1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==============================================
4 Hardware-Feedback Interface for scheduling on
5 ==============================================
6
7 Overview
8 --------
9
10 Intel has described the Hardware Feedback Inte
11 IA-32 Architectures Software Developer's Manua
12 14.6 [1]_.
13
14 The HFI gives the operating system a performan
15 capability data for each CPU in the system. Li
16 the HFI to influence task placement decisions.
17
18 The Hardware Feedback Interface
19 -------------------------------
20
21 The Hardware Feedback Interface provides to th
22 about the performance and energy efficiency of
23 capability is given as a unit-less quantity in
24 indicate higher capability. Energy efficiency
25 separate capabilities. Even though on some sys
26 related, they are specified as independent cap
27
28 These capabilities may change at runtime as a
29 operating conditions of the system or the acti
30 at which these capabilities are updated is spe
31 some models, capabilities are set at boot time
32 capabilities may change every tens of millisec
33 mechanism may be used to lower Thermal Design
34 reflected in the HFI. Likewise, if the system
35 excessive heat, the HFI may reflect reduced pe
36
37 The kernel or a userspace policy daemon can us
38 task placement decisions. For instance, if eit
39 capabilities of a given logical processor beco
40 the hardware recommends to the operating syste
41 that processor for performance or energy effic
42
43 Implementation details for Linux
44 --------------------------------
45
46 The infrastructure to handle thermal event int
47 Local Vector Table of a CPU's local APIC, ther
48 Thermal Monitor Register. This register contro
49 to a CPU when the thermal monitor generates an
50 can be found in the Intel SDM Vol. 3 Section 1
51
52 The thermal monitor may generate interrupts pe
53 generates package-level interrupts. This monit
54 via a set of machine-specific registers. Speci
55 status are controlled via designated bits in t
56 and IA32_PACKAGE_THERM_STATUS registers, respe
57 table per package. Further details can be foun
58 Section 14.9 [1]_.
59
60 The hardware issues an HFI interrupt after upd
61 for the operating system to consume it. CPUs r
62 thermal entry in the Local APIC's Local Vector
63
64 When servicing such interrupt, the HFI driver
65 relays the update to userspace using the therm
66 that there may be many HFI updates every secon
67 userspace are throttled at a rate of CONFIG_HZ
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.