[Qemu-devel] KVM VM hangs forever in R state within lock

[Andrey Korolyov]

Hi,

I`ve observed this issue previously on an old 3.10 branch but wrote it
off due to inability to reproduce in any meaningful way. Currently I
am seeing it on 3.10 branch where all KVM-related and RCU-related
issues are patched more or less for well-known issues.

Way to obtain a problematic state:
 - run a hypervisor for essentially long time, it took a year and half
previously for issue to come on the mentioned old branch, but for
newer kernel and probably due to higher load it took roughly a half of
a year,
 - suddenly a single VM obtains a lock and became unresponsive while
all threads displaying Running state, under this lock VM is neither
not killable via SIGKILL and not freezeable via freezer cgroup, the
only obvious symptoms is that it does not consume any cpu cycles
anymore (no counter inside sched info ) and of course it is
non-debuggable anymore. As it follows, it is quite impossible to say
at a glance where lock sits, as there is no distinctive processes
which are at least sleeping and could be moved out of sight.

It looks like I could have met pure scheduler issue, so if nothing
from attached recursive stack/status dump would click on an idea, I`d
CC scheduler folks. Timer/RCU configs are attached for the
convenience.

Thanks for looking into this!

stack:
http://xdel.ru/downloads/vm-sched-hang/stack.txt
status:
http://xdel.ru/downloads/vm-sched-hang/status.txt

timers.txt
Description: Text document

rcu.txt
Description: Text document