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 DF06CC433F5 for ; Thu, 3 Feb 2022 23:26:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355643AbiBCX0r (ORCPT ); Thu, 3 Feb 2022 18:26:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41754 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355983AbiBCX0o (ORCPT ); Thu, 3 Feb 2022 18:26:44 -0500 Received: from mail-wr1-x42e.google.com (mail-wr1-x42e.google.com [IPv6:2a00:1450:4864:20::42e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 29A75C061714; Thu, 3 Feb 2022 15:26:44 -0800 (PST) Received: by mail-wr1-x42e.google.com with SMTP id k18so7991367wrg.11; Thu, 03 Feb 2022 15:26:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:content-language:to :cc:references:from:in-reply-to:content-transfer-encoding; bh=myVtROYgMKE1wZAdyLUyq1uGnyhk8OXwXD/l2iqY7sw=; b=k37bXsanbVrjOlLv4wC/AL6qV941j3a0RvFnpqvUUBbhUw3j0z00u1LTEVo1WnPWmj Aigl0CZLcs06jqA6X3kB7begqSBivKkxA8kXy77fkvZWg0AQLJQhJLalkTANXhr9yFft h1ff4RtHvkZgiFszHPk59XHelYWiz5v634gjKOQAj9LyiwWTctoY6WyEFZQfpUqHyorE d+f2Vog1hmnHyojNVvmfz7otrvviQ0LCoogAc+UdcL2GCijcffZntNtGrj92gHVwDOPH MkUpOjs+INdSTBqhkgJeiTr+vMv8d7YT7rfU8VVBMQJEy3bVE/hLVmj33eAgu8v+ZSRk RsbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:in-reply-to :content-transfer-encoding; bh=myVtROYgMKE1wZAdyLUyq1uGnyhk8OXwXD/l2iqY7sw=; b=fiFTvBt/8VkXRnlEjny6AYFthaxv8gyYWZwC8V8Dlg61++hlO1IjzRbF764FXtSh8K NRwqXoH3u4Q6jze3UZemvPBWG92wy77j9uanigbHF+4T5Jel09H05r7trLvPpiQLJ0oc MqvEU9GZk8CQxAhui4L7Ovjs20waHuxaGB7Rijn1/6QSd/UCgjkSmzgOhhD8Qg/OLRwr ZDAF7dYd3jUpdEY5X50lwAUP6VUaNOvrV60YqWGeveq/GcJRuvFqWUJhbsYlWLznr4hg hzjt+upvTFflV3oSXKJ8omRxdl/2XSxDTpoirL5MJTZCkkGax6JHxGpJiBOIHri0mXEd 6ohw== X-Gm-Message-State: AOAM531GnKfA5CKrvfFhmW4eVA1ix96GN1ePZKAsKskHVzETGcq/FrZh UAJoVDMjWrxnFJwoE5etqtQ= X-Google-Smtp-Source: ABdhPJwzGTdqmsldZcdMUarsFpmwzYrK9XBhdJLM6+BLH5WHBw6etuBseYorYuEzeMq2sZ1lC5s0ww== X-Received: by 2002:a5d:500c:: with SMTP id e12mr177221wrt.193.1643930802640; Thu, 03 Feb 2022 15:26:42 -0800 (PST) Received: from [192.168.8.198] ([85.255.232.204]) by smtp.gmail.com with ESMTPSA id p2sm120572wmc.33.2022.02.03.15.26.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 03 Feb 2022 15:26:42 -0800 (PST) Message-ID: <2104dd91-676b-84af-3005-a40c996b9403@gmail.com> Date: Thu, 3 Feb 2022 23:21:41 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd Content-Language: en-US To: Jens Axboe , Usama Arif , io-uring@vger.kernel.org, linux-kernel@vger.kernel.org Cc: fam.zheng@bytedance.com References: <20220203174108.668549-1-usama.arif@bytedance.com> <20220203174108.668549-3-usama.arif@bytedance.com> <877d54b9-5baa-f0b5-23fe-25aef78e37c4@bytedance.com> <582c8c6f-cbcf-f8d7-4976-e70d0d51c42d@kernel.dk> From: Pavel Begunkov In-Reply-To: <582c8c6f-cbcf-f8d7-4976-e70d0d51c42d@kernel.dk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 2/3/22 22:16, Jens Axboe wrote: > On 2/3/22 2:47 PM, Pavel Begunkov wrote: >> On 2/3/22 19:54, Usama Arif wrote: >>> On 03/02/2022 19:06, Jens Axboe wrote: >>>> On 2/3/22 12:00 PM, Pavel Begunkov wrote: >>>>> On 2/3/22 18:29, Jens Axboe wrote: >>>>>> On 2/3/22 11:26 AM, Usama Arif wrote: >>>>>>> Hmm, maybe i didn't understand you and Pavel correctly. Are you >>>>>>> suggesting to do the below diff over patch 3? I dont think that would be >>>>>>> correct, as it is possible that just after checking if ctx->io_ev_fd is >>>>>>> present unregister can be called by another thread and set ctx->io_ev_fd >>>>>>> to NULL that would cause a NULL pointer exception later? In the current >>>>>>> patch, the check of whether ev_fd exists happens as the first thing >>>>>>> after rcu_read_lock and the rcu_read_lock are extremely cheap i believe. >>>>>> >>>>>> They are cheap, but they are still noticeable at high requests/sec >>>>>> rates. So would be best to avoid them. >>>>>> >>>>>> And yes it's obviously racy, there's the potential to miss an eventfd >>>>>> notification if it races with registering an eventfd descriptor. But >>>>>> that's not really a concern, as if you