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 4D1EEC433F5 for ; Sun, 15 May 2022 18:34:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238138AbiEOSer (ORCPT ); Sun, 15 May 2022 14:34:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48602 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238123AbiEOSeo (ORCPT ); Sun, 15 May 2022 14:34:44 -0400 Received: from wout5-smtp.messagingengine.com (wout5-smtp.messagingengine.com [64.147.123.21]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3294CDED2; Sun, 15 May 2022 11:34:43 -0700 (PDT) Received: from compute3.internal (compute3.nyi.internal [10.202.2.43]) by mailout.west.internal (Postfix) with ESMTP id EBFF93200972; Sun, 15 May 2022 14:34:39 -0400 (EDT) Received: from mailfrontend1 ([10.202.2.162]) by compute3.internal (MEProxy); Sun, 15 May 2022 14:34:40 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=living180.net; h=cc:cc:content-transfer-encoding:content-type:date:date:from :from:in-reply-to:in-reply-to:message-id:mime-version:references :reply-to:sender:subject:subject:to:to; s=fm2; t=1652639679; x= 1652726079; bh=1tqwKGxvROapik2SZuXrpiyKuwJSO346wMqf0Svvi3s=; b=V 7RDZwhnjNHoUl+SnGUB32icJcjYPZerNpiRLx0H0J4qKV5OKbJqag1s4EIRoZxHV CfMVNMNwUBnm97fepkMdA0HrJpjZO7b+963bC3IHDeXsfqPd9xIcGPz1YjqOYGJ/ iV1i41LwsH17UiEnaF/NJC+WZdwkWNt7vvJrFPYY5A80iaGvkBohVwza6QKqwCWc 1ZUvQabiLMvx7Vmbnyqg/HdlX62VL6WllbBQBhYPy+57Jx0iimnV8vzpBWorflFK G2OUbsJA5ZM2qCdAVjcDGIj6eaYFZlBwnYzj9+gTTIQuGAyACO25YeGiM1QpCp3l B3C8MkyhnjPiWVTowhLeA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=cc:cc:content-transfer-encoding :content-type:date:date:from:from:in-reply-to:in-reply-to :message-id:mime-version:references:reply-to:sender:subject :subject:to:to:x-me-proxy:x-me-proxy:x-me-sender:x-me-sender :x-sasl-enc; s=fm1; t=1652639679; x=1652726079; bh=1tqwKGxvROapi k2SZuXrpiyKuwJSO346wMqf0Svvi3s=; b=n3ic4cjP6oMiDgJdmmVbSY8k/m0kA 4YNNyMu1sJW0h05D3IRB1pFtBTP1rQ6apKZipklWVkjAQGopz1iU9ilUFIWvs6fr YW3VmxDqMbllzJn8VpyiiskW3eBwLCE2Fkni3p5UK4fmYbFzWWM0V+iTgOB+Pd9S K2aJrCN0KxAn3ZwWTBX5opXDqW8oJ3FzUp7+FqLgdhzNB5mPYyoj/CBKBoNVZVh/ z1z8MxoC6ASta1b8kZxv2RZu2yXUdYEwmWj77WDkmSQ91YgHT/fNXHfvEPXAhGBz mtAVMmAmK/c4E/FNSHuh7foEiYbdRQMiT9wAcYfdGwBuO5quyfEG2c1IA== X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedvfedrheefgdduvdeiucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfqfgfvpdfurfetoffkrfgpnffqhgen uceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmne cujfgurhepkfffgggfuffvvehfhfgjtgfgsehtkeertddtfeejnecuhfhrohhmpeffrghn ihgvlhcujfgrrhguihhnghcuoeguhhgrrhguihhngheslhhivhhinhhgudektddrnhgvth eqnecuggftrfgrthhtvghrnhepfeffjeejffegffeuveekkeffiefghefhtefgvdehveei geeuffegfeeileeihfevnecuffhomhgrihhnpehgihhthhhusgdrtghomhenucevlhhush htvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpeguhhgrrhguihhnghes lhhivhhinhhgudektddrnhgvth X-ME-Proxy: Received: by mail.messagingengine.com (Postfix) with ESMTPA; Sun, 15 May 2022 14:34:37 -0400 (EDT) Message-ID: Date: Sun, 15 May 2022 21:34:35 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels Content-Language: en-US To: Thorsten Leemhuis , Pavel Begunkov , Jens Axboe Cc: regressions@lists.linux.dev, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org References: <7925e262-e0d4-6791-e43b-d37e9d693414@living180.net> <6ad38ecc-b2a9-f0e9-f7c7-f312a2763f97@kernel.dk> <371c01dd-258c-e428-7428-ff390b664752@kernel.dk> <2436d42c-85ca-d060-6508-350c769804f1@gmail.com> <12a57dd9-4423-a13d-559b-2b1dd2fb0ef3@living180.net> <897dc597-fc0a-34ec-84b8-7e1c4901e0fc@leemhuis.info> From: Daniel Harding In-Reply-To: <897dc597-fc0a-34ec-84b8-7e1c4901e0fc@leemhuis.info> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/15/22 