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 34DF5C433EF for ; Thu, 3 Feb 2022 21:52:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355516AbiBCVwQ (ORCPT ); Thu, 3 Feb 2022 16:52:16 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49094 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355311AbiBCVwP (ORCPT ); Thu, 3 Feb 2022 16:52:15 -0500 Received: from mail-wm1-x32d.google.com (mail-wm1-x32d.google.com [IPv6:2a00:1450:4864:20::32d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 169BBC061714; Thu, 3 Feb 2022 13:52:15 -0800 (PST) Received: by mail-wm1-x32d.google.com with SMTP id o1-20020a1c4d01000000b0034d95625e1fso8128950wmh.4; Thu, 03 Feb 2022 13:52:15 -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=buRa9dy1somA+KRSylXRCFC8cwfGjuaHgmtJ/vC9N5I=; b=AJNb52/UXg2h/IDNv/EvUdWeUh6pcQr6BUVY2EpdPy9u87X12BXD2JXN0P4WCGQtPN vvV6SVqmjQz7RJxChS7bwJl65p5kVU69BBes0C+l84m1I/++PUxcpPCV5bHd5FZu7WdC C4x4i1MfPCF44QyGdPdxgKDeKrQ5l0+QTxg85Pfpmepaa/fCpHb+yL5XlVYqCNKuOUTf OGCTZCnAASFiJ2kc/PRnajASxWLpiOgZtZebOmwzPxLzHs3WarP/uPY3XB6EketqJiYv kXe/cyB6tqWk0LqgtIuVIalJUlhrMiKvohA/evJaxHjrSdXNuH0c8gbu8efqAZw+47SD mUlQ== 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=buRa9dy1somA+KRSylXRCFC8cwfGjuaHgmtJ/vC9N5I=; b=etszp/6QbEpBplZhHZbf3LtK3CoxjI9WOBt4igzoD+uyGuTvh0l1aoc6eigRqZQGWm 5MIPnSYZS3GySmciHClDyeYkxwM75iDDUf5k8JpIcNDARlT9mcPH6SMWY7BZRchnHd6h +d+hkQQ05vhLrZVmgOv9oZYGT72NLNOEuBkfXCkClUNk0K3FGb2K1W7e3dELQ/6CRB/a 4qLV0JePbU/PTlUA1WwFZQ5RmK7T9jQBbhihNsRJUH0ZFiUAZHF1mnfP+xSfO3ny3gnI dr9JF5WwlsDBj/0oK1aRtx6KkITQOgzDsX/pntSRwuAoJ0IAbbrTeng7Yv+cmM0ksJ+T Ey0A== X-Gm-Message-State: AOAM532irNy0iiz0iPvpP5f7UQfwt8GSkzjD0ZBU8zz94MO+AvT+zbUF PZLLI+uvvq9WYkBU2Q/OCwsBO1KjfTU= X-Google-Smtp-Source: ABdhPJzPWwnqVZ6nC+SkSCc23HRV11PNduKuSsjkFgoYPxIiyQtPgz7ZqNcgsVUX7Sh0vdGTTA/TPw== X-Received: by 2002:a05:600c:3593:: with SMTP id p19mr11818890wmq.172.1643925133525; Thu, 03 Feb 2022 13:52:13 -0800 (PST) Received: from [192.168.8.198] ([85.255.232.204]) by smtp.gmail.com with ESMTPSA id u14sm20350wrs.55.2022.02.03.13.52.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 03 Feb 2022 13:52:13 -0800 (PST) Message-ID: Date: Thu, 3 Feb 2022 21:47:21 +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: Usama Arif , Jens Axboe , 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> From: Pavel Begunkov In-Reply-To: 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 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. 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. fwiw, it alters userpace visible behaviour in either case, shouldn't be as important here but there is always a chance to break userspace -- Pavel Begunkov