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