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=-7.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, 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 CF3D2C433FE for ; Fri, 10 Sep 2021 15:04:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B0AFE60F92 for ; Fri, 10 Sep 2021 15:04:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234157AbhIJPFk (ORCPT ); Fri, 10 Sep 2021 11:05:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36952 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233907AbhIJPFj (ORCPT ); Fri, 10 Sep 2021 11:05:39 -0400 Received: from mail-oi1-x22e.google.com (mail-oi1-x22e.google.com [IPv6:2607:f8b0:4864:20::22e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B0D97C061756 for ; Fri, 10 Sep 2021 08:04:28 -0700 (PDT) Received: by mail-oi1-x22e.google.com with SMTP id w144so3252286oie.13 for ; Fri, 10 Sep 2021 08:04:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:from:to:cc:references:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=1pwlVxDkfCa0su9+pm6vroWkQFMoA+AryU161sjcxfk=; b=Upka5ZzTcAbnaXhLgktHI/hEk3sA+dwVQL8Kt+LeTHdDwSOL5teaXo51VLxQLrhTY0 zHaWBnbao+/ZYQx7iQ8KNffO8Bwbw3ZrHKOfKEOWTxxgq+zd9iZ0xBxL2zkqLVuGUR6C 3I1+nnsaV1qxKKvfek19l7XxI5oG5T1gDXUHzxFkcqwMbgCJD1MazII9eIFRVbqjJsXO IKiOtRNl9mjhkvh46uw34cfWYOwMduNhvPj40XmfrZaa2b96CUkLr8iDIYwlLZmWoFtc ooHZx5cKluVeam5DJudtqN6O9eSdqoCM3BIAn/oNpmSD+wPhZDmiiJ+yRcmjPefk7x/9 n5MA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:subject:from:to:cc:references:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=1pwlVxDkfCa0su9+pm6vroWkQFMoA+AryU161sjcxfk=; b=b4ydDre0AlCfXa4lzkefpA+uvCaov4QQnP5NSmtv49hYP+QDbhInm2d6QZGz7keYDV LTNhF7EhzuKkIbYxXO98xztOf3Yijh9WrYQOoGXShiE2SguajHhLC+lrzglmSpZF1yFe HCPSso7tt8vrtqS0yPee+U5q79l/CEpPNR45/u/wLsuIbBFms4MxLZLPQnKKFqQgATol bPCqe2NAi4l7tEVvGCSDAF1Hwso+5AEN7+Ey0U5Sw/mlqDigPu8t3jR0T0J1d7coYaOv 69p52lB0DjcxRRh9twt4qo3Oshc6dqWCt+W10Ib2BzSbX5NrFNCUOKig7AsPL4DOS6c+ WOiQ== X-Gm-Message-State: AOAM532tznbEeFyT9ynNV/Grer30osgjB95Z+v2eZExBVF38XWAqf1D9 0L1jHcWZwzmbHL3il+Dk/dd1MWD0S/DyW8PXJZY= X-Google-Smtp-Source: ABdhPJxSGAPtW3Oy2T5TFXP6JxfWfln/a1Zc6o7Sj8HhmrsyQ7Zv1HwU0sge2r1VlGeYw6LAnrGExA== X-Received: by 2002:aca:d02:: with SMTP id 2mr4489656oin.126.1631286267793; Fri, 10 Sep 2021 08:04:27 -0700 (PDT) Received: from [192.168.1.30] ([207.135.234.126]) by smtp.gmail.com with ESMTPSA id 99sm1276993otd.2.2021.09.10.08.04.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Sep 2021 08:04:27 -0700 (PDT) Subject: Re: [git pull] iov_iter fixes From: Jens Axboe To: Al Viro Cc: Linus Torvalds , Pavel Begunkov , Linux Kernel Mailing List , linux-fsdevel References: <5971af96-78b7-8304-3e25-00dc2da3c538@kernel.dk> <9ae5f07f-f4c5-69eb-bcb1-8bcbc15cbd09@kernel.dk> <9855f69b-e67e-f7d9-88b8-8941666ab02f@kernel.dk> Message-ID: <4b26d8cd-c3fa-8536-a295-850ecf052ecd@kernel.dk> Date: Fri, 10 Sep 2021 09:04:26 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <9855f69b-e67e-f7d9-88b8-8941666ab02f@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 9/10/21 7:57 AM, Jens Axboe wrote: > On 9/9/21 9:36 PM, Al Viro wrote: >> On Thu, Sep 09, 2021 at 09:30:03PM -0600, Jens Axboe wrote: >> >>>> Again, we should never, ever modify the iovec (or bvec, etc.) array in >>>> ->read_iter()/->write_iter()/->sendmsg()/etc. instances. If