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General Information¶

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intel_pstate is a part of theCPU performance scaling subsystem in the Linux kernel(CPUFreq). It is a scaling driver for the Sandy Bridge and latergenerations of Intel processors. Note, however, that some of those processorsmay not be supported. [To understand intel_pstate it is necessary to knowhow CPUFreq works in general, so this is the time to read CPU Performance Scaling ifyou have not done that yet.]

For the processors supported by intel_pstate, the P-state concept is broaderthan just an operating frequency or an operating performance point (see theLinuxCon Europe 2015 presentation by Kristen Accardi [1] for moreinformation about that). For this reason, the representation of P-states usedby intel_pstate internally follows the hardware specification (for detailsrefer to Intel Software Developer’s Manual [2]). However, the CPUFreq coreuses frequencies for identifying operating performance points of CPUs andfrequencies are involved in the user space interface exposed by it, sointel_pstate maps its internal representation of P-states to frequencies too(fortunately, that mapping is unambiguous). At the same time, it would not bepractical for intel_pstate to supply the CPUFreq core with a table ofavailable frequencies due to the possible size of it, so the driver does not dothat. Some functionality of the core is limited by that.

Since the hardware P-state selection interface used by intel_pstate isavailable at the logical CPU level, the driver always works with individualCPUs. Consequently, if intel_pstate is in use, every CPUFreq policyobject corresponds to one logical CPU and CPUFreq policies are effectivelyequivalent to CPUs. In particular, this means that they become “inactive” everytime the corresponding CPU is taken offline and need to be re-initialized whenit goes back online.

intel_pstate is not modular, so it cannot be unloaded, which means that theonly way to pass early-configuration-time parameters to it is via the kernelcommand line. However, its configuration can be adjusted via sysfs to agreat extent. In some configurations it even is possible to unregister it viasysfs which allows another CPUFreq scaling driver to be loaded andregistered (see below).

Operation Modes¶

intel_pstate can operate in two different modes, active or passive. In theactive mode, it uses its own internal performance scaling governor algorithm orallows the hardware to do performance scaling by itself, while in the passivemode it responds to requests made by a generic CPUFreq governor implementinga certain performance scaling algorithm. Which of them will be in effectdepends on what kernel command line options are used and on the capabilities ofthe processor.

Active Mode¶

This is the default operation mode of intel_pstate for processors withhardware-managed P-states (HWP) support. If it works in this mode, thescaling_driver policy attribute in sysfs for all CPUFreq policiescontains the string “intel_pstate”.

In this mode the driver bypasses the scaling governors layer of CPUFreq andprovides its own scaling algorithms for P-state selection. Those algorithmscan be applied to CPUFreq policies in the same way as generic scalinggovernors (that is, through the scaling_governor policy attribute insysfs). [Note that different P-state selection algorithms may be chosen fordifferent policies, but that is not recommended.]

They are not generic scaling governors, but their names are the same as thenames of some of those governors. Moreover, confusingly enough, they generallydo not work in the same way as the generic governors they share the names with.For example, the powersave P-state selection algorithm provided byintel_pstate is not a counterpart of the generic powersave governor(roughly, it corresponds to the schedutil and ondemand governors).

There are two P-state selection algorithms provided by intel_pstate in theactive mode: powersave and performance. The way they both operatedepends on whether or not the hardware-managed P-states (HWP) feature has beenenabled in the processor and possibly on the processor model.

Which of the P-state selection algorithms is used by default depends on theCONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE kernel configuration option.Namely, if that option is set, the performance algorithm will be used bydefault, and the other one will be used by default if it is not set.

Active Mode With HWP¶

If the processor supports the HWP feature, it will be enabled during theprocessor initialization and cannot be disabled after that. It is possibleto avoid enabling it by passing the intel_pstate=no_hwp argument to thekernel in the command line.

If the HWP feature has been enabled, intel_pstate relies on the processor toselect P-states by itself, but still it can give hints to the processor’sinternal P-state selection logic. What those hints are depends on which P-stateselection algorithm has been applied to the given policy (or to the CPU itcorresponds to).

