1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================================================ 4 Hardware-Feedback Interface for scheduling on Intel Hardware 5 ============================================================ 6 7 Overview 8 -------- 9 10 Intel has described the Hardware Feedback Interface (HFI) in the Intel 64 and 11 IA-32 Architectures Software Developer's Manual (Intel SDM) Volume 3 Section 12 14.6 [1]_. 13 14 The HFI gives the operating system a performance and energy efficiency 15 capability data for each CPU in the system. Linux can use the information from 16 the HFI to influence task placement decisions. 17 18 The Hardware Feedback Interface 19 ------------------------------- 20 21 The Hardware Feedback Interface provides to the operating system information 22 about the performance and energy efficiency of each CPU in the system. Each 23 capability is given as a unit-less quantity in the range [0-255]. Higher values 24 indicate higher capability. Energy efficiency and performance are reported in 25 separate capabilities. Even though on some systems these two metrics may be 26 related, they are specified as independent capabilities in the Intel SDM. 27 28 These capabilities may change at runtime as a result of changes in the 29 operating conditions of the system or the action of external factors. The rate 30 at which these capabilities are updated is specific to each processor model. On 31 some models, capabilities are set at boot time and never change. On others, 32 capabilities may change every tens of milliseconds. For instance, a remote 33 mechanism may be used to lower Thermal Design Power. Such change can be 34 reflected in the HFI. Likewise, if the system needs to be throttled due to 35 excessive heat, the HFI may reflect reduced performance on specific CPUs. 36 37 The kernel or a userspace policy daemon can use these capabilities to modify 38 task placement decisions. For instance, if either the performance or energy 39 capabilities of a given logical processor becomes zero, it is an indication that 40 the hardware recommends to the operating system to not schedule any tasks on 41 that processor for performance or energy efficiency reasons, respectively. 42 43 Implementation details for Linux 44 -------------------------------- 45 46 The infrastructure to handle thermal event interrupts has two parts. In the 47 Local Vector Table of a CPU's local APIC, there exists a register for the 48 Thermal Monitor Register. This register controls how interrupts are delivered 49 to a CPU when the thermal monitor generates and interrupt. Further details 50 can be found in the Intel SDM Vol. 3 Section 10.5 [1]_. 51 52 The thermal monitor may generate interrupts per CPU or per package. The HFI 53 generates package-level interrupts. This monitor is configured and initialized 54 via a set of machine-specific registers. Specifically, the HFI interrupt and 55 status are controlled via designated bits in the IA32_PACKAGE_THERM_INTERRUPT 56 and IA32_PACKAGE_THERM_STATUS registers, respectively. There exists one HFI 57 table per package. Further details can be found in the Intel SDM Vol. 3 58 Section 14.9 [1]_. 59 60 The hardware issues an HFI interrupt after updating the HFI table and is ready 61 for the operating system to consume it. CPUs receive such interrupt via the 62 thermal entry in the Local APIC's Local Vector Table. 63 64 When servicing such interrupt, the HFI driver parses the updated table and 65 relays the update to userspace using the thermal notification framework. Given 66 that there may be many HFI updates every second, the updates relayed to 67 userspace are throttled at a rate of CONFIG_HZ jiffies. 68 69 References 70 ---------- 71 72 .. [1] https://www.intel.com/sdm
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.