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=-7.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 93003C43603 for ; Wed, 11 Dec 2019 17:00:48 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id DD6EB214AF for ; Wed, 11 Dec 2019 17:00:47 +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="gTkHIIYI" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DD6EB214AF Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.dk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-17497-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 7342 invoked by uid 550); 11 Dec 2019 17:00:41 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 7299 invoked from network); 11 Dec 2019 17:00:40 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=aiOSRjp/3HRFxheJM9EeY6VMMDKekJfhGlBwlo3tspE=; b=gTkHIIYImCIIs+Sd4NnGSmtGhaKLxnlMBjccL6/IgXDrB/wB8uNsrg/i0JLEagoSsu cIoGQ0ZsbqQ5t3O+l9ZEb0/bXZJQS34p6qvYUWvL2dlxRiCs416JTB+llaKJBlgBeXiA Cwep7MyFsmJrTu0hJC3akH4s7NBjF0VBn5eaAl0jAbNgB1JNITonTVM4FLHd6FBMrix0 xZ6ayP0NPjoxzsbN1EGpemPOc8KcAMBplju+2oMFyELw4lqOxOeEOENgksXwR9I2IZNT L8WWilRzBmUJuXzZ+zT6x7w3Q4QPvpxei83DLlsL1vbBeVW7fyCwm+p1OM79PBQ1NBii MdAg== 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:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=aiOSRjp/3HRFxheJM9EeY6VMMDKekJfhGlBwlo3tspE=; b=i8HM7T3HmGJ2hkXs9OQmyDTyDUjLdfMGdImkJa2b0isyUeo/szvtXv3F+SDxaBmOo0 YY+xIq68ho2aiaD9FafTYZ0N/u9kMPHHqvJaCbWloOnf2195fVb5HduMnlWqBpsoMRX9 rzUWxqfKqu1infO0hyJaqLNUQWqY7FaQaos74reGzhv0/TJGOpZdW1U59xfJTT8aOYny ufcdpeyoDPGobgd/S4EKRJYS/4wCuUYMw6KjH0YwRKYMGYNORB0+35D0AidQiST5cwfo MHyhz2y44z8YxhZwTsraWTfuRmFBBip/XgQ0nm0cLWLD6tRQYV7H0qQzjI+3mlazWnMe IBEQ== X-Gm-Message-State: APjAAAXQEzHGD1Fn8x9DegjH96lYWeRXy3LsVfHn/YcZapMlDFxJUs20 4ruZBm7Cxyf/ULNEJvzujDjyD9RzMis= X-Google-Smtp-Source: APXvYqx2pjygH4+O3cdwmFd5louaeyGg/v/caErz92Souf53k6sOCvu3wtWU3GP7dpW0kGXbcmRmIA== X-Received: by 2002:a63:d351:: with SMTP id u17mr5144691pgi.84.1576083627779; Wed, 11 Dec 2019 09:00:27 -0800 (PST) Subject: Re: [PATCH 07/11] io_uring: use atomic_t for refcounts To: Kees Cook , Will Deacon Cc: Jann Horn , io-uring , Kernel Hardening References: <20191210155742.5844-1-axboe@kernel.dk> <20191210155742.5844-8-axboe@kernel.dk> <02ba41a9-14f2-e3be-f43f-99f311c662ef@kernel.dk> <201912101445.CF208B717@keescook> <20191211102012.GA4123@willie-the-truck> <201912110851.88536F3F@keescook> From: Jens Axboe Message-ID: <82a67aff-08a7-48f4-2cd1-17ea465c9123@kernel.dk> Date: Wed, 11 Dec 2019 10:00:25 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <201912110851.88536F3F@keescook> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 12/11/19 9:56 AM, Kees Cook wrote: > On Wed, Dec 11, 2019 at 10:20:13AM +0000, Will Deacon wrote: >> On Tue, Dec 10, 2019 at 03:55:05PM -0700, Jens Axboe wrote: >>> On 12/10/19 3:46 PM, Kees Cook wrote: >>>> On Tue, Dec 10, 2019 at 03:21:04PM -0700, Jens Axboe wrote: >>>>> On 12/10/19 3:04 PM, Jann Horn wrote: >>>>>> [context preserved for additional CCs] >>>>>> >>>>>> On Tue, Dec 10, 2019 at 4:57 PM Jens Axboe wrote: >>>>>>> Recently had a regression that turned out to be because >>>>>>> CONFIG_REFCOUNT_FULL was set. >>>>>> >>>>>> I assume "regression" here refers to a performance regression? Do you >>>>>> have more concrete numbers on this? Is one of the refcounting calls >>>>>> particularly problematic compared to the others? >>>>> >>>>> Yes, a performance regression. io_uring is using io-wq now, which does >>>>> an extra get/put on the work item to make it safe against async cancel. >>>>> That get/put translates into a refcount_inc and refcount_dec per work >>>>> item, and meant that we went from 0.5% refcount CPU in the test case to >>>>> 1.5%. That's a pretty substantial increase. >>>>> >>>>>> I really don't like it when raw atomic_t is used for refcounting >>>>>> purposes - not only because that gets rid of the overflow checks, but >>>>>> also because it is less clear semantically. >>>>> >>>>> Not a huge fan either, but... It's hard to give up 1% of extra CPU. You >>>>> could argue I could just turn off REFCOUNT_FULL, and I could. Maybe >>>>> that's what I should do. But I'd prefer to just drop the refcount on the >>>>> io_uring side and keep it on for other potential useful cases. >>>> >>>> There is no CONFIG_REFCOUNT_FULL any more. Will Deacon's version came >>>> out as nearly identical to the x86 asm version. Can you share the >>>> workload where you saw this? We really don't want to regression refcount >>>> protections, especially in the face of new APIs. >>>> >>>> Will, do you have a moment to dig into this? >>> >>> Ah, hopefully it'll work out ok, then. The patch came from testing the >>> full backport on 5.2. > > Oh good! I thought we had some kind of impossible workload. :) > >>> Do you have a link to the "nearly identical"? I can backport that >>> patch and try on 5.2. >> >> You could try my refcount/full branch, which is what ended up getting merged >> during the merge window: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=refcount/full > > Yeah, as you can see in the measured tight-loop timings in > https://git.kernel.org/linus/dcb786493f3e48da3272b710028d42ec608cfda1 > there was 0.1% difference for Will's series compared to the x86 assembly > version, where as the old FULL was almost 70%. That looks very promising! Hopefully the patch is moot at that point, I dropped it from the series yesterday in any case. I'll revisit as soon as I can and holler if there's an issue. -- Jens Axboe