Even though the P-state selection is carried out by the processor automatically,intel_pstate registers utilization update callbacks with the CPU schedulerin this mode. However, they are not used for running a P-state selectionalgorithm, but for periodic updates of the current CPU frequency information tobe made available from the scaling_cur_freq policy attribute in sysfs.

HWP + performance¶

In this configuration intel_pstate will write 0 to the processor’sEnergy-Performance Preference (EPP) knob (if supported) or itsEnergy-Performance Bias (EPB) knob (otherwise), which means that the processor’sinternal P-state selection logic is expected to focus entirely on performance.

This will override the EPP/EPB setting coming from the sysfs interface(see Energy vs Performance Hints below). Moreover, any attempts to changethe EPP/EPB to a value different from 0 (“performance”) via sysfs in thisconfiguration will be rejected.

Also, in this configuration the range of P-states available to the processor’sinternal P-state selection logic is always restricted to the upper boundary(that is, the maximum P-state that the driver is allowed to use).

HWP + powersave¶

In this configuration intel_pstate will set the processor’sEnergy-Performance Preference (EPP) knob (if supported) or itsEnergy-Performance Bias (EPB) knob (otherwise) to whatever value it waspreviously set to via sysfs (or whatever default value it wasset to by the platform firmware). This usually causes the processor’sinternal P-state selection logic to be less performance-focused.

Active Mode Without HWP¶

This operation mode is optional for processors that do not support the HWPfeature or when the intel_pstate=no_hwp argument is passed to the kernel inthe command line. The active mode is used in those cases if theintel_pstate=active argument is passed to the kernel in the command line.In this mode intel_pstate may refuse to work with processors that are notrecognized by it. [Note that intel_pstate will never refuse to work withany processor with the HWP feature enabled.]

In this mode intel_pstate registers utilization update callbacks with theCPU scheduler in order to run a P-state selection algorithm, eitherpowersave or performance, depending on the scaling_governor policysetting in sysfs. The current CPU frequency information to be madeavailable from the scaling_cur_freq policy attribute in sysfs isperiodically updated by those utilization update callbacks too.

performance¶

Without HWP, this P-state selection algorithm is always the same regardless ofthe processor model and platform configuration.

It selects the maximum P-state it is allowed to use, subject to limits set viasysfs, every time the driver configuration for the given CPU is updated(e.g. via sysfs).

This is the default P-state selection algorithm if theCONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE kernel configuration optionis set.

powersave¶

Without HWP, this P-state selection algorithm is similar to the algorithmimplemented by the generic schedutil scaling governor except that theutilization metric used by it is based on numbers coming from feedbackregisters of the CPU. It generally selects P-states proportional to thecurrent CPU utilization.

This algorithm is run by the driver’s utilization update callback for thegiven CPU when it is invoked by the CPU scheduler, but not more often thanevery 10 ms. Like in the performance case, the hardware configurationis not touched if the new P-state turns out to be the same as the currentone.

This is the default P-state selection algorithm if theCONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE kernel configuration optionis not set.

Passive Mode¶

This is the default operation mode of intel_pstate for processors withouthardware-managed P-states (HWP) support. It is always used if theintel_pstate=passive argument is passed to the kernel in the command lineregardless of whether or not the given processor supports HWP. [Note that theintel_pstate=no_hwp setting causes the driver to start in the passive modeif it is not combined with intel_pstate=active.] Like in the active modewithout HWP support, in this mode intel_pstate may refuse to work withprocessors that are not recognized by it if HWP is prevented from being enabledthrough the kernel command line.

If the driver works in this mode, the scaling_driver policy attribute insysfs for all CPUFreq policies contains the string “intel_cpufreq”.Then, the driver behaves like a regular CPUFreq scaling driver. That is,it is invoked by generic scaling governors when necessary to talk to thehardware in order to change the P-state of a CPU (in particular, theschedutil governor can invoke it directly from scheduler context).

While in this mode, intel_pstate can be used with all of the (generic)scaling governors listed by the scaling_available_governors policy attributein sysfs (and the P-state selection algorithms described above are notused). Then, it is responsible for the configuration of policy objectscorresponding to CPUs and provides the CPUFreq core (and the scalinggovernors attached to the policy objects) with accurate information on themaximum and minimum operating frequencies supported by the hardware (includingthe so-called “turbo” frequency ranges). In other words, in the passive modethe entire range of available P-states is exposed by intel_pstate to theCPUFreq core. However, in this mode the driver does not registerutilization update callbacks with the CPU scheduler and the scaling_cur_freqinformation comes from the CPUFreq core (and is the last frequency selectedby the current scaling governor for the given policy).

