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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+.. |intel_pstate| replace:: :doc:`intel_pstate <intel_pstate>`
+
+=======================
+CPU Performance Scaling
+=======================
+
+:Copyright: |copy| 2017 Intel Corporation
+
+:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+
+
+The Concept of CPU Performance Scaling
+======================================
+
+The majority of modern processors are capable of operating in a number of
+different clock frequency and voltage configurations, often referred to as
+Operating Performance Points or P-states (in ACPI terminology).  As a rule,
+the higher the clock frequency and the higher the voltage, the more instructions
+can be retired by the CPU over a unit of time, but also the higher the clock
+frequency and the higher the voltage, the more energy is consumed over a unit of
+time (or the more power is drawn) by the CPU in the given P-state.  Therefore
+there is a natural tradeoff between the CPU capacity (the number of instructions
+that can be executed over a unit of time) and the power drawn by the CPU.
+
+In some situations it is desirable or even necessary to run the program as fast
+as possible and then there is no reason to use any P-states different from the
+highest one (i.e. the highest-performance frequency/voltage configuration
+available).  In some other cases, however, it may not be necessary to execute
+instructions so quickly and maintaining the highest available CPU capacity for a
+relatively long time without utilizing it entirely may be regarded as wasteful.
+It also may not be physically possible to maintain maximum CPU capacity for too
+long for thermal or power supply capacity reasons or similar.  To cover those
+cases, there are hardware interfaces allowing CPUs to be switched between
+different frequency/voltage configurations or (in the ACPI terminology) to be
+put into different P-states.
+
+Typically, they are used along with algorithms to estimate the required CPU
+capacity, so as to decide which P-states to put the CPUs into.  Of course, since
+the utilization of the system generally changes over time, that has to be done
+repeatedly on a regular basis.  The activity by which this happens is referred
+to as CPU performance scaling or CPU frequency scaling (because it involves
+adjusting the CPU clock frequency).
+
+
+CPU Performance Scaling in Linux
+================================
+
+The Linux kernel supports CPU performance scaling by means of the ``CPUFreq``
+(CPU Frequency scaling) subsystem that consists of three layers of code: the
+core, scaling governors and scaling drivers.
+
+The ``CPUFreq`` core provides the common code infrastructure and user space
+interfaces for all platforms that support CPU performance scaling.  It defines
+the basic framework in which the other components operate.
+
+Scaling governors implement algorithms to estimate the required CPU capacity.
+As a rule, each governor implements one, possibly parametrized, scaling
+algorithm.
+
+Scaling drivers talk to the hardware.  They provide scaling governors with
+information on the available P-states (or P-state ranges in some cases) and
+access platform-specific hardware interfaces to change CPU P-states as requested
+by scaling governors.
+
+In principle, all available scaling governors can be used with every scaling
+driver.  That design is based on the observation that the information used by
+performance scaling algorithms for P-state selection can be represented in a
+platform-independent form in the majority of cases, so it should be possible
+to use the same performance scaling algorithm implemented in exactly the same
+way regardless of which scaling driver is used.  Consequently, the same set of
+scaling governors should be suitable for every supported platform.
+
+However, that observation may not hold for performance scaling algorithms
+based on information provided by the hardware itself, for example through
+feedback registers, as that information is typically specific to the hardware
+interface it comes from and may not be easily represented in an abstract,
+platform-independent way.  For this reason, ``CPUFreq`` allows scaling drivers
+to bypass the governor layer and implement their own performance scaling
+algorithms.  That is done by the |intel_pstate| scaling driver.
+
+
+``CPUFreq`` Policy Objects
+==========================
+
+In some cases the hardware interface for P-state control is shared by multiple
+CPUs.  That is, for example, the same register (or set of registers) is used to
+control the P-state of multiple CPUs at the same time and writing to it affects
+all of those CPUs simultaneously.
+
+Sets of CPUs sharing hardware P-state control interfaces are represented by
+``CPUFreq`` as struct cpufreq_policy objects.  For consistency,
+struct cpufreq_policy is also used when there is only one CPU in the given
+set.
+
+The ``CPUFreq`` core maintains a pointer to a struct cpufreq_policy object for
+every CPU in the system, including CPUs that are currently offline.  If multiple
+CPUs share the same hardware P-state control interface, all of the pointers
+corresponding to them point to the same struct cpufreq_policy object.
