amd-pstate CPU Performance Scaling Driver¶
- Copyright
© 2021 Advanced Micro Devices, Inc.
- Author
Huang Rui <ray.huang@amd.com>
Introduction¶
amd-pstate is the AMD CPU performance scaling driver that introduces a
new CPU frequency control mechanism on modern AMD APU and CPU series in
Linux kernel. The new mechanism is based on Collaborative Processor
Performance Control (CPPC) which provides finer grain frequency management
than legacy ACPI hardware P-States. Current AMD CPU/APU platforms are using
the ACPI P-states driver to manage CPU frequency and clocks with switching
only in 3 P-states. CPPC replaces the ACPI P-states controls and allows a
flexible, low-latency interface for the Linux kernel to directly
communicate the performance hints to hardware.
amd-pstate leverages the Linux kernel governors such as schedutil,
ondemand, etc. to manage the performance hints which are provided by
CPPC hardware functionality that internally follows the hardware
specification (for details refer to AMD64 Architecture Programmer’s Manual
Volume 2: System Programming 1). Currently, amd-pstate supports basic
frequency control function according to kernel governors on some of the
Zen2 and Zen3 processors, and we will implement more AMD specific functions
in future after we verify them on the hardware and SBIOS.
AMD CPPC Overview¶
Collaborative Processor Performance Control (CPPC) interface enumerates a
continuous, abstract, and unit-less performance value in a scale that is
not tied to a specific performance state / frequency. This is an ACPI
standard 2 which software can specify application performance goals and
hints as a relative target to the infrastructure limits. AMD processors
provide the low latency register model (MSR) instead of an AML code
interpreter for performance adjustments. amd-pstate will initialize a
struct cpufreq_driver instance, amd_pstate_driver, with the callbacks
to manage each performance update behavior.
Highest Perf ------>+-----------------------+ +-----------------------+
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| | Max Perf ---->| |
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Nominal Perf ------>+-----------------------+ +-----------------------+
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| | Desired Perf ---->| |
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Lowest non- | | | |
linear perf ------>+-----------------------+ +-----------------------+
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| | Lowest perf ---->| |
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Lowest perf ------>+-----------------------+ +-----------------------+
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0 ------>+-----------------------+ +-----------------------+
AMD P-States Performance Scale
AMD CPPC Performance Capability¶
Highest Performance (RO)¶
This is the absolute maximum performance an individual processor may reach, assuming ideal conditions. This performance level may not be sustainable for long durations and may only be achievable if other platform components are in a specific state; for example, it may require other processors to be in an idle state. This would be equivalent to the highest frequencies supported by the processor.
Nominal (Guaranteed) Performance (RO)¶
This is the maximum sustained performance level of the processor, assuming ideal operating conditions. In the absence of an external constraint (power, thermal, etc.), this is the performance level the processor is expected to be able to maintain continuously. All cores/processors are expected to be able to sustain their nominal performance state simultaneously.
Lowest non-linear Performance (RO)¶
This is the lowest performance level at which nonlinear power savings are
achieved, for example, due to the combined effects of voltage and frequency
scaling. Above this threshold, lower performance levels should be generally
more energy efficient than higher performance levels. This register
effectively conveys the most efficient performance level to amd-pstate.
Lowest Performance (RO)¶
This is the absolute lowest performance level of the processor. Selecting a performance level lower than the lowest nonlinear performance level may cause an efficiency penalty but should reduce the instantaneous power consumption of the processor.
AMD CPPC Performance Control¶
amd-pstate passes performance goals through these registers. The
register drives the behavior of the desired performance target.
Minimum requested performance (RW)¶
amd-pstate specifies the minimum allowed performance level.
Maximum requested performance (RW)¶
amd-pstate specifies a limit the maximum performance that is expected
to be supplied by the hardware.
Desired performance target (RW)¶
amd-pstate specifies a desired target in the CPPC performance scale as
a relative number. This can be expressed as percentage of nominal
performance (infrastructure max). Below the nominal sustained performance
level, desired performance expresses the average performance level of the
processor subject to hardware. Above the nominal performance level,
the processor must provide at least nominal performance requested and go higher
if current operating conditions allow.
Energy Performance Preference (EPP) (RW)¶
This attribute provides a hint to the hardware if software wants to bias toward performance (0x0) or energy efficiency (0xff).
