[Qemu-devel] [Bug 607204] Re: New qemu instances often cannot be started if host system is under load

[Guido Winkelmann]

Forgot to mention:

The bug is still present in the latest git-version as of this writing.

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New qemu instances often cannot be started if host system is under load
https://bugs.launchpad.net/bugs/607204
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Status in QEMU: New

Bug description:
I've got a problem where I cannot start any new VMs with qemu-kvm if the host 
machine is under high CPU load. The problem is not 100% reproducible (it works 
sometimes), but under load conditions, it happens most of the time - roughly 
95%.

I'm usually using libvirt to start and stop KVM VMs. When using virsh to start 
a new VM under those conditions, the output looks like this:

virsh # start testserver-a
error: Failed to start domain testserver-a
error: monitor socket did not show up.: Connection refused

(There is a very long wait after the command has been sent until the error 
message shows up.)

This is (an example of) the command line that libvirtd uses to start up qemu:

----- snip -----
LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin HOME=/root USER=root LOGNAME=root 
QEMU_AUDIO_DRV=none /usr/bin/qemu-kvm -S -M pc-0.12 -enable-kvm -m 256 -smp 
1,sockets=1,cores=1,threads=1 -name testserver-a -uuid 
7cbb3665-4d58-86b8-ce8f-20541995a99c -nodefaults -chardev 
socket,id=monitor,path=/usr/local/var/lib/libvirt/qemu/testserver-a.monitor,server,nowait
 -mon chardev=monitor,mode=readline -rtc base=utc -no-acpi -boot c -device 
lsi,id=scsi0,bus=pci.0,addr=0x7 -drive 
file=/data/testserver-a-system.img,if=none,id=drive-scsi0-0-1,boot=on -device 
scsi-disk,bus=scsi0.0,scsi-id=1,drive=drive-scsi0-0-1,id=scsi0-0-1 -drive 
file=/data/testserver-a-data1.img,if=none,id=drive-virtio-disk1 -device 
virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk1,id=virtio-disk1 
-drive file=/data/testserver-a-data2.img,if=none,id=drive-virtio-disk2 -device 
virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk2,id=virtio-disk2 
-drive 
file=/data/gentoo-install-amd64-minimal-20100408.iso,if=none,media=cdrom,id=drive-ide0-0-0,readonly=on
 -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -drive 
file=/data/testserver-a_configfloppy.img,if=none,id=drive-fdc0-0-0 -global 
isa-fdc.driveA=drive-fdc0-0-0 -device 
e1000,vlan=0,id=net0,mac=52:54:00:84:6d:69,bus=pci.0,addr=0x6 -net 
tap,fd=24,vlan=0,name=hostnet0 -usb -vnc 127.0.0.1:1,password -k de -vga cirrus 
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3
----- snip -----

Copy-pasting this to a commandline on the host to start qemu manually leads to 
a non-functional qemu process that "just sits there" with nothing happening. 
The monitor socket /usr/local/var/lib/libvirt/qemu/testserver-a.monitor will, 
indeed, not show up.

I've tried starting qemu with the same commandline but without the parameters 
for redirecting the monitor to a socket, without the fd parameter for the 
network interface and without the vnc parameter. This resulted in a black 
window with the title "QEMU (testserver-a) [Stopped]". I could not access the 
monitor console in graphical mode either. When I press Ctrl-Alt-2 in graphical 
mode to access the monitor console, qemu will sometimes (but not always) crash 
with a segfault about 2 seconds after.

Some experimentation I've done suggests that this problem only happens if the 
high cpu load is caused by another qemu process, not if it is caused by 
something else running on the machine.

The bug appears much less often if I leave off the -nodefaults parameter.

The bug will still appear if I start qemu as qemu-system-x86_64 instead of 
qemu-kvm and replace the -enable-kvm parameter with -no-kvm.

The host machine I'm running this on has got 16 cores in total. It looks like 
it is sufficient for this bug to surface if at least one of these cores is 
brought to near 100% use by a qemu process.

The version of qemu I'm using is qemu-kvm 0.12.4, built from source. Libvirt is 
version 0.8.1, built from source as well. The host OS is Fedora 12. The Kernel 
version is 2.6.32.12-115.fc12.x86_64.

Attached is an strace of attempting to start qemu which I hope will help 
someone with a better understanding of qemu's internals see what's actually 
going on there.