11:20, Thorsten Leemhuis wrote: > On 04.05.22 08:54, Daniel Harding wrote: >> On 5/3/22 17:14, Pavel Begunkov wrote: >>> On 5/3/22 08:37, Daniel Harding wrote: >>>> [Resend with a smaller trace] >>>> On 5/3/22 02:14, Pavel Begunkov wrote: >>>>> On 5/2/22 19:49, Daniel Harding wrote: >>>>>> On 5/2/22 20:40, Pavel Begunkov wrote: >>>>>>> On 5/2/22 18:00, Jens Axboe wrote: >>>>>>>> On 5/2/22 7:59 AM, Jens Axboe wrote: >>>>>>>>> On 5/2/22 7:36 AM, Daniel Harding wrote: >>>>>>>>>> On 5/2/22 16:26, Jens Axboe wrote: >>>>>>>>>>> On 5/2/22 7:17 AM, Daniel Harding wrote: >>>>>>>>>>>> I use lxc-4.0.12 on Gentoo, built with io-uring support >>>>>>>>>>>> (--enable-liburing), targeting liburing-2.1.  My kernel >>>>>>>>>>>> config is a >>>>>>>>>>>> very lightly modified version of Fedora's generic kernel >>>>>>>>>>>> config. After >>>>>>>>>>>> moving from the 5.16.x series to the 5.17.x kernel series, I >>>>>>>>>>>> started >>>>>>>>>>>> noticed frequent hangs in lxc-stop.  It doesn't happen 100% >>>>>>>>>>>> of the >>>>>>>>>>>> time, but definitely more than 50% of the time. Bisecting >>>>>>>>>>>> narrowed >>>>>>>>>>>> down the issue to commit >>>>>>>>>>>> aa43477b040251f451db0d844073ac00a8ab66ee: >>>>>>>>>>>> io_uring: poll rework. Testing indicates the problem is still >>>>>>>>>>>> present >>>>>>>>>>>> in 5.18-rc5. Unfortunately I do not have the expertise with the >>>>>>>>>>>> codebases of either lxc or io-uring to try to debug the problem >>>>>>>>>>>> further on my own, but I can easily apply patches to any of the >>>>>>>>>>>> involved components (lxc, liburing, kernel) and rebuild for >>>>>>>>>>>> testing or >>>>>>>>>>>> validation.  I am also happy to provide any further >>>>>>>>>>>> information that >>>>>>>>>>>> would be helpful with reproducing or debugging the problem. >>>>>>>>>>> Do you have a recipe to reproduce the hang? That would make it >>>>>>>>>>> significantly easier to figure out. >>>>>>>>>> I can reproduce it with just the following: >>>>>>>>>> >>>>>>>>>>      sudo lxc-create --n lxc-test --template download --bdev >>>>>>>>>> dir --dir /var/lib/lxc/lxc-test/rootfs -- -d ubuntu -r bionic >>>>>>>>>> -a amd64 >>>>>>>>>>      sudo lxc-start -n lxc-test >>>>>>>>>>      sudo lxc-stop -n lxc-test >>>>>>>>>> >>>>>>>>>> The lxc-stop command never exits and the container continues >>>>>>>>>> running. >>>>>>>>>> If that isn't sufficient to reproduce, please let me know. >>>>>>>>> Thanks, that's useful! I'm at a conference this week and hence have >>>>>>>>> limited amount of time to debug, hopefully Pavel has time to >>>>>>>>> take a look >>>>>>>>> at this. >>>>>>>> Didn't manage to reproduce. Can you try, on both the good and bad >>>>>>>> kernel, to do: >>>>>>> Same here, it doesn't reproduce for me >>>>>> OK, sorry it wasn't something simple. >>>>>>> # echo 1 > /sys/kernel/debug/tracing/events/io_uring/enable >>>>>>>> run lxc-stop >>>>>>>> >>>>>>>> # cp /sys/kernel/debug/tracing/trace ~/iou-trace >>>>>>>> >>>>>>>> so we can see what's going on? Looking at the source, lxc is just >>>>>>>> using >>>>>>>> plain POLL_ADD, so I'm guessing it's not getting a notification >>>>>>>> when it >>>>>>>> expects to, or it's POLL_REMOVE not doing its job. If we have a >>>>>>>> trace >>>>>>>> from both a working and broken kernel, that might shed some light >>>>>>>> on it. >>>>>> It's late in my timezone, but I'll try to work on getting those >>>>>> traces tomorrow. >>>>> I think I got it, I've attached a trace. >>>>> >>>>> What's interesting is that it issues a multi shot poll but I don't >>>>> see any kind of cancellation, neither cancel requests nor task/ring >>>>> exit. Perhaps have to go look at lxc to see how it's supposed >>>>> to work >>>> Yes, that looks exactly like my bad trace.  