Linux supported с states

System Sleep States¶

Sleep states are global low-power states of the entire system in which user space code cannot be executed and the overall system activity is significantly reduced.

Sleep States That Can Be Supported¶

Depending on its configuration and the capabilities of the platform it runs on, the Linux kernel can support up to four system sleep states, includig hibernation and up to three variants of system suspend. The sleep states that can be supported by the kernel are listed below.

Suspend-to-Idle¶

This is a generic, pure software, light-weight variant of system suspend (also referred to as S2I or S2Idle). It allows more energy to be saved relative to runtime idle by freezing user space, suspending the timekeeping and putting all I/O devices into low-power states (possibly lower-power than available in the working state), such that the processors can spend time in their deepest idle states while the system is suspended.

The system is woken up from this state by in-band interrupts, so theoretically any devices that can cause interrupts to be generated in the working state can also be set up as wakeup devices for S2Idle.

This state can be used on platforms without support for standby or suspend-to-RAM , or it can be used in addition to any of the deeper system suspend variants to provide reduced resume latency. It is always supported if the CONFIG_SUSPEND kernel configuration option is set.

Standby¶

This state, if supported, offers moderate, but real, energy savings, while providing a relatively straightforward transition back to the working state. No operating state is lost (the system core logic retains power), so the system can go back to where it left off easily enough.

In addition to freezing user space, suspending the timekeeping and putting all I/O devices into low-power states, which is done for suspend-to-idle too, nonboot CPUs are taken offline and all low-level system functions are suspended during transitions into this state. For this reason, it should allow more energy to be saved relative to suspend-to-idle , but the resume latency will generally be greater than for that state.

The set of devices that can wake up the system from this state usually is reduced relative to suspend-to-idle and it may be necessary to rely on the platform for setting up the wakeup functionality as appropriate.

This state is supported if the CONFIG_SUSPEND kernel configuration option is set and the support for it is registered by the platform with the core system suspend subsystem. On ACPI-based systems this state is mapped to the S1 system state defined by ACPI.

Suspend-to-RAM¶

This state (also referred to as STR or S2RAM), if supported, offers significant energy savings as everything in the system is put into a low-power state, except for memory, which should be placed into the self-refresh mode to retain its contents. All of the steps carried out when entering standby are also carried out during transitions to S2RAM. Additional operations may take place depending on the platform capabilities. In particular, on ACPI-based systems the kernel passes control to the platform firmware (BIOS) as the last step during S2RAM transitions and that usually results in powering down some more low-level components that are not directly controlled by the kernel.

The state of devices and CPUs is saved and held in memory. All devices are suspended and put into low-power states. In many cases, all peripheral buses lose power when entering S2RAM, so devices must be able to handle the transition back to the “on” state.

On ACPI-based systems S2RAM requires some minimal boot-strapping code in the platform firmware to resume the system from it. This may be the case on other platforms too.

The set of devices that can wake up the system from S2RAM usually is reduced relative to suspend-to-idle and standby and it may be necessary to rely on the platform for setting up the wakeup functionality as appropriate.

S2RAM is supported if the CONFIG_SUSPEND kernel configuration option is set and the support for it is registered by the platform with the core system suspend subsystem. On ACPI-based systems it is mapped to the S3 system state defined by ACPI.

Hibernation¶

This state (also referred to as Suspend-to-Disk or STD) offers the greatest energy savings and can be used even in the absence of low-level platform support for system suspend. However, it requires some low-level code for resuming the system to be present for the underlying CPU architecture.

Hibernation is significantly different from any of the system suspend variants. It takes three system state changes to put it into hibernation and two system state changes to resume it.

First, when hibernation is triggered, the kernel stops all system activity and creates a snapshot image of memory to be written into persistent storage. Next, the system goes into a state in which the snapshot image can be saved, the image is written out and finally the system goes into the target low-power state in which power is cut from almost all of its hardware components, including memory, except for a limited set of wakeup devices.

Once the snapshot image has been written out, the system may either enter a special low-power state (like ACPI S4), or it may simply power down itself. Powering down means minimum power draw and it allows this mechanism to work on any system. However, entering a special low-power state may allow additional means of system wakeup to be used (e.g. pressing a key on the keyboard or opening a laptop lid).

After wakeup, control goes to the platform firmware that runs a boot loader which boots a fresh instance of the kernel (control may also go directly to the boot loader, depending on the system configuration, but anyway it causes a fresh instance of the kernel to be booted). That new instance of the kernel (referred to as the restore kernel ) looks for a hibernation image in persistent storage and if one is found, it is loaded into memory. Next, all activity in the system is stopped and the restore kernel overwrites itself with the image contents and jumps into a special trampoline area in the original kernel stored in the image (referred to as the image kernel ), which is where the special architecture-specific low-level code is needed. Finally, the image kernel restores the system to the pre-hibernation state and allows user space to run again.

