Qemuppc Boot Hangs

qemuppc is randomly hanging at boot during sanity testing. For example:

• https://autobuilder.yocto.io/builders/nightly-ppc-lsb/builds/581
• https://autobuilder.yocto.io/builders/nightly-ppc/builds/592

Despite initial suspicions this was not due to entropy problems.

It is *not*:

• Specific to a host OS (observed on centos7, debian8, ubuntu1710)
• Specific to image recipe (happens sato, sato-sdk, lsb, lsb-sdk)
• Specific to DISTRO (poky and poky-lsb)

The hang usually occurs after:

[    6.667445] udevd[104]: starting version 3.2.2
[    6.743262] udevd[105]: starting eudev-3.2.2

and is hanging in the poll() call in udevadm-settle.c:adm_settle(), it doesn't seem to return from the syscall.

In some cases the boot proceeds further and hangs in the dropbear key creation.

The timing of the usb-tablet device does vary in the boot logs however the hang has been observed with the usb-tablet device removed.

It seems that running a build in parallel with testimage does help the issue occur more frequently.

I wondered about a connection with host cpufreq changes. In order to test, I booted with "intel_pstate=disable" on the kernel commandline and tried: while true; do cpupower frequency-set -f 1200000 > /dev/null; sleep 0.25; cpupower frequency-set -f 2200000 > /dev/null; done along with setting to max cpu freq and also setting just to min cpu freq. I've observed hangs in all combinations although the likelyhood of a hang seemed higher with the alternating frequency. I also tried the script:

import time
import random
import subprocess 

cpus = range(0,87)
freqs = range(1200000, 2200000, 100000)

while True:
    for cpunum in cpus:
        f = random.choice(freqs)
        with open("/sys/devices/system/cpu/cpu%d/cpufreq/scaling_setspeed" % cpunum, 'w') as speed:
            speed.write("%d\n" % f)

    time.sleep(0.05)

which also seems to make hangs more likely somehow.