+
+``CPUFreq`` uses struct cpufreq_policy as its basic data type and the design
+of its user space interface is based on the policy concept.
+
+
+CPU Initialization
+==================
+
+First of all, a scaling driver has to be registered for ``CPUFreq`` to work.
+It is only possible to register one scaling driver at a time, so the scaling
+driver is expected to be able to handle all CPUs in the system.
+
+The scaling driver may be registered before or after CPU registration.  If
+CPUs are registered earlier, the driver core invokes the ``CPUFreq`` core to
+take a note of all of the already registered CPUs during the registration of the
+scaling driver.  In turn, if any CPUs are registered after the registration of
+the scaling driver, the ``CPUFreq`` core will be invoked to take note of them
+at their registration time.
+
+In any case, the ``CPUFreq`` core is invoked to take note of any logical CPU it
+has not seen so far as soon as it is ready to handle that CPU.  [Note that the
+logical CPU may be a physical single-core processor, or a single core in a
+multicore processor, or a hardware thread in a physical processor or processor
+core.  In what follows "CPU" always means "logical CPU" unless explicitly stated
+otherwise and the word "processor" is used to refer to the physical part
+possibly including multiple logical CPUs.]
+
+Once invoked, the ``CPUFreq`` core checks if the policy pointer is already set
+for the given CPU and if so, it skips the policy object creation.  Otherwise,
+a new policy object is created and initialized, which involves the creation of
+a new policy directory in ``sysfs``, and the policy pointer corresponding to
+the given CPU is set to the new policy object's address in memory.
+
+Next, the scaling driver's ``->init()`` callback is invoked with the policy
+pointer of the new CPU passed to it as the argument.  That callback is expected
+to initialize the performance scaling hardware interface for the given CPU (or,
+more precisely, for the set of CPUs sharing the hardware interface it belongs
+to, represented by its policy object) and, if the policy object it has been
+called for is new, to set parameters of the policy, like the minimum and maximum
+frequencies supported by the hardware, the table of available frequencies (if
+the set of supported P-states is not a continuous range), and the mask of CPUs
+that belong to the same policy (including both online and offline CPUs).  That
+mask is then used by the core to populate the policy pointers for all of the
+CPUs in it.
+
+The next major initialization step for a new policy object is to attach a
+scaling governor to it (to begin with, that is the default scaling governor
+determined by the kernel command line or configuration, but it may be changed
+later via ``sysfs``).  First, a pointer to the new policy object is passed to
+the governor's ``->init()`` callback which is expected to initialize all of the
+data structures necessary to handle the given policy and, possibly, to add
+a governor ``sysfs`` interface to it.  Next, the governor is started by
+invoking its ``->start()`` callback.
+
+That callback is expected to register per-CPU utilization update callbacks for
+all of the online CPUs belonging to the given policy with the CPU scheduler.
+The utilization update callbacks will be invoked by the CPU scheduler on
+important events, like task enqueue and dequeue, on every iteration of the
+scheduler tick or generally whenever the CPU utilization may change (from the
+scheduler's perspective).  They are expected to carry out computations needed
+to determine the P-state to use for the given policy going forward and to
+invoke the scaling driver to make changes to the hardware in accordance with
+the P-state selection.  The scaling driver may be invoked directly from
+scheduler context or asynchronously, via a kernel thread or workqueue, depending
+on the configuration and capabilities of the scaling driver and the governor.
+
+Similar steps are taken for policy objects that are not new, but were "inactive"
+previously, meaning that all of the CPUs belonging to them were offline.  The
+only practical difference in that case is that the ``CPUFreq`` core will attempt
+to use the scaling governor previously used with the policy that became
+"inactive" (and is re-initialized now) instead of the default governor.
+
+In turn, if a previously offline CPU is being brought back online, but some
+other CPUs sharing the policy object with it are online already, there is no
+need to re-initialize the policy object at all.  In that case, it only is
+necessary to restart the scaling governor so that it can take the new online CPU
+into account.  That is achieved by invoking the governor's ``->stop`` and
+``->start()`` callbacks, in this order, for the entire policy.
