From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B353BC43334 for ; Thu, 30 Jun 2022 07:16:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230061AbiF3HQ3 (ORCPT ); Thu, 30 Jun 2022 03:16:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44922 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229676AbiF3HQ2 (ORCPT ); Thu, 30 Jun 2022 03:16:28 -0400 Received: from out30-131.freemail.mail.aliyun.com (out30-131.freemail.mail.aliyun.com [115.124.30.131]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EAF7437A00; Thu, 30 Jun 2022 00:16:25 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R181e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045168;MF=ziyangzhang@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0VHqj5dO_1656573382; Received: from 30.97.57.27(mailfrom:ZiyangZhang@linux.alibaba.com fp:SMTPD_---0VHqj5dO_1656573382) by smtp.aliyun-inc.com; Thu, 30 Jun 2022 15:16:23 +0800 Message-ID: Date: Thu, 30 Jun 2022 15:16:21 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [RFC] libubd: library for ubd(userspace block driver based on io_uring passthrough) Content-Language: en-US To: Ming Lei Cc: linux-block@vger.kernel.org, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, Gabriel Krisman Bertazi , Xiaoguang Wang , joseph.qi@linux.alibaba.com References: From: Ziyang Zhang In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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