Date:   Tue, 3 May 2022 15:14:02 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Daniel Harding <dharding@...ing180.net>,
        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 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.

-- 
Pavel Begunkov

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