On Mon, Jul 04, 2022 at 05:49:34PM +0800, Ziyang Zhang wrote: > On 2022/7/4 12:08, Ming Lei wrote: > > On Thu, Jun 30, 2022 at 05:29:07PM +0800, Ziyang Zhang wrote: > >> 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. :-( > > > > Both the two are not hard to do, and turns out that making libubdsrv is actually > > one big cleanup. > > > > All these works[1] are basically done: > > > > 1) libublksrv > > - built .so and .a are under lib/ > > - exported header file is include/ublksrv.h > > - so any other application can make ublk device against this library > > - eventfd notification is added too, so io handling doesn't have to > > be done via io_uring, one callback of ->handle_event(), and two APIs > > are added for this support > > > > 2) ublk/ubd utility > > - built against libublksrv, meantime it uses the private header of the > > library too, which is fine, since the two are in same project > > > > 3) two examples > > - demo_null.c: one < 200 LOC standalone example to show how to make > > a ubd/null block device against libublksrv > > > > - demo_event.c: one simple standalone example(~300LOC) to make one ublk > > disk by handling io via another pthread(not by io_uring) against > > libublksrv > > > > Any comments/feedback/tests are welcome. > > > > > > [1] libublk > > https://github.com/ming1/ubdsrv/tree/libublk > > > > Hi Ming, > > Thanks for your libublk code. > You almost have done everything we discussed before :) > > One small question: > Could you please write another demo_event_loop.c: That is actually easy, but I'd suggest you to do it, and I can apply the patch on ublksrv tree once it is ready. It is also one good chance for you to understand the recent change of libublksrv. I have written one loop based on io_uring, which performs very well. The eventfd interface is actually for existed project which can't use io_uring or io_uring may not implement the function, that should be your use case. > > 1) based on demo_event.c(handling io via another pthread) > but the loop backend is a fd(file). User calls pwrite/pread with the fd. > > 2) User-provided buffer: data buffer is allocated by backend > (not posix_memalign() in ublksrv.c) > and it is passed to ublksrv(finally to ubd_drv in kernel) by libublksrv's API > ( maybe passed as an arg to ublksrv_complete_io()? ) > > 3) I think pread/pwrite should be in demo_event_real_io_handler_fn() > for all IOs of each pending list. Am I correct? Exactly, the pthread of demo_event_real_io_handler_fn is for handling io from /dev/ublkbN. > > 4) Shall we consider multiple pthreads calling demo_event_real_io_handler_fn() > (they are io worker threads actually) > so we can handle IO concurrently? That can be done, but sync among these pthreads have to be dealt with well. > > > I think designing more examples helps us design libublk's API more clearly. The example of demo_event.c is enough for implementing the loop/event. Thanks, Ming
