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=-9.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 30FCEC2BBCA for ; Tue, 15 Dec 2020 14:00:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EEC6A224D2 for ; Tue, 15 Dec 2020 14:00:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729134AbgLOOAh (ORCPT ); Tue, 15 Dec 2020 09:00:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37820 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727200AbgLOOA1 (ORCPT ); Tue, 15 Dec 2020 09:00:27 -0500 Received: from mail-wm1-x341.google.com (mail-wm1-x341.google.com [IPv6:2a00:1450:4864:20::341]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2D33FC0617A6; Tue, 15 Dec 2020 05:59:47 -0800 (PST) Received: by mail-wm1-x341.google.com with SMTP id e25so18588491wme.0; Tue, 15 Dec 2020 05:59:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:autocrypt:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=xzFQ3fAAmyTrUxg6AtOBJwbOPrlweQwSN+UmczMjICw=; b=enJXGu6bN1ze2eKV1slYMB62l6gxp4DbOYF2ruxOTHRHRY+0PXa5OJLd5bPstfCZSA zyLBauA9dq7SgXfTH/wBwv/1HvkzxVx7bVmirWkQoZ+Fdq5u/dL3Kvy7Go09T5sbtWVV 58MZy3owhyRxJ89VRTjachdprvecnW+w8zSsC7vphjEY/Pvw1Ff8mvUDfjvvquqSbNlF dVgAg3WCUdM5IJ9zC82N1T89U4MKuwlh5rc06WgJI7hcCU6c7y3S5X40k+a6qtH0M0iG wP3KlhkXivgMMi7MZXY+3Cvu/Q+0+aZexEr7AqZi7tl2i4HJ/AkmL7UgQ7Fjodh39mqY kzkQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:autocrypt :message-id:date:user-agent:mime-version:in-reply-to :content-language:content-transfer-encoding; bh=xzFQ3fAAmyTrUxg6AtOBJwbOPrlweQwSN+UmczMjICw=; b=W9fUXsfU07qRbLYcFyIJ1KMrLQ3trH10IKdGLYe/HxV0b22YVoYZI5Ovga4zn3i9ly buqWc1d8FxRRdzcf2qSHbMrBgI+C5EKXMFQ/uqJvHQiNEnQZR1C0CXanVY0sEBejuQeY pXaVcgNgAilT136YD0OFGyV1/cOe6hfJ6ut2DwTYQyWY4NItp9GjepP8k6DHZYpSX2CA XgTTtMkmUQvCNogedVcMUycEHbcYc1gSLWceZ1ehFvWaT/mZ2EZ/1s52yfQ9IHtZZ4Ub XalNuy21R6Ax+M1WDYraZMFgz0n6Ma2JtWnDcCW1KQOH6Nm18OlX/mvIMxRETgJK83r9 qYRw== X-Gm-Message-State: AOAM533tR9LJNzXEbsimRDgaFVpiVqWd73tZiDwa550R+bzfDD+Zp77G VeqjgzS9aICk9oA6oUYEikLUe/0KjWPrFajG X-Google-Smtp-Source: ABdhPJw09wd7odQNp6dejR+hFMlJ9y4tAGzkhuSzXXzbmuLWuxdwCJaNqV8exIq1nWVIqPRrGFsJFQ== X-Received: by 2002:a1c:e084:: with SMTP id x126mr32733413wmg.109.1608040785708; Tue, 15 Dec 2020 05:59:45 -0800 (PST) Received: from [192.168.8.128] ([185.69.144.228]) by smtp.gmail.com with ESMTPSA id a12sm11925845wrh.71.2020.12.15.05.59.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Dec 2020 05:59:44 -0800 (PST) Subject: Re: [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance() To: David Laight , "linux-block@vger.kernel.org" Cc: Jens Axboe , Christoph Hellwig , Matthew Wilcox , Ming Lei , Johannes Weiner , Alexander Viro , "Darrick J . Wong" , "Martin K . Petersen" , Jonathan Corbet , "linux-xfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "io-uring@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "target-devel@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-doc@vger.kernel.org" References: <5c9c22dbeecad883ca29b31896c262a8d2a77132.1607976425.git.asml.silence@gmail.com> <262132648a8f4e7a9d1c79003ea74b3f@AcuMS.aculab.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Message-ID: <2d45795e-c077-2ea0-c38d-f9a4736bccd7@gmail.com> Date: Tue, 15 Dec 2020 13:56:25 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 15/12/2020 13:54, David Laight wrote: > From: Pavel Begunkov >> Sent: 15 December 2020 11:24 >> >> On 15/12/2020 09:37, David Laight wrote: >>> From: Pavel Begunkov >>>> Sent: 15 December 2020 00:20 >>>> >>>> iov_iter_advance() is heavily used, but implemented through generic >>>> iteration. As bvecs have a specifically crafted advance() function, i.e. >>>> bvec_iter_advance(), which is faster and slimmer, use it instead. >>>> >>>> lib/iov_iter.c | 19 +++++++++++++++++++ >> [...] >>>> void iov_iter_advance(struct iov_iter *i, size_t size) >>>> { >>>> if (unlikely(iov_iter_is_pipe(i))) { >>>> @@ -1077,6 +1092,10 @@ void iov_iter_advance(struct iov_iter *i, size_t size) >>>> i->count -= size; >>>> return; >>>> } >>>> + if (iov_iter_is_bvec(i)) { >>>> + iov_iter_bvec_advance(i, size); >>>> + return; >>>> + } >>>> iterate_and_advance(i, size, v, 0, 0, 0) >>>> } >>> >>> This seems to add yet another comparison before what is probably >>> the common case on an IOVEC (ie normal userspace buffer). >> >> If Al finally takes the patch for iov_iter_is_*() helpers it would >> be completely optimised out. > > I knew I didn't have that path - the sources I looked at aren't that new. > Didn't know its state. > > In any case that just stops the same test being done twice. > In still changes the order of the tests. > > The three 'unlikely' cases should really be inside a single > 'unlikely' test for all three bits. > Then there is only one mis-predictable jump prior to the usual path. > > By adding the test before iterate_and_advance() you are (effectively) > optimising for the bvec (and discard) cases. Take a closer look, bvec check is already first in iterate_and_advance(). Anyway, that all is an unrelated story. > Adding 'unlikely()' won't make any difference on some architectures. > IIRC recent intel x86 don't have a 'static prediction' for unknown > branches - they just use whatever in is the branch predictor tables. -- Pavel Begunkov