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AJvYcCWff1NKEJB7Mw+o3C509rXf5p/DKk1Yt+gRZCCon5F9PDV/MQKXSYNWZOqg0wiAlJ/7n28ripYrAcLqdkoE2sMALPOjp6aqyUa9mnAV X-Gm-Message-State: AOJu0YxiT4X+0Bc8aQLD+oIdPh+YlLPnIFBLGs6v7IrSAUVNk6Hs2FsE QO2HJaBZsqxGwSsF1q3hQVClO91BU3pmSpb/tzp2hufyfKHPAEVZ X-Google-Smtp-Source: AGHT+IHlye3fkEXgNAEnAcc4XvQBBFhLqkNN+i3MEIkF9i/jIbO0V4cW5uqs6VLNdBzYv5r0qfxaPA== X-Received: by 2002:a2e:a545:0:b0:2d8:58b6:c10d with SMTP id e5-20020a2ea545000000b002d858b6c10dmr2267357ljn.18.1712928030896; Fri, 12 Apr 2024 06:20:30 -0700 (PDT) Received: from [192.168.42.203] ([163.114.131.193]) by smtp.gmail.com with ESMTPSA id yz15-20020a170906dc4f00b00a4673706b4dsm1830435ejb.78.2024.04.12.06.20.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Apr 2024 06:20:30 -0700 (PDT) Message-ID: <0eca7b8a-5ea5-4c9a-b6a7-6920b93d27b1@gmail.com> Date: Fri, 12 Apr 2024 14:20:34 +0100 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] io_uring: Add REQ_F_CQE_SKIP support for io_uring zerocopy To: Oliver Crumrine , axboe@kernel.dk Cc: io-uring@vger.kernel.org, linux-kernel@vger.kernel.org References: <6850f08d-0e89-4eb3-bbfb-bdcc5d4e1b78@gmail.com> <09f1a8e9-d9ad-4b40-885b-21e1c2ba147b@gmail.com> <8666ff9d-1cb6-4e92-a1b3-4f3b1fb0ac79@gmail.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 4/11/24 01:52, Oliver Crumrine wrote: > Pavel Begunkov wrote: >> On 4/9/24 02:33, Oliver Crumrine wrote: >>> Pavel Begunkov wrote: >>>> On 4/7/24 20:14, Oliver Crumrine wrote: >>>>> Oliver Crumrine wrote: >>>>>> Pavel Begunkov wrote: >>>>>>> On 4/5/24 21:04, Oliver Crumrine wrote: >>>>>>>> Pavel Begunkov wrote: >>>>>>>>> On 4/4/24 23:17, Oliver Crumrine wrote: >>>>>>>>>> In his patch to enable zerocopy networking for io_uring, Pavel Begunkov >>>>>>>>>> specifically disabled REQ_F_CQE_SKIP, as (at least from my >>>>>>>>>> understanding) the userspace program wouldn't receive the >>>>>>>>>> IORING_CQE_F_MORE flag in the result value. >>>>>>>>> >>>>>>>>> No. IORING_CQE_F_MORE means there will be another CQE from this >>>>>>>>> request, so a single CQE without IORING_CQE_F_MORE is trivially >>>>>>>>> fine. >>>>>>>>> >>>>>>>>> The problem is the semantics, because by suppressing the first >>>>>>>>> CQE you're loosing the result value. You might rely on WAITALL >>>>>>>> That's already happening with io_send. >>>>>>> >>>>>>> Right, and it's still annoying and hard to use >>>>>> Another solution might be something where there is a counter that stores >>>>>> how many CQEs with REQ_F_CQE_SKIP have been processed. Before exiting, >>>>>> userspace could call a function like: io_wait_completions(int completions) >>>>>> which would wait until everything is done, and then userspace could peek >>>>>> the completion ring. >>>>>>> >>>>>>>>> as other sends and "fail" (in terms of io_uring) the request >>>>>>>>> in case of a partial send posting 2 CQEs, but that's not a great >>>>>>>>> way and it's getting userspace complicated pretty easily. >>>>>>>>> >>>>>>>>> In short, it was left out for later because there is a >>>>>>>>> better way to implement it, but it should be done carefully >>>>>>>> Maybe we could put the return values in the notifs? That would be a >>>>>>>> discrepancy between io_send and io_send_zc, though. >>>>>>> >>>>>>> Yes. And yes, having a custom flavour is not good. It'd only >>>>>>> be well usable if apart from returning the actual result >>>>>>> it also guarantees there will be one and only one CQE, then >>>>>>> the userspace doesn't have to do the dancing with counting >>>>>>> and checking F_MORE. In fact, I outlined before how a generic >>>>>>> solution may looks like: >>>>>>> >>>>>>> https://github.com/axboe/liburing/issues/824 >>>>>>> >>>>>>> The only interesting part, IMHO, is to be able to merge the >>>>>>> main completion with its notification. Below is an old stash >>>>>>> rebased onto for-6.10. The only thing missing is relinking, >>>>>>> but maybe we don't even care about it. I need to cover it >>>>>>> well with tests. >>>>>> The patch looks pretty good. The only potential issue is that you store >>>>>> the res of the normal CQE into the notif CQE. This overwrites the >>>>>> IORING_CQE_F_NOTIF with