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 > > > > >> > >> > >> > >>> > >>>> > >>>> (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 Thanks, Ming
