During the migration of my build system for fedora packages, from plague to koji, I set up all the machines needed by this infrastructure as virtual boxes. This is interesting for builder machines, as it is possible to adjust their dedicated resources dynamically, depending on the number of packages to be built by the system. Unfortunately, I've quickly been bitten by a recurring bug in my kvm builders : as soon as they are configured with more than one virtual processor, processes begin to stall, the virtual machine becomes unresponsive, although there's no sign of abnormal disk, net, or cpu activity.

After some research, looking for the "soft lockup" expression that the guest kernel puts in the log files, which seems to be a rather common cause of problems, I narrowed the possibilities down to the clock source being used in the guest kernel. Booting with "clocksource=acpi_pm", and forcing the use of this clock source provided much more stability in my VM so far...

# echo acpi_pm > /sys/devices/system/clocksource/clocksource0/current_clocksource