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

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/16/22 13:12, Pavel Begunkov wrote:
> 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.

How it was with oneshots:

1: kernel: poll fires, add a CQE
2: kernel: remove poll
3: userspace: get CQE
4: userspace: read(terminal_fd);
5: userspace: add new poll
6: goto 1)

What might happen and actually happens with multishot:

1: kernel: poll fires, add CQE1
2: kernel: poll fires again, add CQE2
3: userspace: get CQE1
4: userspace: read(terminal_fd); // reads all data, for both CQE1 and CQE2
5: userspace: get CQE2
6: userspace: read(terminal_fd); // nothing to read, hangs here

It should be the read in lxc_terminal_ptx_io().

IMHO, it's not a regression but a not perfect feature API and/or
an API misuse.

Cc: Christian Brauner

Christian, in case you may have some input on the LXC side of things.
Daniel reported an LXC problem when it uses io_uring multishot poll requests.
Before aa43477b04025 ("io_uring: poll rework"), multishot poll requests for
tty/pty and some other files were always downgraded to oneshots, which had
been fixed by the commit and exposed the problem. I hope the example above
explains it, but please let me know if it needs more details

-- 
Pavel Begunkov