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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2269C3F2C6 for ; Tue, 3 Mar 2020 16:04:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C4B8A20CC7 for ; Tue, 3 Mar 2020 16:04:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=kernel-dk.20150623.gappssmtp.com header.i=@kernel-dk.20150623.gappssmtp.com header.b="DTLtzuz7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729898AbgCCQES (ORCPT ); Tue, 3 Mar 2020 11:04:18 -0500 Received: from mail-il1-f196.google.com ([209.85.166.196]:43223 "EHLO mail-il1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729543AbgCCQES (ORCPT ); Tue, 3 Mar 2020 11:04:18 -0500 Received: by mail-il1-f196.google.com with SMTP id o18so3189020ilg.10 for ; Tue, 03 Mar 2020 08:04:17 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:content-language:content-transfer-encoding; bh=XNptDHf3B9MrspnBaDkgfKgyPkZSg0/a7ZTbis3qJW4=; b=DTLtzuz7Dp0xKxV+FonbxARhkpLzpHhck+yNDuY7gHlYzctkaPgZqQJP/udCFnPwb4 sHkly8itVnmxHs52SD803yQKTlfO4VqBM0hq42zIEhz6mL8ELRE/UsVSHsp/5HdgK9tx xUuAUdTvNIpafnczzNKn0yOEY8cs7NUc0orIgDf9XUSr0MhJA2NQGgIVkgb3h8JtR+F1 uSKNe3KfTroRIHwmuU1xm2ABSfemzBy2nZQl+ysZc6RQkJ9n6iIImNpO8WIe2CEZLdGa HTtdvp6Uu5pbtdEOtRHRKJq/PJXLVRnqBRZbyvse3nPc8ltRUYsurqjDJTUsAk1ay39z f3GA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=XNptDHf3B9MrspnBaDkgfKgyPkZSg0/a7ZTbis3qJW4=; b=ghhr3Lk5AbOo1jxD8bh4fnd75TWV6kezQhsRhGqxYAlh0jVKukMicT6U6GkpNw9rIc w5jbpC12a1iS56ZfFbSEK4SdCje9fdv5RtXVNG9FWHqFfTo5RgQJTrfqlMu1ci62u/Sd I3aYCiqwlk4Z70hl/Cryojvl1mBG4BvOW2JE1fVd0dzsicq3LVbm2D4xjOLjYo9dYgga VL5Q/Hstfu5Frb0ctpGb4EghcmjnlkMyzpIST16JC2RcimVQXafH9SO7PpxmHJz3AMVR JSKQeoTOnEez2ldR1I1mfevIbLlLZBRlDd8evvDg2RcriTwXNS5kbPJLg/+cx6G1nz5G 8nPQ== X-Gm-Message-State: ANhLgQ1M5Xy2wyiHD7laPHgCHEGsZ22BXB4OK9SJsc0JTF/6QE18KeTW m2T821vFraa/KYV5GiC/hq8Klw== X-Google-Smtp-Source: ADFU+vu6PipftNRJq0p/JIHkf9QKrsEkxQTEizleEIbWFUvIukFuGzs0v5q/jcYySuktXSmLO0DySg== X-Received: by 2002:a92:9c52:: with SMTP id h79mr5150905ili.213.1583251457276; Tue, 03 Mar 2020 08:04:17 -0800 (PST) Received: from [192.168.1.159] ([65.144.74.34]) by smtp.gmail.com with ESMTPSA id c12sm7991082ile.12.2020.03.03.08.04.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Mar 2020 08:04:16 -0800 (PST) Subject: Re: [PATCH v2 4/4] io_uring: get next req on subm ref drop To: Pavel Begunkov , io-uring@vger.kernel.org, linux-kernel@vger.kernel.org References: <444aef98f849d947d7f10e88f30244fa0bc82360.1583181841.git.asml.silence@gmail.com> <3ab75953-ee39-2c4e-99e2-f8c18ceb6a8d@kernel.dk> <52b282f5-50f3-2ee6-a055-6ef0c2c39e93@gmail.com> <9ead66eb-cb5d-2dab-1a78-02466958674a@gmail.com> From: Jens Axboe Message-ID: <153662f4-0ab9-8dac-1577-0df1ce35f320@kernel.dk> Date: Tue, 3 Mar 2020 09:04:16 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <9ead66eb-cb5d-2dab-1a78-02466958674a@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 3/3/20 3:46 AM, Pavel Begunkov wrote: > On 3/3/2020 9:54 AM, Pavel Begunkov wrote: >> On 03/03/2020 07:26, Jens Axboe wrote: >>> On 3/2/20 1:45 PM, Pavel Begunkov wrote: >>>> Get next request when dropping the submission reference. However, if >>>> there is an asynchronous counterpart (i.e. read/write, timeout, etc), >>>> that would be dangerous to do, so ignore them using new >>>> REQ_F_DONT_STEAL_NEXT flag. >>> >>> Hmm, not so sure I like this one. It's not quite clear to me where we >>> need REQ_F_DONT_STEAL_NEXT. If we have an async component, then we set >>> REQ_F_DONT_STEAL_NEXT. So this is generally the case where our >>> io_put_req() for submit is not the last drop. And for the other case, >>> the put is generally in the caller anyway. So I don't really see what >>> this extra flag buys us? >> >> Because io_put_work() holds a reference, no async handler can achive req->refs >> == 0, so it won't return next upon dropping the submission ref (i.e. by >> put_find_nxt()). And I want to have next before io_put_work(), to, instead of as >> currently: >> >> run_work(work); >> assign_cur_work(NULL); // spinlock + unlock worker->lock >> new_work = put_work(work); >> assign_cur_work(new_work); // the second time >> >> do: >> >> new_work = run_work(work); >> assign_cur_work(new_work); // need new_work here >> put_work(work); >> >> >> The other way: >> >> io_wq_submit_work() // for all async handlers >> { >> ... >> // Drop submission reference. >> // One extra ref will be put in io_put_work() right >> // after return, and it'll be done in the same thread >> if (atomic_dec_and_get(req) == 1) >> steal_next(req); >> } >> >> Maybe cleaner, but looks fragile as well. Would you prefer it? > > Any chance you've measured your next-work fix? I wonder how much does it > hurt performance, and whether we need a terse patch for 5.6. Unless I'm missing something, the worker will pick up the next work without sleeping, since the request will have finished. So it really should not add any extra overhead, except you'll do an extra wqe lock roundtrip. But I'll run some testing to be totally sure. -- Jens Axboe