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Date:   Mon, 16 May 2022 16:57:05 +0300
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Subject: Re: [REGRESSION] lxc-stop hang on 5.17.x kernels
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To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     regressions@lists.linux.dev, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Thorsten Leemhuis <regressions@leemhuis.info>,
        Jens Axboe <axboe@kernel.dk>,
        Christian Brauner <christian.brauner@ubuntu.com>
References: <7925e262-e0d4-6791-e43b-d37e9d693414@living180.net>
 <6ad38ecc-b2a9-f0e9-f7c7-f312a2763f97@kernel.dk>
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 <da56fa5f-0624-413e-74a1-545993940d27@gmail.com>
From:   Daniel Harding <dharding@living180.net>
In-Reply-To: <da56fa5f-0624-413e-74a1-545993940d27@gmail.com>
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On 5/16/22 16:25, Pavel Begunkov wrote:
> 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, I had actually just started a draft email with the same theory 
(although you stated it much more clearly than I could have).  I'm 
working on debugging the LXC side, but I'm pretty sure the issue is due 
to LXC using blocking reads and getting stuck exactly as you describe.  
If I can confirm this, I'll go ahead and mark this regression as invalid 
and file an issue with LXC.  Thanks for your help and patience.

-- 
Regards,

Daniel Harding