register with inflight IO >>>>>> pending, then that always exists just depending on timing. The only >>>>>> thing I care about here is that it's always _safe_. Hence something ala >>>>>> what you did below is totally fine, as we're re-evaluating under rcu >>>>>> protection. >>>>> >>>>> Indeed, the patch doesn't have any formal guarantees for propagation >>>>> to already inflight requests, so this extra unsynchronised check >>>>> doesn't change anything. >>>>> >>>>> I'm still more сurious why we need RCU and extra complexity when >>>>> apparently there is no use case for that. If it's only about >>>>> initial initialisation, then as I described there is a much >>>>> simpler approach. >>>> >>>> Would be nice if we could get rid of the quiesce code in general, but I >>>> haven't done a check to see what'd be missing after this... >>>> >>> >>> I had checked! I had posted below in in reply to v1 (https://lore.kernel.org/io-uring/02fb0bc3-fc38-b8f0-3067-edd2a525ef29@gmail.com/T/#m5ac7867ac61d86fe62c099be793ffe5a9a334976), but i think it got missed! Copy-pasting here for reference: >> >> May have missed it then, apologies >> >>> " >>> I see that if we remove ring quiesce from the the above 3 opcodes, then >>> only IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS is >>> left for ring quiesce. I just had a quick look at those, and from what i >>> see we might not need to enter ring quiesce in >>> IORING_REGISTER_ENABLE_RINGS as the ring is already disabled at that point? >>> And for IORING_REGISTER_RESTRICTIONS if we do a similar approach to >>> IORING_REGISTER_EVENTFD, i.e. wrap ctx->restrictions inside an RCU >>> protected data structure, use spin_lock to prevent multiple >>> io_register_restrictions calls at the same time, and use read_rcu_lock >>> in io_check_restriction, then we can remove ring quiesce from >>> io_uring_register altogether? >>> >>> My usecase only uses IORING_REGISTER_EVENTFD, but i think entering ring >>> quiesce costs similar in other opcodes. If the above sounds reasonable, >>> please let me know and i can send patches for removing ring quiesce for >>> io_uring_register. >>> " >>> >>> Let me know if above makes sense, i can add patches on top of the current patchset, or we can do it after they get merged. >>> >>> As for why, quiesce state is very expensive. its making io_uring_register the most expensive syscall in my usecase (~15ms) compared to ~0.1ms now with RCU, which is why i started investigating this. And this patchset avoids ring quiesce for 3 of the opcodes, so it would generally be quite helpful if someone does registers and unregisters eventfd multiple times. >> >> I agree that 15ms for initial setup is silly and it has to be >> reduced. However, I'm trying weight the extra complexity against >> potential benefits of _also_ optimising [de,re]-registration >> >> Considering that you only register it one time at the beginning, >> we risk adding a yet another feature that nobody is going to ever >> use. This doesn't give me a nice feeling, well, unless you do >> have a use case. > > It's not really a new feature, it's just making the existing one not > suck quite as much... Does it matter when nobody uses it? My point is that does not. >> To emphasise, I'm comparing 15->0.1 improvement for only initial >> registration (which is simpler) vs 15->0.1 for both registration >> and unregistration. > > reg+unreg should be way faster too, if done properly with the assignment > tricks. > >> fwiw, it alters userpace visible behaviour in either case, shouldn't >> be as important here but there is always a chance to break userspace > > It doesn't alter userspace behavior, if the registration works like I > described with being able to assign a new one while the old one is being > torn down. > > Or do you mean wrt inflight IO? I don't think the risk is very high > there, to be honest. Right, if somebody tries such a trick it'll be pretty confusing to get randomly firing eventfd, though it's rather a marginal case. -- Pavel Begunkov