Elevated CPU usage after adding extra memory hotplug on Linux on IBM Z Series (s390x)

Solution In Progress - Updated -

Issue

When memory is added online by

$ echo online > ./memory<#>/state

the CPU usage jumps from near zero to around 25% even on a system without any application load and the elevated CPU usage stays elevated for the duration of the extra memory in online state. The CPU usage should drop back to the original level after the transition period when kernel structures are being set up for the extra memory

When the extra memory is taken offline, the CPU usage drops back down, meaning the elevated CPU usage remains only for the duration of the extra memory presence.

With the guest loaded by actual production application load, adding extra memory online like this has a dramatic effect leading to a totally unresponsive system, effectively a hang requiring a restart of the VM.

Environment

  • Red Hat Enterprise Linux 6
  • IBM Z Series architecture (s390x)

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