As expected, the thermal subsystem updates for the in-development Linux 5.18 kernel bring the new Hardware Feedback Interface (HFI) to benefit from their hybrid architecture processors, as recently introduced with Alder Lake.
The Intel Hardware Feedback Interface is used to communicate the performance and power efficiency capabilities of individual processor cores in the system. The Linux kernel will use the HFI data to improve task placement when deciding where to place tasks among available CPU cores/threads. Intel engineers have been working on this new Linux driver for several months since last year.
The Intel HFI driver must be part of the Linux 5.18 kernel.
Intel Hardware Feedback Interface power/thermal data is dynamically updated in real time based on current system conditions and other factors. From the kernel documentation with the new Intel HFI driver:
The hardware feedback interface provides the operating system with information about the performance and power efficiency of each processor in the system. Each capacity is given as a unitless quantity in the range [0-255]. Higher values indicate higher capacity. Energy efficiency and performance are reported in separate capacities. Although on some systems these two metrics may be related, they are specified as independent capabilities in Intel SDM.
These capabilities may change during runtime due to changes in system operating conditions or the action of external factors. The frequency at which these features are updated is specific to each processor model. On some models, features are set at startup and never change. On others, the capacities can change every tens of milliseconds. For example, a remote mechanism can be used to reduce thermal design power. Such a change can be reflected in the HFI. Likewise, if the system should be throttling due to excessive heat, the HFI may reflect reduced performance on specific processors.
The kernel or a userspace policy daemon can use these capabilities to modify task placement decisions. For example, if the performance or power capabilities of a given logical processor drops to zero, this indicates that the hardware recommends the operating system not to schedule any tasks on that processor for performance or power efficiency reasons, respectively. .
Linux driver for Intel HFI was submitted today as part of thermal updates for Linux 5.18. With the driver himself, the intel-speed-select The utility has also been updated to handle Intel HFI data. There are also fixes for other thermal drivers and other normal code changes as part of this pull request, but Intel HFI is the most notable addition for end users.