Key Governors Support¶
amd-pstate can be used with all the (generic) scaling governors listed
by the scaling_available_governors policy attribute in sysfs. Then,
it is responsible for the configuration of policy objects corresponding to
CPUs and provides the CPUFreq core (and the scaling governors attached
to the policy objects) with accurate information on the maximum and minimum
operating frequencies supported by the hardware. Users can check the
scaling_cur_freq information comes from the CPUFreq core.
amd-pstate mainly supports schedutil and ondemand for dynamic
frequency control. It is to fine tune the processor configuration on
amd-pstate to the schedutil with CPU CFS scheduler. amd-pstate
registers the adjust_perf callback to implement performance update behavior
similar to CPPC. It is initialized by sugov_start and then populates the
CPU’s update_util_data pointer to assign sugov_update_single_perf as the
utilization update callback function in the CPU scheduler. The CPU scheduler
will call cpufreq_update_util and assigns the target performance according
to the struct sugov_cpu that the utilization update belongs to.
Then, amd-pstate updates the desired performance according to the CPU
scheduler assigned.
Processor Support¶
The amd-pstate initialization will fail if the _CPC entry in the ACPI
SBIOS does not exist in the detected processor. It uses acpi_cpc_valid
to check the existence of _CPC. All Zen based processors support the legacy
ACPI hardware P-States function, so when amd-pstate fails initialization,
the kernel will fall back to initialize the acpi-cpufreq driver.
There are two types of hardware implementations for amd-pstate: one is
Full MSR Support and another is Shared Memory Support. It can use the X86_FEATURE_CPPC feature flag to
indicate the different types. (For details, refer to the Processor Programming
Reference (PPR) for AMD Family 19h Model 51h, Revision A1 Processors 3.)
amd-pstate is to register different static_call instances for different
hardware implementations.
Currently, some of the Zen2 and Zen3 processors support amd-pstate. In the
future, it will be supported on more and more AMD processors.
Full MSR Support¶
Some new Zen3 processors such as Cezanne provide the MSR registers directly
while the X86_FEATURE_CPPC CPU feature flag is set.
amd-pstate can handle the MSR register to implement the fast switch
function in CPUFreq that can reduce the latency of frequency control in
interrupt context. The functions with a pstate_xxx prefix represent the
operations on MSR registers.
User Space Interface in sysfs¶
amd-pstate exposes several global attributes (files) in sysfs to
control its functionality at the system level. They are located in the
/sys/devices/system/cpu/cpufreq/policyX/ directory and affect all CPUs.
root@hr-test1:/home/ray# ls /sys/devices/system/cpu/cpufreq/policy0/*amd*
/sys/devices/system/cpu/cpufreq/policy0/amd_pstate_highest_perf
/sys/devices/system/cpu/cpufreq/policy0/amd_pstate_lowest_nonlinear_freq
/sys/devices/system/cpu/cpufreq/policy0/amd_pstate_max_freq
amd_pstate_highest_perf / amd_pstate_max_freq
Maximum CPPC performance and CPU frequency that the driver is allowed to
set, in percent of the maximum supported CPPC performance level (the highest
performance supported in AMD CPPC Performance Capability).
In some ASICs, the highest CPPC performance is not the one in the _CPC
table, so we need to expose it to sysfs. If boost is not active, but
still supported, this maximum frequency will be larger than the one in
cpuinfo.
This attribute is read-only.
amd_pstate_lowest_nonlinear_freq
The lowest non-linear CPPC CPU frequency that the driver is allowed to set, in percent of the maximum supported CPPC performance level. (Please see the lowest non-linear performance in AMD CPPC Performance Capability.) This attribute is read-only.
Other performance and frequency values can be read back from
/sys/devices/system/cpu/cpuX/acpi_cppc/, see CPPC.
amd-pstate vs acpi-cpufreq¶
On the majority of AMD platforms supported by acpi-cpufreq, the ACPI tables
provided by the platform firmware are used for CPU performance scaling, but
only provide 3 P-states on AMD processors.
However, on modern AMD APU and CPU series, hardware provides the Collaborative
Processor Performance Control according to the ACPI protocol and customizes this
for AMD platforms. That is, fine-grained and continuous frequency ranges
instead of the legacy hardware P-states. amd-pstate is the kernel
module which supports the new AMD P-States mechanism on most of the future AMD
platforms. The AMD P-States mechanism is the more performance and energy
efficiency frequency management method on AMD processors.
Kernel Module Options for amd-pstate¶
shared_mem
Use a module param (shared_mem) to enable related processors manually with
amd_pstate.shared_mem=1.
Due to the performance issue on the processors with Shared Memory Support, we disable it presently and will re-enable this by default
once we address performance issue with this solution.
