fixed-size(size is max io size, 256KiB) pre-allocated data buffers in UBDSRV
>> may consume too much memory. Do you mean these pages can be reclaimed by sth like madvise()?
>> If (1)swap is not set and (2)madvise() is not called, these pages may not be reclaimed."
>>
>> I observed that your ubdsrv use posix_memalign() to pre-allocate data buffers,
>> and I have already noticed the memory cost while testing your ubdsrv with hundreds of /dev/ubdbX.
>
> Usually posix_memalign just allocates virtual memory which is unlimited
> in 64bit arch, and pages should be allocated until the buffer is read or write.
> After the READ/WRITE is done, kernel still can reclaim the pages in this
> virtual memory.
>
> In future, we still may optimize the memory uses via madvise, such as
> MADV_DONTNEED, after the slot is idle for long enough.

Ok, thanks for explanation.

>
>>
>> Another IMPORTANT problem is your commit:
>> https://github.com/ming1/linux/commit/0a964a1700e11ba50227b6d633edf233bdd8a07d
>> may be not helpful for WRITE requests if I understand correctly.
>>
>> Consider this data flow:
>>
>> 1. ubdsrv commits an IO req(req1, a READ req).
>>
>> 2. ubdsrv issues a sqe(UBD_IO_COMMIT_AND_FETCH_REQ), and sets io->addr to addr1.
>> addr1 is the addr of buffer user passed.
>>
>>
>> 3. ubd gets the sqe and commits req1, sets io->addr to addr1.
>>
>> 4. ubd gets IO req(req2, a WRITE req) from blk-mq(queue_rq) and commit a cqe.
>>
>> 5. ubd copys data to be written from biovec to addr1 in a task_work.
>>
>> 6. ubdsrv gets the cqe and tell the IO target to handle req2.
>>
>> 7. IO target handles req2. It is a WRITE req so target issues a io_uring write
>> cmd(with buffer set to addr1).
>>
>>
>>
>> The problem happens in 5). You cannot know the actual data_len of an blk-mq req
>> until you get one in queue_rq. So length of addr1 may be less than data_len.
>
> So far, the actual length of buffer has to be set as at least rq_max_blocks, since
> we set it as ubd queue's max hw sectors. Yeah, you may argue memory
> waste, but process virtual address is unlimited for 64bit arch, and
> pages are allocated until actual read/write is started.

Ok, since I allow users to config rq_max_blocks in libubd,
it's users' responsibility to ensure length of user buffers
is at least rq_max_blocks.

Now I agree on your commit:
https://github.com/ming1/linux/commit/0a964a1700e11ba50227b6d633edf233bdd8a07d

Provide WRITE buffer in advance(when sending COMMIT_AND_FETCH) seems OK :)

>
>>>
>>>>
>>>> To better use ubd in more complicated scenarios, we have developed libubd.
>>>> It does not assume implementation of backend and can be embedded into it.
>>>> We refer to the code structure of tcmu-runner[4],
>>>> which includes a library(libtcmu) for users
>>>> to embed tcmu-runner inside backend's code.
>>>> It:
>>>>
>>>> (1) Does not fork/pthread_create but embedded in backend's threads
>>>
>>> That is because your backend may not use io_uring, I guess.
>>>
>>> But it is pretty easy to move the decision of creating pthread to target
>>> code, which can be done in the interface of .prepare_target().
>>
>> I think the library should not create any thread if we want a libubd.
>
> I Agree.
>
>>
>>>
>>>>
>>>> (2) Provides libubd APIs for backend to add/delete ubd devices
>>>> and fetch/commit IO requests
>>>
>>> The above could be the main job of libubd.
>>
>> indeed.
>>
>>>
>>>>
>>>> (3) simply passes backend-provided data buffers to ubd_drv.c in kernel,
>>>> since the backend actually has no knowledge
>>>> on incoming data size until it gets an IO descriptor.
>>>
>>> I can understand your requirement, not look at your code yet, but libubd
>>> should be pretty thin from function viewpoint, and there are lots of common
>>> things to abstract/share among all drivers, please see recent ubdsrv change:
>>>
>>> https://github.com/ming1/ubdsrv/commits/master
>>>
>>> in which:
>>> - coroutine is added for handling target io
>>> - the target interface(ubdsrv_tgt_type) has been cleaned/improved for
>>> supporting complicated target
>>> - c++ support
>>
>> Yes, I have read your coroutine code but I am not an expert of C++ 20.:(
>> I think it is actually target(backend) design and ubd should not assume
>> how the backend handle IOs.
>>
>> The work ubd in userspace has to be done is:
>>
>> 1) give some IO descriptors to backend, such as ubd_get_io_requests()
>>
>> 2) get IO completion form backend, such as ubd_complete_io_requests()
>
> Or the user provides/registers two callbacks: handle_io_async() and
> io_complete(), the former is called when one request comes from ubd
> driver, the latter(optional) is called when one io is done.
>
> Also you didn't mention how you notify io_uring about io completion after
> io_uring_enter() is slept if your backend code doesn't use io_uring to
> handle io.
>
> I think one communication mechanism(such as eventfd) is needed for your
> case.

