73 lines
3.5 KiB
ReStructuredText
73 lines
3.5 KiB
ReStructuredText
.. 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
|