From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr1-f51.google.com (mail-wr1-f51.google.com [209.85.221.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D1535D51E; Thu, 11 Apr 2024 00:52:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712796769; cv=none; b=Q/zQoZsPeBBK4ud/FW8vIVurwLv/DN536vgCAf2SUYKkmdwM37TiO+2VtbPhW66CPZHMN9iK/XtPb5TPYdDt8VDB6A7k8p/H3eZcMVOeWgxT09hd9acWzs55umhI7MctrJd5UQxf+l1Nz42aWqE4LDs+7xVRoCLpcz3qB0DJJkc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712796769; c=relaxed/simple; bh=tDbOaO/1XU/356k6Y8UmDzuMfxh0VhDDmH7UHMOqrzM=; h=From:In-Reply-To:References:Mime-Version:Date:Message-ID:Subject: To:Cc:Content-Type; b=iEHuIkJzfi+nKZ56HNOaaeCXADq5ln/J4TkHsEcLsUeAA46rZ00AYZDTHOJQuwO4LRHKCpHHXl3T+shhlF5GLl/yxbJZTwx/ZTNR/byGr2eoxv4oYbwYDx2iX4rq9Ma17OQP3ZcDt+zVIOxEeaqJt9tUlAvURW1WQarxktnNPNE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=FFcC2sUB; arc=none smtp.client-ip=209.85.221.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="FFcC2sUB" Received: by mail-wr1-f51.google.com with SMTP id ffacd0b85a97d-346406a5fb9so1556668f8f.1; Wed, 10 Apr 2024 17:52:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1712796766; x=1713401566; darn=vger.kernel.org; h=cc:to:subject:message-id:date:mime-version:references:in-reply-to :from:from:to:cc:subject:date:message-id:reply-to; bh=tDbOaO/1XU/356k6Y8UmDzuMfxh0VhDDmH7UHMOqrzM=; b=FFcC2sUByX7Jwko4178i0m7PMwIlYvDoIkRPjAOnrg5ov7xTqla+AQmxl8sLJ3HSVa fmtKasR0tEvWvyyVEtW9ugB3Gg8j0MuAjLmfGQWNe0oiH4vfd+n7RPk1r4eRDK2caR5G T//NVt9qVnrWMnPEZIms/xmN5u+Bolkvs2vgjr+BUlScgocxfPVgMbVVLfIdjh5n31p9 /iGllII+J02h4J9aVi+uOM+1QvMIyvdgeKoy+wF+gPrAIneK0k/Dgwz+r3ydhJRjnX2G 8y8iAzyit4QUN4X8tIuwUMS9gd2UJpxg73GVqDEAwaVIMFN11I9G7dY8oCvGZ10Z3w44 AqGw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1712796766; x=1713401566; h=cc:to:subject:message-id:date:mime-version:references:in-reply-to :from:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=tDbOaO/1XU/356k6Y8UmDzuMfxh0VhDDmH7UHMOqrzM=; b=I500Sm6cCRc/Ozpx7wp6svQV5hg5gFX7DD2HTSmhc2jFYjHLltxeN6VJucRAFfX1V+ SiIYENG5dbC2dw6Kwjv2ZfDMbg8I5e8zuQ2l6sTwiwtfKKLlgms5bn7c6k2pWvJBD0Tt j/dYAkoXk67eZxhpHTBHi2pZQDC2dqEvR/kJ4oUMtLtiCbJWaKO8nJSi1P9D6vf1uVBu VFK9/P+8SrnokVCSqzt215DHC0HrphwfLZkmP4dKXV8b6q7mkAqbp008EaEQ82R9cjsr nf6ioZY31/HsyQz2ZGTvyc2fnar8U8KBaoOSSTAxvS/sQWVrGmYLXC3ihw1Z1pcexaZG mFjA== X-Forwarded-Encrypted: i=1; AJvYcCVtyZj51crUKrx733lvHtMU1coAqJOMEZh+ckClyeZpeBQNVTLyT1VoGm/xQKOzG+gZC/VJulXgfoKkmnQfbqSA4xJZAIoxTk/7EM6N X-Gm-Message-State: AOJu0Yw9Tjp3GAu1/61fH/L/BnIu+ffx+C84gZbptlXsLxgc3JKYXtu5 JSQfyYQrhTioVXRVVSCf+1cmKvUKZ9xiaYbGiimQlH0ERV540LRJPiOaqqAnLA67fiDPbZYcvTk wwZdgtcUgTXInfSN0x7Go5ARlRPM= X-Google-Smtp-Source: AGHT+IHO4ErRa1hFQmFIoMyeFaR/i7qecuC11H78hkhdTewP1Qypp4aPxjXvBg/t8fNJ4AmDGblwU+cafaUWNRqW91g= X-Received: by 2002:a5d:6d81:0:b0:346:75ae:471b with SMTP id l1-20020a5d6d81000000b0034675ae471bmr2294597wrs.51.1712796765761; Wed, 10 Apr 2024 17:52:45 -0700 (PDT) Received: from 753933720722 named unknown by gmailapi.google.com with HTTPREST; Wed, 10 Apr 2024 17:52:45 -0700 From: Oliver Crumrine In-Reply-To: <8666ff9d-1cb6-4e92-a1b3-4f3b1fb0ac79@gmail.com> References: <6850f08d-0e89-4eb3-bbfb-bdcc5d4e1b78@gmail.com> <09f1a8e9-d9ad-4b40-885b-21e1c2ba147b@gmail.com> <8666ff9d-1cb6-4e92-a1b3-4f3b1fb0ac79@gmail.com> Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Date: Wed, 10 Apr 2024 17:52:45 -0700 Message-ID: Subject: Re: [PATCH 1/3] io_uring: Add REQ_F_CQE_SKIP support for io_uring zerocopy To: Pavel Begunkov , Oliver Crumrine , axboe@kernel.dk Cc: io-uring@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" 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. > 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. > 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. > improving it a little bit for TCP because of a report, not changing > uapi but might change performance math. > > -- > Pavel Begunkov