To check whether the current processor is using Full MSR Support or Shared Memory Support :
ray@hr-test1:~$ lscpu | grep cppc
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 erms invpcid cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr rdpru wbnoinvd cppc arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip pku ospke vaes vpclmulqdq rdpid overflow_recov succor smca fsrm
If the CPU flags have cppc, then this processor supports Full MSR Support. Otherwise, it supports Shared Memory Support.
cpupower tool support for amd-pstate¶
amd-pstate is supported by the cpupower tool, which can be used to dump
frequency information. Development is in progress to support more and more
operations for the new amd-pstate module with this tool.
root@hr-test1:/home/ray# cpupower frequency-info
analyzing CPU 0:
driver: amd-pstate
CPUs which run at the same hardware frequency: 0
CPUs which need to have their frequency coordinated by software: 0
maximum transition latency: 131 us
hardware limits: 400 MHz - 4.68 GHz
available cpufreq governors: ondemand conservative powersave userspace performance schedutil
current policy: frequency should be within 400 MHz and 4.68 GHz.
The governor "schedutil" may decide which speed to use
within this range.
current CPU frequency: Unable to call hardware
current CPU frequency: 4.02 GHz (asserted by call to kernel)
boost state support:
Supported: yes
Active: yes
AMD PSTATE Highest Performance: 166. Maximum Frequency: 4.68 GHz.
AMD PSTATE Nominal Performance: 117. Nominal Frequency: 3.30 GHz.
AMD PSTATE Lowest Non-linear Performance: 39. Lowest Non-linear Frequency: 1.10 GHz.
AMD PSTATE Lowest Performance: 15. Lowest Frequency: 400 MHz.
Diagnostics and Tuning¶
Trace Events¶
There are two static trace events that can be used for amd-pstate
diagnostics. One of them is the cpu_frequency trace event generally used
by CPUFreq, and the other one is the amd_pstate_perf trace event
specific to amd-pstate. The following sequence of shell commands can
be used to enable them and see their output (if the kernel is
configured to support event tracing).
root@hr-test1:/home/ray# cd /sys/kernel/tracing/
root@hr-test1:/sys/kernel/tracing# echo 1 > events/amd_cpu/enable
root@hr-test1:/sys/kernel/tracing# cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 47827/42233061 #P:2
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
<idle>-0 [015] dN... 4995.979886: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=15 changed=false fast_switch=true
<idle>-0 [007] d.h.. 4995.979893: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=7 changed=false fast_switch=true
cat-2161 [000] d.... 4995.980841: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=0 changed=false fast_switch=true
sshd-2125 [004] d.s.. 4995.980968: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=4 changed=false fast_switch=true
<idle>-0 [007] d.s.. 4995.980968: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=7 changed=false fast_switch=true
<idle>-0 [003] d.s.. 4995.980971: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=3 changed=false fast_switch=true
<idle>-0 [011] d.s.. 4995.980996: amd_pstate_perf: amd_min_perf=85 amd_des_perf=85 amd_max_perf=166 cpu_id=11 changed=false fast_switch=true
The cpu_frequency trace event will be triggered either by the schedutil scaling
governor (for the policies it is attached to), or by the CPUFreq core (for the
policies with other scaling governors).
Tracer Tool¶
amd_pstate_tracer.py can record and parse amd-pstate trace log, then
generate performance plots. This utility can be used to debug and tune the
performance of amd-pstate driver. The tracer tool needs to import intel
pstate tracer.
Tracer tool located in linux/tools/power/x86/amd_pstate_tracer. It can be
used in two ways. If trace file is available, then directly parse the file
with command
./amd_pstate_trace.py [-c cpus] -t <trace_file> -n <test_name>
Or generate trace file with root privilege, then parse and plot with command
sudo ./amd_pstate_trace.py [-c cpus] -n <test_name> -i <interval> [-m kbytes]
The test result can be found in results/test_name. Following is the example
about part of the output.
common_cpu common_secs common_usecs min_perf des_perf max_perf freq mperf apef tsc load duration_ms sample_num elapsed_time common_comm
CPU_005 712 116384 39 49 166 0.7565 9645075 2214891 38431470 25.1 11.646 469 2.496 kworker/5:0-40
CPU_006 712 116408 39 49 166 0.6769 8950227 1839034 37192089 24.06 11.272 470 2.496 kworker/6:0-1264
Reference¶
- 1
AMD64 Architecture Programmer’s Manual Volume 2: System Programming, https://www.amd.com/system/files/TechDocs/24593.pdf
- 2
Advanced Configuration and Power Interface Specification, https://uefi.org/sites/default/files/resources/ACPI_Spec_6_4_Jan22.pdf
- 3
Processor Programming Reference (PPR) for AMD Family 19h Model 51h, Revision A1 Processors https://www.amd.com/system/files/TechDocs/56569-A1-PUB.zip