On 5/16/22 16:57, Daniel Harding wrote: > 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. Yes, it does appear that was the problem.  The attach POC patch against LXC fixes the hang.  The kernel is working as intended. #regzbot invalid:  userspace programming error -- Regards, Daniel Harding