Date:   Wed, 4 May 2022 09:54:10 +0300
From:   Daniel Harding <dharding@...ing180.net>
To:     Pavel Begunkov <asml.silence@...il.com>,
        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 5/3/22 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

-- 
Regards,

Daniel Harding

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