+
+As mentioned before, the |intel_pstate| scaling driver bypasses the scaling
+governor layer of ``CPUFreq`` and provides its own P-state selection algorithms.
+Consequently, if |intel_pstate| is used, scaling governors are not attached to
+new policy objects.  Instead, the driver's ``->setpolicy()`` callback is invoked
+to register per-CPU utilization update callbacks for each policy.  These
+callbacks are invoked by the CPU scheduler in the same way as for scaling
+governors, but in the |intel_pstate| case they both determine the P-state to
+use and change the hardware configuration accordingly in one go from scheduler
+context.
+
+The policy objects created during CPU initialization and other data structures
+associated with them are torn down when the scaling driver is unregistered
+(which happens when the kernel module containing it is unloaded, for example) or
+when the last CPU belonging to the given policy in unregistered.
+
+
+Policy Interface in ``sysfs``
+=============================
+
+During the initialization of the kernel, the ``CPUFreq`` core creates a
+``sysfs`` directory (kobject) called ``cpufreq`` under
+:file:`/sys/devices/system/cpu/`.
+
+That directory contains a ``policyX`` subdirectory (where ``X`` represents an
+integer number) for every policy object maintained by the ``CPUFreq`` core.
+Each ``policyX`` directory is pointed to by ``cpufreq`` symbolic links
+under :file:`/sys/devices/system/cpu/cpuY/` (where ``Y`` represents an integer
+that may be different from the one represented by ``X``) for all of the CPUs
+associated with (or belonging to) the given policy.  The ``policyX`` directories
+in :file:`/sys/devices/system/cpu/cpufreq` each contain policy-specific
+attributes (files) to control ``CPUFreq`` behavior for the corresponding policy
+objects (that is, for all of the CPUs associated with them).
+
+Some of those attributes are generic.  They are created by the ``CPUFreq`` core
+and their behavior generally does not depend on what scaling driver is in use
+and what scaling governor is attached to the given policy.  Some scaling drivers
+also add driver-specific attributes to the policy directories in ``sysfs`` to
+control policy-specific aspects of driver behavior.
+
+The generic attributes under :file:`/sys/devices/system/cpu/cpufreq/policyX/`
+are the following:
+
+``affected_cpus``
+	List of online CPUs belonging to this policy (i.e. sharing the hardware
+	performance scaling interface represented by the ``policyX`` policy
+	object).
+
+``bios_limit``
+	If the platform firmware (BIOS) tells the OS to apply an upper limit to
+	CPU frequencies, that limit will be reported through this attribute (if
+	present).
+
+	The existence of the limit may be a result of some (often unintentional)
+	BIOS settings, restrictions coming from a service processor or another
+	BIOS/HW-based mechanisms.
+
+	This does not cover ACPI thermal limitations which can be discovered
+	through a generic thermal driver.
+
+	This attribute is not present if the scaling driver in use does not
+	support it.
+
+``cpuinfo_cur_freq``
+	Current frequency of the CPUs belonging to this policy as obtained from
+	the hardware (in KHz).
+
+	This is expected to be the frequency the hardware actually runs at.
+	If that frequency cannot be determined, this attribute should not
+	be present.
+
+``cpuinfo_max_freq``
+	Maximum possible operating frequency the CPUs belonging to this policy
+	can run at (in kHz).
+
+``cpuinfo_min_freq``
+	Minimum possible operating frequency the CPUs belonging to this policy
+	can run at (in kHz).
+
+``cpuinfo_transition_latency``
+	The time it takes to switch the CPUs belonging to this policy from one
+	P-state to another, in nanoseconds.
+
+	If unknown or if known to be so high that the scaling driver does not
+	work with the `ondemand`_ governor, -1 (:c:macro:`CPUFREQ_ETERNAL`)
+	will be returned by reads from this attribute.
+
+``related_cpus``
+	List of all (online and offline) CPUs belonging to this policy.
+
+``scaling_available_governors``
+	List of ``CPUFreq`` scaling governors present in the kernel that can
+	be attached to this policy or (if the |intel_pstate| scaling driver is
+	in use) list of scaling algorithms provided by the driver that can be
+	applied to this policy.
+
+	[Note that some governors are modular and it may be necessary to load a
+	kernel module for the governor held by it to become available and be
+	listed by this attribute.]