IORING_CQE_F_MORE. This means that the notif would >>>>>> indicate to userspace that there will be another CQE, of which there >>>>>> won't. >>>>> I was wrong here; Mixed up flags and result value. >>>> >>>> Right, it's fine. And it's synchronised by the ubuf refcounting, >>>> though it might get more complicated if I'd try out some counting >>>> optimisations. >>>> >>>> FWIW, it shouldn't give any performance wins. The heavy stuff is >>>> notifications waking the task, which is still there. I can even >>>> imagine that having separate CQEs might be more flexible and would >>>> allow more efficient CQ batching. >>> I've actaully been working on this issue for a little while now. My current >>> idea is that an id is put into the optval section of the SQE, and then it >>> can be used to tag that req with a certain group. When a req has a flag >>> set on it, it can request for all of group's notifs to be "flushed" in one >>> notif that encompasses that entire group. If the id is zero, it won't be >>> associated with a group and will generate a notif. LMK if you see anything >>> in here that could overcomplicate userspace. I think it's pretty simple, >>> but you've had a crack at this before so I'd like to hear your opinion. >> >> You can take a look at early versions of the IORING_OP_SEND_ZC, e.g. >> patchset v1, probably even later ones. It was basically doing what >> you've described with minor uapi changes, like you had to declare groups >> (slots) in advance, i.e. register them. > My idea is that insead of allocating slots before making requests, "slots" > will be allocated as the group ids show up. Instead of an array of slots, a > linked list can be used so things can be kmalloc'ed on the fly to make > the uapi simpler. >> >> More flexible and so performant in some circumstances, but the overall >> feedback from people trying it is that it's complicated. The user should >> allocate group ids, track bound requests / buffers, do other management. >> The next question is how the user should decide what bind to what. There >> is some nastiness in using the same group for multiple sockets, and then > Then maybe we find a way to prevent multiple sockets on one group. You don't have to explicitly prevent it unless there are other reasons, it's just not given a real app would be able to use it this way. >> what's the cut line to flush the previous notif? I probably forgot a > I'd make it the max for a u32 -- I'm (probably) going to use an atomic_t > to store the counter of how many reqs have been completed, so a u32 max > would make sense. To be clear, the question raised is entirely for userspace to decide if we're talking about the design when the user has to flush a group notificaiton via flag or so. Atomics or not is a performance side, that's separate. >> couple more complaints. >> >> TL;DR; >> >> The performance is a bit of a longer story, problems are mostly coming >> from the async nature of io_uring, and it'd be nice to solve at least a >> part of it generically, not only for sendzc. The expensive stuff is >> waking up the task, it's not unique to notifications, recv will trigger >> it with polling as well as other opcodes. Then the key is completion >> batching. > Maybe the interface is made for sendzc first, and people could test it > there. Then if it is considered beneficial to other places, it could be > implemented there. >> >> What's interesting, take for example some tx only toy benchmark with >> DEFER_TASKRUN (recommended to use in any case). If you always wait for >> sends without notifications and add eventual *_get_events(), that would >> completely avoid the wake up overhead if there are enough buffers, >> and if it's not it can 1:1 replace tx polling. > Seems like an interesting way to eliminate waiting overhead. >> >> Try groups, see if numbers are good. And a heads up, I'm looking at > I will. Working hard to have the code done by Sunday. Good, and here is the patchset I mentioned: https://lore.kernel.org/io-uring/cover.1712923998.git.asml.silence@gmail.com/T/ >> improving it a little bit for TCP because of a report, not changing >> uapi but might change performance math. -- Pavel Begunkov