Messages in this thread | | | Date | Tue, 3 May 2022 15:14:02 +0100 | Subject | Re: [REGRESSION] lxc-stop hang on 5.17.x kernels | From | Pavel Begunkov <> |
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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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