Hibernation is supported if the CONFIG_HIBERNATION kernel configuration option is set. However, this option can only be set if support for the given CPU architecture includes the low-level code for system resume.

Basic sysfs Interfaces for System Suspend and Hibernation¶

The following files located in the /sys/power/ directory can be used by user space for sleep states control.

This file contains a list of strings representing sleep states supported by the kernel. Writing one of these strings into it causes the kernel to start a transition of the system into the sleep state represented by that string.

In particular, the strings “disk”, “freeze” and “standby” represent the hibernation , suspend-to-idle and standby sleep states, respectively. The string “mem” is interpreted in accordance with the contents of the mem_sleep file described below.

If the kernel does not support any system sleep states, this file is not present.

This file contains a list of strings representing supported system suspend variants and allows user space to select the variant to be associated with the “mem” string in the state file described above.

The strings that may be present in this file are “s2idle”, “shallow” and “deep”. The string “s2idle” always represents suspend-to-idle and, by convention, “shallow” and “deep” represent standby and suspend-to-RAM , respectively.

Writing one of the listed strings into this file causes the system suspend variant represented by it to be associated with the “mem” string in the state file. The string representing the suspend variant currently associated with the “mem” string in the state file is listed in square brackets.

If the kernel does not support system suspend, this file is not present.

This file contains a list of strings representing different operations that can be carried out after the hibernation image has been saved. The possible options are as follows:

platform Put the system into a special low-power state (e.g. ACPI S4) to make additional wakeup options available and possibly allow the platform firmware to take a simplified initialization path after wakeup. shutdown Power off the system. reboot Reboot the system (useful for diagnostics mostly). suspend Hybrid system suspend. Put the system into the suspend sleep state selected through the mem_sleep file described above. If the system is successfully woken up from that state, discard the hibernation image and continue. Otherwise, use the image to restore the previous state of the system. test_resume Diagnostic operation. Load the image as though the system had just woken up from hibernation and the currently running kernel instance was a restore kernel and follow up with full system resume.

Writing one of the listed strings into this file causes the option represented by it to be selected.

The currently selected option is shown in square brackets which means that the operation represented by it will be carried out after creating and saving the image next time hibernation is triggered by writing disk to /sys/power/state .

If the kernel does not support hibernation, this file is not present.

According to the above, there are two ways to make the system go into the suspend-to-idle state. The first one is to write “freeze” directly to /sys/power/state . The second one is to write “s2idle” to /sys/power/mem_sleep and then to write “mem” to /sys/power/state . Likewise, there are two ways to make the system go into the standby state (the strings to write to the control files in that case are “standby” or “shallow” and “mem”, respectively) if that state is supported by the platform. However, there is only one way to make the system go into the suspend-to-RAM state (write “deep” into /sys/power/mem_sleep and “mem” into /sys/power/state ).

The default suspend variant (ie. the one to be used without writing anything into /sys/power/mem_sleep ) is either “deep” (on the majority of systems supporting suspend-to-RAM ) or “s2idle”, but it can be overridden by the value of the “mem_sleep_default” parameter in the kernel command line. On some ACPI-based systems, depending on the information in the ACPI tables, the default may be “s2idle” even if suspend-to-RAM is supported.

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intel_pstate CPU Performance Scaling Driver¶

General Information¶

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

For the processors supported by intel_pstate , the P-state concept is broader than just an operating frequency or an operating performance point (see the LinuxCon Europe 2015 presentation by Kristen Accardi for more information about that). For this reason, the representation of P-states used by intel_pstate internally follows the hardware specification (for details refer to Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3: System Programming Guide). However, the CPUFreq core uses frequencies for identifying operating performance points of CPUs and frequencies are involved in the user space interface exposed by it, so intel_pstate maps its internal representation of P-states to frequencies too (fortunately, that mapping is unambiguous). At the same time, it would not be practical for intel_pstate to supply the CPUFreq core with a table of available frequencies due to the possible size of it, so the driver does not do that. Some functionality of the core is limited by that.

Since the hardware P-state selection interface used by intel_pstate is available at the logical CPU level, the driver always works with individual CPUs. Consequently, if intel_pstate is in use, every CPUFreq policy object corresponds to one logical CPU and CPUFreq policies are effectively equivalent to CPUs. In particular, this means that they become “inactive” every time the corresponding CPU is taken offline and need to be re-initialized when it goes back online.

intel_pstate is not modular, so it cannot be unloaded, which means that the only way to pass early-configuration-time parameters to it is via the kernel command line. However, its configuration can be adjusted via sysfs to a great extent. In some configurations it even is possible to unregister it via sysfs which allows another CPUFreq scaling driver to be loaded and registered (see below).

