Message-ID: <41c86189-0d1f-60f0-ca8e-f80b3ccf5130@gmail.com>
Date:   Mon, 16 May 2022 13:12:00 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Daniel Harding <dharding@...ing180.net>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Jens Axboe <axboe@...nel.dk>
Cc:     regressions@...ts.linux.dev, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels

On 5/15/22 19:34, Daniel Harding wrote:
> 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.

Haven't had time to debug it. Apparently LXC is stuck on
read(2) terminal fd. Not yet clear what is the reason.

-- 
Pavel Begunkov

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