+
+``scaling_cur_freq``
+	Current frequency of all of the CPUs belonging to this policy (in kHz).
+
+	In the majority of cases, this is the frequency of the last P-state
+	requested by the scaling driver from the hardware using the scaling
+	interface provided by it, which may or may not reflect the frequency
+	the CPU is actually running at (due to hardware design and other
+	limitations).
+
+	Some architectures (e.g. ``x86``) may attempt to provide information
+	more precisely reflecting the current CPU frequency through this
+	attribute, but that still may not be the exact current CPU frequency as
+	seen by the hardware at the moment.
+
+``scaling_driver``
+	The scaling driver currently in use.
+
+``scaling_governor``
+	The scaling governor currently attached to this policy or (if the
+	|intel_pstate| scaling driver is in use) the scaling algorithm
+	provided by the driver that is currently applied to this policy.
+
+	This attribute is read-write and writing to it will cause a new scaling
+	governor to be attached to this policy or a new scaling algorithm
+	provided by the scaling driver to be applied to it (in the
+	|intel_pstate| case), as indicated by the string written to this
+	attribute (which must be one of the names listed by the
+	``scaling_available_governors`` attribute described above).
+
+``scaling_max_freq``
+	Maximum frequency the CPUs belonging to this policy are allowed to be
+	running at (in kHz).
+
+	This attribute is read-write and writing a string representing an
+	integer to it will cause a new limit to be set (it must not be lower
+	than the value of the ``scaling_min_freq`` attribute).
+
+``scaling_min_freq``
+	Minimum frequency the CPUs belonging to this policy are allowed to be
+	running at (in kHz).
+
+	This attribute is read-write and writing a string representing a
+	non-negative integer to it will cause a new limit to be set (it must not
+	be higher than the value of the ``scaling_max_freq`` attribute).
+
+``scaling_setspeed``
+	This attribute is functional only if the `userspace`_ scaling governor
+	is attached to the given policy.
+
+	It returns the last frequency requested by the governor (in kHz) or can
+	be written to in order to set a new frequency for the policy.
+
+
+Generic Scaling Governors
+=========================
+
+``CPUFreq`` provides generic scaling governors that can be used with all
+scaling drivers.  As stated before, each of them implements a single, possibly
+parametrized, performance scaling algorithm.
+
+Scaling governors are attached to policy objects and different policy objects
+can be handled by different scaling governors at the same time (although that
+may lead to suboptimal results in some cases).
+
+The scaling governor for a given policy object can be changed at any time with
+the help of the ``scaling_governor`` policy attribute in ``sysfs``.
+
+Some governors expose ``sysfs`` attributes to control or fine-tune the scaling
+algorithms implemented by them.  Those attributes, referred to as governor
+tunables, can be either global (system-wide) or per-policy, depending on the
+scaling driver in use.  If the driver requires governor tunables to be
+per-policy, they are located in a subdirectory of each policy directory.
+Otherwise, they are located in a subdirectory under
+:file:`/sys/devices/system/cpu/cpufreq/`.  In either case the name of the
+subdirectory containing the governor tunables is the name of the governor
+providing them.
+
+``performance``
+---------------
+
+When attached to a policy object, this governor causes the highest frequency,
+within the ``scaling_max_freq`` policy limit, to be requested for that policy.
+
+The request is made once at that time the governor for the policy is set to
+``performance`` and whenever the ``scaling_max_freq`` or ``scaling_min_freq``
+policy limits change after that.
+
+``powersave``
+-------------
+
+When attached to a policy object, this governor causes the lowest frequency,
+within the ``scaling_min_freq`` policy limit, to be requested for that policy.
+
+The request is made once at that time the governor for the policy is set to
+``powersave`` and whenever the ``scaling_max_freq`` or ``scaling_min_freq``
+policy limits change after that.
+
+``userspace``
+-------------
+
+This governor does not do anything by itself.  Instead, it allows user space
+to set the CPU frequency for the policy it is attached to by writing to the
+``scaling_setspeed`` attribute of that policy.
+
+``schedutil``
+-------------
+
+This governor uses CPU utilization data available from the CPU scheduler.  It
+generally is regarded as a part of the CPU scheduler, so it can access the
+scheduler's internal data structures directly.
