[Qemu-devel] [Bug 1805256] Re: qemu-img hangs on high core count ARM system

[Rafael David Tinoco]

Hello Liz,

I'll try to reproduce this issue in a Cortex-A53 aarch64 real
environment (w/ 24 HW threads) AND in a virtual environment w/ lots of
vCPUs... but, if it's a barrier missing - or the lack of atomicity
and/or ordering in a primitive - then, I'm afraid the context switch in
between vCPUs might not be the same as in real CPUs (IPIs are sent and
handled differently and host kernel delays IPI delivery because of its
own callbacks, before scheduling, etc...) and I could need a qemu dump
from your environment.

Would that be feasible ? Can you reproduce this nowadays ? This bug has
aged a little, so I'm now sure!

Could you provide me the dump caused by latest package available for
your Ubuntu version ? This way I have the debug symbols to work with.

Meanwhile, I'll be trying to reproduce on my side.

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https://bugs.launchpad.net/bugs/1805256

Title:
  qemu-img hangs on high core count ARM system

Status in QEMU:
  Confirmed
Status in qemu package in Ubuntu:
  In Progress

Bug description:
  On the HiSilicon D06 system - a 96 core NUMA arm64 box - qemu-img
  frequently hangs (~50% of the time) with this command:

  qemu-img convert -f qcow2 -O qcow2 /tmp/cloudimg /tmp/cloudimg2

  Where "cloudimg" is a standard qcow2 Ubuntu cloud image. This
  qcow2->qcow2 conversion happens to be something uvtool does every time
  it fetches images.

  Once hung, attaching gdb gives the following backtrace:

  (gdb) bt
  #0  0x0000ffffae4f8154 in __GI_ppoll (fds=0xaaaae8a67dc0, 
nfds=187650274213760, 
      timeout=<optimized out>, timeout@entry=0x0, sigmask=0xffffc123b950)
      at ../sysdeps/unix/sysv/linux/ppoll.c:39
  #1  0x0000aaaabbefaf00 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized 
out>, 
      __fds=<optimized out>) at /usr/include/aarch64-linux-gnu/bits/poll2.h:77
  #2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, 
      timeout=timeout@entry=-1) at util/qemu-timer.c:322
  #3  0x0000aaaabbefbf80 in os_host_main_loop_wait (timeout=-1)
      at util/main-loop.c:233
  #4  main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:497
  #5  0x0000aaaabbe2aa30 in convert_do_copy (s=0xffffc123bb58) at 
qemu-img.c:1980
  #6  img_convert (argc=<optimized out>, argv=<optimized out>) at 
qemu-img.c:2456
  #7  0x0000aaaabbe2333c in main (argc=7, argv=<optimized out>) at 
qemu-img.c:4975

  Reproduced w/ latest QEMU git (@ 53744e0a182)

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