Turbo P-states Support¶

In the majority of cases, the entire range of P-states available tointel_pstate can be divided into two sub-ranges that correspond todifferent types of processor behavior, above and below a boundary thatwill be referred to as the “turbo threshold” in what follows.

The P-states above the turbo threshold are referred to as “turbo P-states” andthe whole sub-range of P-states they belong to is referred to as the “turborange”. These names are related to the Turbo Boost technology allowing amulticore processor to opportunistically increase the P-state of one or morecores if there is enough power to do that and if that is not going to cause thethermal envelope of the processor package to be exceeded.

Specifically, if software sets the P-state of a CPU core within the turbo range(that is, above the turbo threshold), the processor is permitted to take overperformance scaling control for that core and put it into turbo P-states of itschoice going forward. However, that permission is interpreted differently bydifferent processor generations. Namely, the Sandy Bridge generation ofprocessors will never use any P-states above the last one set by software forthe given core, even if it is within the turbo range, whereas all of the laterprocessor generations will take it as a license to use any P-states from theturbo range, even above the one set by software. In other words, on thoseprocessors setting any P-state from the turbo range will enable the processorto put the given core into all turbo P-states up to and including the maximumsupported one as it sees fit.

One important property of turbo P-states is that they are not sustainable. Moreprecisely, there is no guarantee that any CPUs will be able to stay in any ofthose states indefinitely, because the power distribution within the processorpackage may change over time or the thermal envelope it was designed for mightbe exceeded if a turbo P-state was used for too long.

In turn, the P-states below the turbo threshold generally are sustainable. Infact, if one of them is set by software, the processor is not expected to changeit to a lower one unless in a thermal stress or a power limit violationsituation (a higher P-state may still be used if it is set for another CPU inthe same package at the same time, for example).

Some processors allow multiple cores to be in turbo P-states at the same time,but the maximum P-state that can be set for them generally depends on the numberof cores running concurrently. The maximum turbo P-state that can be set for 3cores at the same time usually is lower than the analogous maximum P-state for2 cores, which in turn usually is lower than the maximum turbo P-state that canbe set for 1 core. The one-core maximum turbo P-state is thus the maximumsupported one overall.

The maximum supported turbo P-state, the turbo threshold (the maximum supportednon-turbo P-state) and the minimum supported P-state are specific to theprocessor model and can be determined by reading the processor’s model-specificregisters (MSRs). Moreover, some processors support the Configurable TDP(Thermal Design Power) feature and, when that feature is enabled, the turbothreshold effectively becomes a configurable value that can be set by theplatform firmware.

Unlike _PSS objects in the ACPI tables, intel_pstate always exposesthe entire range of available P-states, including the whole turbo range, to theCPUFreq core and (in the passive mode) to generic scaling governors. Thisgenerally causes turbo P-states to be set more often when intel_pstate isused relative to ACPI-based CPU performance scaling (see belowfor more information).

Moreover, since intel_pstate always knows what the real turbo threshold is(even if the Configurable TDP feature is enabled in the processor), itsno_turbo attribute in sysfs (described below) shouldwork as expected in all cases (that is, if set to disable turbo P-states, italways should prevent intel_pstate from using them).

Processor Support¶

To handle a given processor intel_pstate requires a number of differentpieces of information on it to be known, including:

• The minimum supported P-state.
• The maximum supported non-turbo P-state.
• Whether or not turbo P-states are supported at all.
• The maximum supported one-core turbo P-state (if turbo P-statesare supported).
• The scaling formula to translate the driver’s internal representationof P-states into frequencies and the other way around.

Generally, ways to obtain that information are specific to the processor modelor family. Although it often is possible to obtain all of it from the processoritself (using model-specific registers), there are cases in which hardwaremanuals need to be consulted to get to it too.