I've attached good trace >>>> (captured with linux-5.16.19) and a bad trace (captured with >>>> linux-5.17.5).  These are the differences I noticed with just a >>>> visual scan: >>>> >>>> * Both traces have three io_uring_submit_sqe calls at the very >>>> beginning, but in the good trace, there are further >>>> io_uring_submit_sqe calls throughout the trace, while in the bad >>>> trace, there are none. >>>> * The good trace uses a mask of c3 for io_uring_task_add much more >>>> often than the bad trace:  the bad trace uses a mask of c3 only for >>>> the very last call to io_uring_task_add, but a mask of 41 for the >>>> other calls. >>>> * In the good trace, many of the io_uring_complete calls have a >>>> result of 195, while in the bad trace, they all have a result of 1. >>>> >>>> I don't know whether any of those things are significant or not, but >>>> that's what jumped out at me. >>>> >>>> I have also attached a copy of the script I used to generate the >>>> traces.  If there is anything further I can to do help debug, please >>>> let me know. >>> Good observations! thanks for traces. >>> >>> It sounds like multi-shot poll requests were getting downgraded >>> to one-shot, which is a valid behaviour and was so because we >>> didn't fully support some cases. If that's the reason, than >>> the userspace/lxc is misusing the ABI. At least, that's the >>> working hypothesis for now, need to check lxc. >> So, I looked at the lxc source code, and it appears to at least try to >> handle the case of multi-shot being downgraded to one-shot.  I don't >> know enough to know if the code is actually correct however: >> >> https://github.com/lxc/lxc/blob/7e37cc96bb94175a8e351025d26cc35dc2d10543/src/lxc/mainloop.c#L165-L189 >> https://github.com/lxc/lxc/blob/7e37cc96bb94175a8e351025d26cc35dc2d10543/src/lxc/mainloop.c#L254 >> https://github.com/lxc/lxc/blob/7e37cc96bb94175a8e351025d26cc35dc2d10543/src/lxc/mainloop.c#L288-L290 > Hi, this is your Linux kernel regression tracker. Nothing happened here > for round about ten days now afaics; or did the discussion continue > somewhere else. > > From what I gathered from this discussion is seems the root cause might > be in LXC, but it was exposed by kernel change. That makes it sill a > kernel regression that should be fixed; or is there a strong reason why > we should let this one slip? No, there hasn't been any discussion since the email you replied to.  I've done a bit more testing on my end, but without anything conclusive.  The one thing I can say is that my testing shows that LXC does correctly handle multi-shot poll requests which were being downgraded to one-shot in 5.16.x kernels, which I think invalidates Pavel's theory.  In 5.17.x kernels, those same poll requests are no longer being downgraded to one-shot requests, and thus under 5.17.x LXC is no longer re-arming those poll requests (but also shouldn't need to, according to what is being returned by the kernel).  I don't know if this change in kernel behavior is related to the hang, or if it is just a side effect of other io-uring changes that made it into 5.17.  Nothing in the LXC's usage of io-uring seems obviously incorrect to me, but I am far from an expert.  I also did some work toward creating a simpler reproducer, without success (I was able to get a simple program using io-uring running, but never could get it to hang).  ISTM that this is still a kernel regression, unless someone can point out a definite fault in the way LXC is using io-uring. -- Regards, Daniel Harding