Operation Modes¶

intel_pstate can operate in three different modes: in the active mode with or without hardware-managed P-states support and in the passive mode. Which of them will be in effect depends on what kernel command line options are used and on the capabilities of the processor.

Active Mode¶

This is the default operation mode of intel_pstate . If it works in this mode, the scaling_driver policy attribute in sysfs for all CPUFreq policies contains the string “intel_pstate”.

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

They are not generic scaling governors, but their names are the same as the names of some of those governors. Moreover, confusingly enough, they generally do not work in the same way as the generic governors they share the names with. For example, the powersave P-state selection algorithm provided by intel_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 the active mode: powersave and performance . The way they both operate depends on whether or not the hardware-managed P-states (HWP) feature has been enabled in the processor and possibly on the processor model.

Which of the P-state selection algorithms is used by default depends on the CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE kernel configuration option. Namely, if that option is set, the performance algorithm will be used by default, 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 the processor initialization and cannot be disabled after that. It is possible to avoid enabling it by passing the intel_pstate=no_hwp argument to the kernel in the command line.

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

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

HWP + performance В¶

In this configuration intel_pstate will write 0 to the processor’s Energy-Performance Preference (EPP) knob (if supported) or its Energy-Performance Bias (EPB) knob (otherwise), which means that the processor’s internal 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).

Also, in this configuration the range of P-states available to the processor’s internal 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 В¶

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

Active Mode Without HWP¶

This is the default operation mode for processors that do not support the HWP feature. It also is used by default with the intel_pstate=no_hwp argument in the kernel command line. However, in this mode intel_pstate may refuse to work with the given processor if it does not recognize it. [Note that intel_pstate will never refuse to work with any processor with the HWP feature enabled.]

In this mode intel_pstate registers utilization update callbacks with the CPU scheduler in order to run a P-state selection algorithm, either powersave or performance , depending on the scaling_cur_freq policy setting in sysfs . The current CPU frequency information to be made available from the scaling_cur_freq policy attribute in sysfs is periodically updated by those utilization update callbacks too.

performance В¶

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

It selects the maximum P-state it is allowed to use, subject to limits set via sysfs , every time the P-state selection computations are carried out by the driver’s utilization update callback for the given CPU (that does not happen more often than every 10 ms), but the hardware configuration will not be changed if the new P-state is the same as the current one.

This is the default P-state selection algorithm if the CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE kernel configuration option is set.

powersave В¶

Without HWP, this P-state selection algorithm generally depends on the processor model and/or the system profile setting in the ACPI tables and there are two variants of it.

One of them is used with processors from the Atom line and (regardless of the processor model) on platforms with the system profile in the ACPI tables set to “mobile” (laptops mostly), “tablet”, “appliance PC”, “desktop”, or “workstation”. It is also used with processors supporting the HWP feature if that feature has not been enabled (that is, with the intel_pstate=no_hwp argument in the kernel command line). It is similar to the algorithm implemented by the generic schedutil scaling governor except that the utilization metric used by it is based on numbers coming from feedback registers of the CPU. It generally selects P-states proportional to the current CPU utilization, so it is referred to as the “proportional” algorithm.

The second variant of the powersave P-state selection algorithm, used in all of the other cases (generally, on processors from the Core line, so it is referred to as the “Core” algorithm), is based on the values read from the APERF and MPERF feedback registers and the previously requested target P-state. It does not really take CPU utilization into account explicitly, but as a rule it causes the CPU P-state to ramp up very quickly in response to increased utilization which is generally desirable in server environments.

Regardless of the variant, this algorithm is run by the driver’s utilization update callback for the given CPU when it is invoked by the CPU scheduler, but not more often than every 10 ms (that can be tweaked via debugfs in this particular case). Like in the performance case, the hardware configuration is not touched if the new P-state turns out to be the same as the current one.

This is the default P-state selection algorithm if the CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE kernel configuration option is not set.

Passive Mode¶

This mode is used if the intel_pstate=passive argument is passed to the kernel in the command line (it implies the intel_pstate=no_hwp setting too). Like in the active mode without HWP support, in this mode intel_pstate may refuse to work with the given processor if it does not recognize it.