you see >>>> such behaviour anywhere, report it immediately. Any such is a blatant >>>> bug. >>> >>> Yes that was wrong, the iovec is obviously const. But that really >>> doesn't change the original point, which was that copying the iov_iter >>> itself unconditionally would be miserable. >> >> Might very well be true, but... won't your patch hit the reimport on >> every short read? And the cost of uaccess in there is *much* higher >> than copying of 48 bytes into local variable... >> >> Or am I misreading your patch? Note that short reads on reaching >> EOF are obviously normal - it's not a rare case at all. > > It was just a quick hack, might very well be too eager to go through > those motions. But pondering this instead of sleeping, we don't need to > copy all of iov_iter in order to restore the state, and we can use the > same advance after restoring. So something like this may be more > palatable. Caveat - again untested, and I haven't tested the performance > impact of this at all. Passes basic testing for me. I added a sysctl switch for this so I can compare performance, running my usual peak-perf-single-core benchmark. That one does ~3.5M IOPS, using polled IO. There's always a slight variability between boots and builds, hence the sysctl so I could toggle this behavior on the fly. Did a few runs, and the differences are very stable. With this enabled, we spend about 0.15% more time in io_read(). That's only worth about 5K IOPS at 3.5M, not enough to notice as the variation for the 1 second reporting window usually swings more than that: Old behavior: IOPS=3536512, IOS/call=32/31, inflight=(75) IOPS=3541888, IOS/call=32/32, inflight=(64) IOPS=3529056, IOS/call=32/31, inflight=(119) IOPS=3521184, IOS/call=32/32, inflight=(96) IOPS=3527456, IOS/call=32/31, inflight=(128) IOPS=3525504, IOS/call=32/32, inflight=(128) IOPS=3524288, IOS/call=32/32, inflight=(128) IOPS=3536192, IOS/call=32/32, inflight=(96) IOPS=3535840, IOS/call=32/32, inflight=(96) IOPS=3533728, IOS/call=32/31, inflight=(128) IOPS=3528384, IOS/call=32/32, inflight=(128) IOPS=3518400, IOS/call=32/32, inflight=(64) Turning it on: IOPS=3533824, IOS/call=32/31, inflight=(64) IOPS=3541408, IOS/call=32/32, inflight=(32) IOPS=3533024, IOS/call=32/31, inflight=(64) IOPS=3528672, IOS/call=32/32, inflight=(35) IOPS=3522272, IOS/call=32/31, inflight=(107) IOPS=3517632, IOS/call=32/32, inflight=(57) IOPS=3516000, IOS/call=32/31, inflight=(96) IOPS=3513568, IOS/call=32/32, inflight=(34) IOPS=3525600, IOS/call=32/31, inflight=(96) IOPS=3527136, IOS/call=32/31, inflight=(101) I think that's tolerable, it was never going to be absolutely free. What do you think of this approach? Parts of iov_iter are going to remain constant, like iter_type and data_source. io_uring already copies iter->count, so that just leaves restoring iov (and unionized friends), nr_segs union, and iov_offset; We could pretty this up and have the state part be explicit in iov_iter, and have the store/restore parts end up in uio.h. That'd tie them closer together, though I don't expect iov_iter changes to be an issue. It would make it more maintainable, though. I'll try and hack up this generic solution, see if that looks any better. -- Jens Axboe