For this reason, there is a list of supported processors in intel_pstate andthe driver initialization will fail if the detected processor is not in thatlist, unless it supports the HWP feature. [The interface to obtain all of theinformation listed above is the same for all of the processors supporting theHWP feature, which is why intel_pstate works with all of them.]

User Space Interface in sysfs¶

Global Attributes¶

intel_pstate exposes several global attributes (files) in sysfs tocontrol its functionality at the system level. They are located in the/sys/devices/system/cpu/intel_pstate/ directory and affect all CPUs.

Some of them are not present if the intel_pstate=per_cpu_perf_limitsargument is passed to the kernel in the command line.

Maximum P-state the driver is allowed to set in percent of themaximum supported performance level (the highest supported turboP-state).

This attribute will not be exposed if theintel_pstate=per_cpu_perf_limits argument is present in the kernelcommand line.

Minimum P-state the driver is allowed to set in percent of themaximum supported performance level (the highest supported turboP-state).

This attribute will not be exposed if theintel_pstate=per_cpu_perf_limits argument is present in the kernelcommand line.

Number of P-states supported by the processor (between 0 and 255inclusive) including both turbo and non-turbo P-states (seeTurbo P-states Support).

The value of this attribute is not affected by the no_turbosetting described below.

This attribute is read-only.

Ratio of the turbo range size to the size of the entirerange of supported P-states, in percent.

This attribute is read-only.

If set (equal to 1), the driver is not allowed to set any turbo P-states(see Turbo P-states Support). If unset (equal to 0, which is thedefault), turbo P-states can be set by the driver.[Note that intel_pstate does not support the general boostattribute (supported by some other scaling drivers) which is replacedby this one.]

This attribute does not affect the maximum supported frequency valuesupplied to the CPUFreq core and exposed via the policy interface,but it affects the maximum possible value of per-policy P-state limits(see Interpretation of Policy Attributes below for details).

This attribute is only present if intel_pstate works in theactive mode with the HWP feature enabled inthe processor. If set (equal to 1), it causes the minimum P-state limitto be increased dynamically for a short time whenever a task previouslywaiting on I/O is selected to run on a given logical CPU (the purposeof this mechanism is to improve performance).

This setting has no effect on logical CPUs whose minimum P-state limitis directly set to the highest non-turbo P-state or above it.

Operation mode of the driver: “active”, “passive” or “off”.

“active”

The driver is functional and in the active mode.

“passive”

The driver is functional and in the passive mode.

“off”

The driver is not functional (it is not registered as a scalingdriver with the CPUFreq core).

This attribute can be written to in order to change the driver’soperation mode or to unregister it. The string written to it must beone of the possible values of it and, if successful, the write willcause the driver to switch over to the operation mode represented bythat string - or to be unregistered in the “off” case. [Actually,switching over from the active mode to the passive mode or the otherway around causes the driver to be unregistered and registered againwith a different set of callbacks, so all of its settings (the globalas well as the per-policy ones) are then reset to their defaultvalues, possibly depending on the target operation mode.]

Interpretation of Policy Attributes¶

The interpretation of some CPUFreq policy attributes described inCPU Performance Scaling is special with intel_pstate as the current scaling driverand it generally depends on the driver’s operation mode.

First of all, the values of the cpuinfo_max_freq, cpuinfo_min_freq andscaling_cur_freq attributes are produced by applying a processor-specificmultiplier to the internal P-state representation used by intel_pstate.Also, the values of the scaling_max_freq and scaling_min_freqattributes are capped by the frequency corresponding to the maximum P-state thatthe driver is allowed to set.

If the no_turboglobal attribute is set, the driver isnot allowed to use turbo P-states, so the maximum value of scaling_max_freqand scaling_min_freq is limited to the maximum non-turbo P-state frequency.Accordingly, setting no_turbo causes scaling_max_freq andscaling_min_freq to go down to that value if they were above it before.However, the old values of scaling_max_freq and scaling_min_freq will berestored after unsetting no_turbo, unless these attributes have been writtento after no_turbo was set.

If no_turbo is not set, the maximum possible value of scaling_max_freqand scaling_min_freq corresponds to the maximum supported turbo P-state,which also is the value of cpuinfo_max_freq in either case.

