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

[Ziyang Zhang]

On 2022/6/30 17:09, Ming Lei wrote:
> On Thu, Jun 30, 2022 at 03:16:21PM +0800, Ziyang Zhang wrote:
>> 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@xxxxxxxxxx/
>>>>>> (4) https://lore.kernel.org/all/20220517055358.3164431-1-ming.lei@xxxxxxxxxx/
>>>>>>
>>>>>>
>>>>>> 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).
> 
> Yeah, that was exactly my question, :-)
> 
>>
>> 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.
> 
> FYI, there is one example about using eventfd to wakeup io_uring, which
> can be added to the library for your usecase:
> 
> https://gist.github.com/1Jo1/6496d1b8b6b363c301271340e2eab95b

Thanks, will take a view.

> 
>>
>>>
>>>>
>>>>
>>>>
>>>>>
>>>>>>
>>>>>> (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).
> 
> I have added libublk branch for working towards this direction, if we
> cowork on libublk, please write patch against this branch, then I can
> apply your patch directly.
> 
> https://github.com/ming1/ubdsrv/tree/libublk

Ok, but It concerns me that libubdsrv may change current ubdsrv project's structure a lot
because:
1) target implementation will be built on top of libubdsrv and the target
   should create pthread(ubdsrv loop) itself.

2) have to remove pthread/process(daemon) in current ubdsrv to build libubdsrv.
   It was really a hard job. :-(

If you agree to adjust current ubdsrv project's structure, I will start right now.

> 
> Thanks, 
> Ming