Cleaning up Linux's CPU hotplug for real time and energy management

Abstract

Linux's CPU-Hotplug facility was originally designed to allow failing hardware to be removed from a running system. Hardware fails quite infrequently, so CPU-hotplug performance (much less real-time response) was not a major consideration. However, CPU hotplug is now used for energy management and (believe it or not!) real-time response, both of which have unsurprisingly exposed some shortcomings in CPU hotplug. This document reviews a number of these shortcomings, and then proposes an alternative CPU-hotplug approach that we believe will address these shortcomings.