If the driver works in this mode, the scaling_driver policy attribute in sysfs 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 the hardware in order to change the P-state of a CPU (in particular, the schedutil 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 attribute in sysfs (and the P-state selection algorithms described above are not used). 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 (including the so-called “turbo” frequency ranges). In other words, in the passive mode the entire range of available P-states is exposed by intel_pstate to the CPUFreq core. However, in this mode the driver does not register utilization update callbacks with the CPU scheduler and the scaling_cur_freq information comes from the CPUFreq core (and is the last frequency selected by the current scaling governor for the given policy).

Turbo P-states Support¶

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

The P-states above the turbo threshold are referred to as “turbo P-states” and the whole sub-range of P-states they belong to is referred to as the “turbo range”. These names are related to the Turbo Boost technology allowing a multicore processor to opportunistically increase the P-state of one or more cores if there is enough power to do that and if that is not going to cause the thermal 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 over performance scaling control for that core and put it into turbo P-states of its choice going forward. However, that permission is interpreted differently by different processor generations. Namely, the Sandy Bridge generation of processors will never use any P-states above the last one set by software for the given core, even if it is within the turbo range, whereas all of the later processor generations will take it as a license to use any P-states from the turbo range, even above the one set by software. In other words, on those processors setting any P-state from the turbo range will enable the processor to put the given core into all turbo P-states up to and including the maximum supported one as it sees fit.

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

In turn, the P-states below the turbo threshold generally are sustainable. In fact, if one of them is set by software, the processor is not expected to change it to a lower one unless in a thermal stress or a power limit violation situation (a higher P-state may still be used if it is set for another CPU in the 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 number of cores running concurrently. The maximum turbo P-state that can be set for 3 cores at the same time usually is lower than the analogous maximum P-state for 2 cores, which in turn usually is lower than the maximum turbo P-state that can be set for 1 core. The one-core maximum turbo P-state is thus the maximum supported one overall.

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

Unlike _PSS objects in the ACPI tables, intel_pstate always exposes the entire range of available P-states, including the whole turbo range, to the CPUFreq core and (in the passive mode) to generic scaling governors. This generally causes turbo P-states to be set more often when intel_pstate is used relative to ACPI-based CPU performance scaling (see below for 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), its no_turbo attribute in sysfs (described below) should work as expected in all cases (that is, if set to disable turbo P-states, it always should prevent intel_pstate from using them).

Processor Support¶

To handle a given processor intel_pstate requires a number of different pieces 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-states are supported).
• The scaling formula to translate the driver’s internal representation of P-states into frequencies and the other way around.

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

For this reason, there is a list of supported processors in intel_pstate and the driver initialization will fail if the detected processor is not in that list, unless it supports the HWP feature. [The interface to obtain all of the information listed above is the same for all of the processors supporting the HWP feature, which is why they all are supported by intel_pstate .]

User Space Interface in sysfs В¶

Global Attributes¶

intel_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/intel_pstate/ directory and affect all CPUs.

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

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

This attribute will not be exposed if the intel_pstate=per_cpu_perf_limits argument is present in the kernel command line.

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

This attribute will not be exposed if the intel_pstate=per_cpu_perf_limits argument is present in the kernel command line.

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

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

This attribute is read-only.

Ratio of the turbo range size to the size of the entire range 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 (equalt to 0, which is the default), turbo P-states can be set by the driver. [Note that intel_pstate does not support the general boost attribute (supported by some other scaling drivers) which is replaced by this one.]

This attrubute does not affect the maximum supported frequency value supplied 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).

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 scaling driver with the CPUFreq core).

This attribute can be written to in order to change the driver’s operation mode or to unregister it. The string written to it must be one of the possible values of it and, if successful, the write will cause the driver to switch over to the operation mode represented by that string — or to be unregistered in the “off” case. [Actually, switching over from the active mode to the passive mode or the other way around causes the driver to be unregistered and registered again with a different set of callbacks, so all of its settings (the global as well as the per-policy ones) are then reset to their default values, possibly depending on the target operation mode.]

That only is supported in some configurations, though (for example, if the HWP feature is enabled in the processor, the operation mode of the driver cannot be changed), and if it is not supported in the current configuration, writes to this attribute with fail with an appropriate error.

Interpretation of Policy Attributes¶

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

First of all, the values of the cpuinfo_max_freq , cpuinfo_min_freq and scaling_cur_freq attributes are produced by applying a processor-specific multiplier to the internal P-state representation used by intel_pstate . Also, the values of the scaling_max_freq and scaling_min_freq attributes are capped by the frequency corresponding to the maximum P-state that the driver is allowed to set.