Next, the following policy attributes have special meaning ifintel_pstate works in the active mode:

scaling_available_governors

List of P-state selection algorithms provided by intel_pstate.

scaling_governor

P-state selection algorithm provided by intel_pstate currently inuse with the given policy.

scaling_cur_freq

Frequency of the average P-state of the CPU represented by the givenpolicy for the time interval between the last two invocations of thedriver’s utilization update callback by the CPU scheduler for that CPU.

One more policy attribute is present if the HWP feature is enabled in theprocessor:

base_frequency

Shows the base frequency of the CPU. Any frequency above this will bein the turbo frequency range.

The meaning of these attributes in the passive mode is thesame as for other scaling drivers.

Additionally, the value of the scaling_driver attribute for intel_pstatedepends on the operation mode of the driver. Namely, it is either“intel_pstate” (in the active mode) or “intel_cpufreq” (in thepassive mode).

Driver Octoplus

Coordination of P-State Limits¶

intel_pstate allows P-state limits to be set in two ways: with the help ofthe max_perf_pct and min_perf_pctglobal attributes or via the scaling_max_freq and scaling_min_freqCPUFreq policy attributes. The coordination between those limits is basedon the following rules, regardless of the current operation mode of the driver:

1. All CPUs are affected by the global limits (that is, none of them can berequested to run faster than the global maximum and none of them can berequested to run slower than the global minimum).
2. Each individual CPU is affected by its own per-policy limits (that is, itcannot be requested to run faster than its own per-policy maximum and itcannot be requested to run slower than its own per-policy minimum). Theeffective performance depends on whether the platform supports per coreP-states, hyper-threading is enabled and on current performance requestsfrom other CPUs. When platform doesn’t support per core P-states, theeffective performance can be more than the policy limits set on a CPU, ifother CPUs are requesting higher performance at that moment. Even with percore P-states support, when hyper-threading is enabled, if the sibling CPUis requesting higher performance, the other siblings will get higherperformance than their policy limits.
3. The global and per-policy limits can be set independently.

In the active mode with the HWP feature enabled, theresulting effective values are written into hardware registers whenever thelimits change in order to request its internal P-state selection logic to alwaysset P-states within these limits. Otherwise, the limits are taken into accountby scaling governors (in the passive mode) and by the driverevery time before setting a new P-state for a CPU.

Additionally, if the intel_pstate=per_cpu_perf_limits command line argumentis passed to the kernel, max_perf_pct and min_perf_pct are not exposedat all and the only way to set the limits is by using the policy attributes.

Energy vs Performance Hints¶

If the hardware-managed P-states (HWP) is enabled in the processor, additionalattributes, intended to allow user space to help intel_pstate to adjust theprocessor’s internal P-state selection logic by focusing it on performance or onenergy-efficiency, or somewhere between the two extremes, are present in everyCPUFreq policy directory in sysfs. They are :

Current value of the energy vs performance hint for the given policy(or the CPU represented by it).

The hint can be changed by writing to this attribute.

List of strings that can be written to theenergy_performance_preference attribute.

They represent different energy vs performance hints and should beself-explanatory, except that default represents whatever hintvalue was set by the platform firmware.

Strings written to the energy_performance_preference attribute areinternally translated to integer values written to the processor’sEnergy-Performance Preference (EPP) knob (if supported) or itsEnergy-Performance Bias (EPB) knob. It is also possible to write a positiveinteger value between 0 to 255, if the EPP feature is present. If the EPPfeature is not present, writing integer value to this attribute is notsupported. In this case, user can use the“/sys/devices/system/cpu/cpu*/power/energy_perf_bias” interface.

[Note that tasks may by migrated from one CPU to another by the scheduler’sload-balancing algorithm and if different energy vs performance hints areset for those CPUs, that may lead to undesirable outcomes. To avoid suchissues it is better to set the same energy vs performance hint for all CPUsor to pin every task potentially sensitive to them to a specific CPU.]

intel_pstate vs acpi-cpufreq¶

On the majority of systems supported by intel_pstate, the ACPI tablesprovided by the platform firmware contain _PSS objects returning informationthat can be used for CPU performance scaling (refer to the ACPI specification[3] for details on the _PSS objects and the format of the informationreturned by them).