Ok, I will try eventfd with io_uring.

>
>>
>>
>>
>>>
>>> IMO, libubd isn't worth of one freshly new project, and it could be integrated
>>> into ubdsrv easily. The potential users could be existed usersapce
>>> block driver projects.
>>
>> Yes, so many userspace storage systems can use ubd!
>> You may look at tcmu-runner. It:
>>
>> 1) provides a library(libtcmu.c) for those who have a existing backend.
>>
>> 2) provides a runner(main.c in tcmu-runner) like your ubdsrv
>> for those who just want to run it.
>> And the runner is build on top of libtcmu.
>>
>>>
>>> If you don't object, I am happy to co-work with you to add the support
>>> for libubd in ubdsrv, then we can avoid to invent a wheel
>>
>> +1 :)
>
> Thinking of further, I'd suggest to split ubdsrv into two parts:
>
> 1) libubdsrv
> - provide APIs like what you did in libubd
> - provide API for notify io_uring(handling io command) that one io is
> completed, and the API should support handling IO from other context
> (not same with the io_uring context for handling io command).
>
> 2) ubd target
> - built on libubdsrv, such as ubd command is built on libubdsrv, and
> specific target implementation is built on the library too.
>
> It shouldn't be hard to work towards this direction, and I guess this
> way should make current target implementation more clean.
>

Yes, this is like tcmu-runner's structure: a libtcmu and some target
Thanks, Ming. Glad to co-work with you.

I will take your advice and improve libubd(the communication mechanism, maybe eventfd).

>>
>>>
>>>>
>>>> Note:
>>>>
>>>> (1) libubd is just a POC demo and is not stick to the principles of
>>>> designing a library and we are still developing it now...
>>>>
>>>> (2) The repo[5] including some useful examples using libubd.
>>>>
>>>> (3) We modify the kernel part: ubd_drv.c and
>>>> it[6] is against Ming Lei's newest branch[2]
>>>> because we forked our branch from his early branch
>>>> (v5.17-ubd-dev).
>>>
>>> Please look at the following tree for ubd driver:
>>>
>>> https://github.com/ming1/linux/tree/my_for-5.19-ubd-devel_v3
>>>
>>> in which most of your change should have been there already.
>>>
>>> I will post v3 soon, please feel free to review after it is out and
>>> see if it is fine for you.
>>
>> Yes, I have read your newest branch.
>> You use some task_work() functions in ubd_drv.c
>> for error-handling such as aborting IO.
>
> The IO aborting has to be supported, otherwise what if the io_uring
> context(pthread) is killed, and what if the device is removed when
> handling IO.

Ok, now libubd just focuses on performance.
I will try to add these error-handling methods.

>
> ubdsrv/tests/generic provides two tests for deleting ubd & killing
> per-queue pthread meantime with heavy IO.
>
>>
>> But I find they are too complicated to understand
>> and it's hard to write libubd code in this branch.
>
> Actually the latest ubdsrv code becomes much clean, and should be
> easier to abstract APIs for external/binary target.
>
>>
>> So I choose your first(easiest to understand)
>> version: v5.17-ubd-dev.
>
> That code is outdated, and full of bugs, :-(

Ok, I will try to follow your newest branch.

>
>
> Thanks,
> Ming