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

[Rafael David Tinoco <rafaeldtinoco@kernelpath.com>]

Alright, I couldn't reproduce this yet, I'm running same test case in a
24 cores box and causing lots of context switches and CPU migrations in
parallel (trying to exhaust the logic).

Will let this running for sometime to check.

Unfortunately this can be related QEMU AIO BH locking/primitives and
cache coherency in the HW in question (which I got specs from:
https://en.wikichip.org/wiki/hisilicon/kunpeng/hi1616):

l1$ size	8 MiB
l1d$ size	4 MiB
l1i$ size	4 MiB
l2$ size	32 MiB
l3$ size	64 MiB

like for example when having 2 threads in different NUMA domains, or
some other situation.

I can't simulate the same since I have a SOC with:

Cortex-A53 MPCore 24cores,

L1 I/D=32KB/32KB
L2 =256KB
L3 =4MB

and I'm not even close to L1/L2/L3 cache numbers from D06 =o).

Just got a note that I'll be able to reproduce this in the real HW, will
get back soon with real gdb debugging.

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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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