Today's "hang" involved a zombie compiz consuming 100% of a cpu, along
with an emacs instance consuming another 100%. Load average around 11,
and climbing. Only 22 zombies currently, but it was 4 when I managed to
get on with ssh.

I was in the process of installing software updates, using the GUI tool
(rather than direct use of apt-get from the shell) when this happened.

Parts of the update still seem to be running.

arlie@ansuz$ ps -Fa -p1 -www
UID        PID  PPID  C    SZ   RSS PSR STIME TTY          TIME CMD
root         1     0  0 30034  4656   2 Apr28 ?        00:00:08 /sbin/init 
splash
root     25826 25775  0  1127  1712   0 07:57 pts/18   00:00:00 /bin/sh -e 
/var/lib/dpkg/info/udev.postrm upgrade 229-4ubuntu17
root     25843 25826  0  6542  1352   0 07:57 pts/18   00:00:00 systemctl 
--system daemon-reload
arlie    25846 22284  0  9342  3232   2 07:57 pts/4    00:00:00 ps -Fa -p1 -www


I'm wondering now whether my first guess of a kernel issue is dead wrong, and 
the root cause is actually compiz. Or perhaps we have multiple causes, for the 
same basic symptom.

Here's the current crop of defunct processes

arlie@ansuz$ ps aux | grep defunct
arlie     2488  0.0  0.0      0     0 ?        Z<l  Apr28   0:00 [pulseaudio] 
<defunct>
arlie     2503  0.8  0.0      0     0 ?        Zsl  Apr28  55:08 [compiz] 
<defunct>
arlie     2692  0.0  0.0      0     0 ?        Z    Apr28   0:00 [gconf-helper] 
<defunct>
root     22212  0.0  0.0      0     0 ?        Z    07:42   0:00 
[check-new-relea] <defunct>
sshd     24480  0.0  0.0      0     0 ?        Z    07:52   0:00 [sshd] 
<defunct>
sshd     24489  0.0  0.0      0     0 ?        Z    07:52   0:00 [sshd] 
<defunct>
sshd     24491  0.0  0.0      0     0 ?        Z    07:52   0:00 [sshd] 
<defunct>
sshd     24494  0.0  0.0      0     0 ?        Z    07:53   0:00 [sshd] 
<defunct>
sshd     24496  0.0  0.0      0     0 ?        Z    07:53   0:00 [sshd] 
<defunct>
sshd     24500  0.0  0.0      0     0 ?        Z    07:53   0:00 [sshd] 
<defunct>
sshd     24504  0.0  0.0      0     0 ?        Z    07:53   0:00 [sshd] 
<defunct>
sshd     24508  0.0  0.0      0     0 ?        Z    07:53   0:00 [sshd] 
<defunct>
sshd     24510  0.0  0.0      0     0 ?        Z    07:54   0:00 [sshd] 
<defunct>
sshd     24514  0.0  0.0      0     0 ?        Z    07:54   0:00 [sshd] 
<defunct>
sshd     24518  0.0  0.0      0     0 ?        Z    07:54   0:00 [sshd] 
<defunct>
sshd     24523  0.0  0.0      0     0 ?        Z    07:54   0:00 [sshd] 
<defunct>
sshd     24532  0.0  0.0      0     0 ?        Z    07:54   0:00 [sshd] 
<defunct>
sshd     24538  0.0  0.0      0     0 ?        Z    07:55   0:00 [sshd] 
<defunct>
sshd     24541  0.0  0.0      0     0 ?        Z    07:55   0:00 [sshd] 
<defunct>
sshd     24543  0.0  0.0      0     0 ?        Z    07:55   0:00 [sshd] 
<defunct>
sshd     25708  0.0  0.0      0     0 ?        Z    07:55   0:00 [sshd] 
<defunct>
sshd     25711  0.0  0.0      0     0 ?        Z    07:56   0:00 [sshd] 
<defunct>
arlie    26946  0.0  0.0  14228   964 pts/4    S+   08:00   0:00 grep defunct

Systemd is in top's state "D" - just like last time. That's an
uninterruptable sleep. It does not appear to have accumulated any cpu
time since I got in via ssh.

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND     
 2503 arlie     20   0       0      0      0 Z 100.0  0.0  61:12.34 compiz      
    7 root      20   0       0      0      0 S   0.3  0.0   0:57.09 rcu_sched   
    1 root      20   0  120136   4656   3204 D   0.0  0.1   0:08.94 systemd     
    2 root      20   0       0      0      0 S   0.0  0.0   0:00.02 kthreadd    

So the root cause might be systemd blocking on something.

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https://bugs.launchpad.net/bugs/1680502

Title:
  Hang after upgrade to 16.04

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