If the no_turbo global attribute is set, the driver is not allowed to use turbo P-states, so the maximum value of scaling_max_freq and scaling_min_freq is limited to the maximum non-turbo P-state frequency. Accordingly, setting no_turbo causes scaling_max_freq and scaling_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 be restored after unsetting no_turbo , unless these attributes have been written to after no_turbo was set.

If no_turbo is not set, the maximum possible value of scaling_max_freq and 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 if intel_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 in use with the given policy. scaling_cur_freq Frequency of the average P-state of the CPU represented by the given policy for the time interval between the last two invocations of the driver’s utilization update callback by the CPU scheduler for that CPU.

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

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

Coordination of P-State Limits¶

intel_pstate allows P-state limits to be set in two ways: with the help of the max_perf_pct and min_perf_pct global attributes or via the scaling_max_freq and scaling_min_freq CPUFreq policy attributes. The coordination between those limits is based on 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 be requested to run faster than the global maximum and none of them can be requested to run slower than the global minimum).
2. Each individual CPU is affected by its own per-policy limits (that is, it cannot be requested to run faster than its own per-policy maximum and it cannot be requested to run slower than its own per-policy minimum).
3. The global and per-policy limits can be set independently.

If the HWP feature is enabled in the processor, the resulting effective values are written into its registers whenever the limits change in order to request its internal P-state selection logic to always set P-states within these limits. Otherwise, the limits are taken into account by scaling governors (in the passive mode) and by the driver every time before setting a new P-state for a CPU.

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

Energy vs Performance Hints¶

If intel_pstate works in the active mode with the HWP feature enabled in the processor, additional attributes are present in every CPUFreq policy directory in sysfs . They are intended to allow user space to help intel_pstate to adjust the processor’s internal P-state selection logic by focusing it on performance or on energy-efficiency, or somewhere between the two extremes:

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 the energy_performance_preference attribute.

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

Strings written to the energy_performance_preference attribute are internally translated to integer values written to the processor’s Energy-Performance Preference (EPP) knob (if supported) or its Energy-Performance Bias (EPB) knob.

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

intel_pstate vs acpi-cpufreq В¶

On the majority of systems supported by intel_pstate , the ACPI tables provided by the platform firmware contain _PSS objects returning information that can be used for CPU performance scaling (refer to the ACPI specification for details on the _PSS objects and the format of the information returned by them).

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

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

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

For example, if a given governor attempts to select a frequency proportional to estimated 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 below the turbo threshold if acpi-cpufreq is used as the scaling driver, because in that case the turbo range corresponds to a small fraction of the frequency band it can use (1 MHz vs 1 GHz or more). In consequence, it will only go to the turbo range for the highest loads and the other loads above 50% that might benefit from running at turbo frequencies will be given non-turbo P-states instead.

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

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

Kernel Command Line Options for intel_pstate В¶

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

disable Do not register intel_pstate as the scaling driver even if the processor is supported by it. passive

Register intel_pstate in the passive mode to start with.

This option implies the no_hwp one described below.

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

This may prevent some platform features (such as thermal controls and power capping) that rely on the availability of ACPI P-states information from functioning as expected, so it should be used with caution.

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

no_hwp Do not enable the hardware-managed P-states (HWP) feature even if it is supported by the processor. hwp_only Register intel_pstate as the scaling driver only if the hardware-managed P-states (HWP) feature is supported by the processor. support_acpi_ppc

Take ACPI _PPC performance limits into account.

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

per_cpu_perf_limits Use per-logical-CPU P-State limits (see Coordination of P-state Limits for details).

Diagnostics and Tuning¶

Trace Events¶

There are two static trace events that can be used for intel_pstate diagnostics. One of them is the cpu_frequency trace event generally used by CPUFreq , and the other one is the pstate_sample trace event specific to intel_pstate . Both of them are triggered by intel_pstate only if it works in the active mode.

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

If intel_pstate works in the passive mode, 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).

ftrace В¶

The ftrace interface can be used for low-level diagnostics of intel_pstate . For example, to check how often the function to set a P-state is called, the ftrace filter can be set to to intel_pstate_set_pstate() :

Tuning Interface in debugfs В¶

The powersave algorithm provided by intel_pstate for the Core line of processors in the active mode is based on a PID controller whose parameters were chosen to address a number of different use cases at the same time. However, it still is possible to fine-tune it to a specific workload and the debugfs interface under /sys/kernel/debug/pstate_snb/ is provided for this purpose. [Note that the pstate_snb directory will be present only if the specific P-state selection algorithm matching the interface in it actually is in use.]

The following files present in that directory can be used to modify the PID controller parameters at run time:

Note, however, that achieving desirable results this way generally requires expert-level understanding of the power vs performance tradeoff, so extra care is recommended when attempting to do that.

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