The information returned by the ACPI _PSS objects is used by theacpi-cpufreq scaling driver. On systems supported by intel_pstatethe acpi-cpufreq driver uses the same hardware CPU performance scalinginterface, but the set of P-states it can use is limited by the _PSSoutput.

On those systems each _PSS object returns a list of P-states supported bythe corresponding CPU which basically is a subset of the P-states range that canbe used by intel_pstate on the same system, with one exception: the wholeturbo range is represented by one item in it (the topmost one). Byconvention, the frequency returned by _PSS for that item is greater by 1 MHzthan the frequency of the highest non-turbo P-state listed by it, but thecorresponding P-state representation (following the hardware specification)returned for it matches the maximum supported turbo P-state (or is thespecial value 255 meaning essentially “go as high as you can get”).

The list of P-states returned by _PSS is reflected by the table ofavailable frequencies supplied by acpi-cpufreq to the CPUFreq core andscaling governors and the minimum and maximum supported frequencies reported byit come from that list as well. In particular, given the special representationof the turbo range described above, this means that the maximum supportedfrequency reported by acpi-cpufreq is higher by 1 MHz than the frequencyof the highest supported non-turbo P-state listed by _PSS which, of course,affects decisions made by the scaling governors, except for powersave andperformance.

For example, if a given governor attempts to select a frequency proportional toestimated CPU load and maps the load of 100% to the maximum supported frequency(possibly multiplied by a constant), then it will tend to choose P-states belowthe turbo threshold if acpi-cpufreq is used as the scaling driver, becausein that case the turbo range corresponds to a small fraction of the frequencyband it can use (1 MHz vs 1 GHz or more). In consequence, it will only go tothe turbo range for the highest loads and the other loads above 50% that mightbenefit from running at turbo frequencies will be given non-turbo P-statesinstead.

One more issue related to that may appear on systems supporting theConfigurable TDP feature allowing the platform firmware to set theturbo threshold. Namely, if that is not coordinated with the lists of P-statesreturned by _PSS properly, there may be more than one item corresponding toa turbo P-state in those lists and there may be a problem with avoiding theturbo range (if desirable or necessary). Usually, to avoid using turboP-states overall, acpi-cpufreq simply avoids using the topmost state listedby _PSS, but that is not sufficient when there are other turbo P-states inthe list returned by it.

Apart from the above, acpi-cpufreq works like intel_pstate in thepassive mode, except that the number of P-states it can setis limited to the ones listed by the ACPI _PSS objects.

Kernel Command Line Options for intel_pstate¶

Several kernel command line options can be used to pass early-configuration-timeparameters to intel_pstate in order to enforce specific behavior of it. Allof them have to be prepended with the intel_pstate= prefix.

Register intel_pstate as the scaling driver instead ofacpi-cpufreq even if the latter is preferred on the given system.

This may prevent some platform features (such as thermal controls andpower capping) that rely on the availability of ACPI P-statesinformation from functioning as expected, so it should be used withcaution.

This option does not work with processors that are not supported byintel_pstate and on platforms where the pcc-cpufreq scalingdriver is used instead of acpi-cpufreq.

Pct2785 Drivers

Take ACPI _PPC performance limits into account.

If the preferred power management profile in the FADT (Fixed ACPIDescription Table) is set to “Enterprise Server” or “PerformanceServer”, the ACPI _PPC limits are taken into account by defaultand this option has no effect.

Diagnostics and Tuning¶

Trace Events¶

There are two static trace events that can be used for intel_pstatediagnostics. One of them is the cpu_frequency trace event generally usedby CPUFreq, and the other one is the pstate_sample trace event specificto intel_pstate. Both of them are triggered by intel_pstate only ifit works in the active mode.

The following sequence of shell commands can be used to enable them and seetheir output (if the kernel is generally configured to support event tracing):

If intel_pstate works in the passive mode, thecpu_frequency trace event will be triggered either by the schedutilscaling governor (for the policies it is attached to), or by the CPUFreqcore (for the policies with other scaling governors).

ftrace¶

The ftrace interface can be used for low-level diagnostics ofintel_pstate. For example, to check how often the function to set aP-state is called, the ftrace filter can be set tointel_pstate_set_pstate():

References¶