The idle checks all happen in the kernel scheduling code which runs on every CPU/core/hyperthread between doing real work. There isn't a separate core which ha

The idle checks all happen in the kernel scheduling code which runs on every CPU/core/hyperthread between doing real work. There isn't a separate core which hands out work or controls them. The kernel knows which CPUs are active because it's the one running things, not because it asks the CPU if it ran any code. On a single CPU system it works because when the current process stops running (calls sleep, does IO, etc...) the kernel looks to see if there's anything else to do, if not, it knows the system is now idle.