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diff --git a/Documentation/x86/intel-hfi.rst b/Documentation/x86/intel-hfi.rst new file mode 100644 index 000000000..49dea58ea --- /dev/null +++ b/Documentation/x86/intel-hfi.rst @@ -0,0 +1,72 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================================================ +Hardware-Feedback Interface for scheduling on Intel Hardware +============================================================ + +Overview +-------- + +Intel has described the Hardware Feedback Interface (HFI) in the Intel 64 and +IA-32 Architectures Software Developer's Manual (Intel SDM) Volume 3 Section +14.6 [1]_. + +The HFI gives the operating system a performance and energy efficiency +capability data for each CPU in the system. Linux can use the information from +the HFI to influence task placement decisions. + +The Hardware Feedback Interface +------------------------------- + +The Hardware Feedback Interface provides to the operating system information +about the performance and energy efficiency of each CPU in the system. Each +capability is given as a unit-less quantity in the range [0-255]. Higher values +indicate higher capability. Energy efficiency and performance are reported in +separate capabilities. Even though on some systems these two metrics may be +related, they are specified as independent capabilities in the Intel SDM. + +These capabilities may change at runtime as a result of changes in the +operating conditions of the system or the action of external factors. The rate +at which these capabilities are updated is specific to each processor model. On +some models, capabilities are set at boot time and never change. On others, +capabilities may change every tens of milliseconds. For instance, a remote +mechanism may be used to lower Thermal Design Power. Such change can be +reflected in the HFI. Likewise, if the system needs to be throttled due to +excessive heat, the HFI may reflect reduced performance on specific CPUs. + +The kernel or a userspace policy daemon can use these capabilities to modify +task placement decisions. For instance, if either the performance or energy +capabilities of a given logical processor becomes zero, it is an indication that +the hardware recommends to the operating system to not schedule any tasks on +that processor for performance or energy efficiency reasons, respectively. + +Implementation details for Linux +-------------------------------- + +The infrastructure to handle thermal event interrupts has two parts. In the +Local Vector Table of a CPU's local APIC, there exists a register for the +Thermal Monitor Register. This register controls how interrupts are delivered +to a CPU when the thermal monitor generates and interrupt. Further details +can be found in the Intel SDM Vol. 3 Section 10.5 [1]_. + +The thermal monitor may generate interrupts per CPU or per package. The HFI +generates package-level interrupts. This monitor is configured and initialized +via a set of machine-specific registers. Specifically, the HFI interrupt and +status are controlled via designated bits in the IA32_PACKAGE_THERM_INTERRUPT +and IA32_PACKAGE_THERM_STATUS registers, respectively. There exists one HFI +table per package. Further details can be found in the Intel SDM Vol. 3 +Section 14.9 [1]_. + +The hardware issues an HFI interrupt after updating the HFI table and is ready +for the operating system to consume it. CPUs receive such interrupt via the +thermal entry in the Local APIC's Local Vector Table. + +When servicing such interrupt, the HFI driver parses the updated table and +relays the update to userspace using the thermal notification framework. Given +that there may be many HFI updates every second, the updates relayed to +userspace are throttled at a rate of CONFIG_HZ jiffies. + +References +---------- + +.. [1] https://www.intel.com/sdm |