linux-kernel - Re: [REGRESSION] lxc-stop hang on 5.17.x kernels

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Date:   Tue, 3 May 2022 10:37:41 +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

[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.

-- 
Regards,

Daniel Harding

View attachment "lxc-trace-good" of type "text/plain" (19774 bytes)

View attachment "lxc-trace-bad" of type "text/plain" (23533 bytes)

View attachment "lxc-record-trace" of type "text/plain" (372 bytes)