Intel HFI Driver Can "Save Tons Of CPU Cycles" By Only Enabling Itself When Needed

The Linux kernel has supported the Intel Hardware Feedback Interface "HFI" via the "intel_hfi" driver since 2022 for bettering supporting Core hybrid processors. The Intel HFI can be used for communicating performance and energy efficiency capabilities of individual CPU cores of the system. In turn the Linux kernel can leverage Intel HFI details for better task placement among the available CPU cores/threads. With a new patch series, the Intel HFI driver can "save tons of CPU cycles" by only enabling it when needed.

A patch series has been undergoing review for the Intel HFI Linux driver to only enable the HFI feature when required. Or in particular, only enabling it when there is user-space consumers active. If Intel Speed Select or the Intel Low-Power daemon is running, the Intel HFI interface is activated but if not it will be disabled and in turn can apparently save significant CPU resources.

The latest patches for the Intel HFI driver explain:

"Implement the notification mechanism in the [intel_hfi] driver, it is utilized to disable the Hardware Feedback Interface (HFI) dynamically. By implementing a thermal genl notify callback, the driver can now enable or disable the HFI based on actual demand, particularly when user-space applications like intel-speed-select or Intel Low Power daemon utilize events related to performance and energy efficiency capabilities.

On machines where Intel HFI is present, but there are no user-space components installed, we can save tons of CPU cycles."

It makes sense to toggle the Intel HFI kernel support dynamically based on need but surprising it wasn't done to start with and that it can apparently save a significant number of CPU cycles. But for those wanting to take most advantage of their modern (Alder Lake / Raptor Lake / Meteor Lake) Intel Core hybrid processors will want the Intel Low Power Mode Daemon (LPMD) software loaded for maximizing power efficiency.