+
+It runs entirely in scheduler context, although in some cases it may need to
+invoke the scaling driver asynchronously when it decides that the CPU frequency
+should be changed for a given policy (that depends on whether or not the driver
+is capable of changing the CPU frequency from scheduler context).
+
+The actions of this governor for a particular CPU depend on the scheduling class
+invoking its utilization update callback for that CPU.  If it is invoked by the
+RT or deadline scheduling classes, the governor will increase the frequency to
+the allowed maximum (that is, the ``scaling_max_freq`` policy limit).  In turn,
+if it is invoked by the CFS scheduling class, the governor will use the
+Per-Entity Load Tracking (PELT) metric for the root control group of the
+given CPU as the CPU utilization estimate (see the *Per-entity load tracking*
+LWN.net article [1]_ for a description of the PELT mechanism).  Then, the new
+CPU frequency to apply is computed in accordance with the formula
+
+	f = 1.25 * ``f_0`` * ``util`` / ``max``
+
+where ``util`` is the PELT number, ``max`` is the theoretical maximum of
+``util``, and ``f_0`` is either the maximum possible CPU frequency for the given
+policy (if the PELT number is frequency-invariant), or the current CPU frequency
+(otherwise).
+
+This governor also employs a mechanism allowing it to temporarily bump up the
+CPU frequency for tasks that have been waiting on I/O most recently, called
+"IO-wait boosting".  That happens when the :c:macro:`SCHED_CPUFREQ_IOWAIT` flag
+is passed by the scheduler to the governor callback which causes the frequency
+to go up to the allowed maximum immediately and then draw back to the value
+returned by the above formula over time.
+
+This governor exposes only one tunable:
+
+``rate_limit_us``
+	Minimum time (in microseconds) that has to pass between two consecutive
+	runs of governor computations (default: 1000 times the scaling driver's
+	transition latency).
+
+	The purpose of this tunable is to reduce the scheduler context overhead
+	of the governor which might be excessive without it.
+
+This governor generally is regarded as a replacement for the older `ondemand`_
+and `conservative`_ governors (described below), as it is simpler and more
+tightly integrated with the CPU scheduler, its overhead in terms of CPU context
+switches and similar is less significant, and it uses the scheduler's own CPU
+utilization metric, so in principle its decisions should not contradict the
+decisions made by the other parts of the scheduler.
+
+``ondemand``
+------------
+
+This governor uses CPU load as a CPU frequency selection metric.
+
+In order to estimate the current CPU load, it measures the time elapsed between
+consecutive invocations of its worker routine and computes the fraction of that
+time in which the given CPU was not idle.  The ratio of the non-idle (active)
+time to the total CPU time is taken as an estimate of the load.
+
+If this governor is attached to a policy shared by multiple CPUs, the load is
+estimated for all of them and the greatest result is taken as the load estimate
+for the entire policy.
+
+The worker routine of this governor has to run in process context, so it is
+invoked asynchronously (via a workqueue) and CPU P-states are updated from
+there if necessary.  As a result, the scheduler context overhead from this
+governor is minimum, but it causes additional CPU context switches to happen
+relatively often and the CPU P-state updates triggered by it can be relatively
+irregular.  Also, it affects its own CPU load metric by running code that
+reduces the CPU idle time (even though the CPU idle time is only reduced very
+slightly by it).
+
+It generally selects CPU frequencies proportional to the estimated load, so that
+the value of the ``cpuinfo_max_freq`` policy attribute corresponds to the load of
+1 (or 100%), and the value of the ``cpuinfo_min_freq`` policy attribute
+corresponds to the load of 0, unless when the load exceeds a (configurable)
+speedup threshold, in which case it will go straight for the highest frequency
+it is allowed to use (the ``scaling_max_freq`` policy limit).
+
+This governor exposes the following tunables:
+
+``sampling_rate``
+	This is how often the governor's worker routine should run, in
+	microseconds.
+
+	Typically, it is set to values of the order of 10000 (10 ms).  Its
+	default value is equal to the value of ``cpuinfo_transition_latency``
+	for each policy this governor is attached to (but since the unit here
+	is greater by 1000, this means that the time represented by
+	``sampling_rate`` is 1000 times greater than the transition latency by
+	default).
+
+	If this tunable is per-policy, the following shell command sets the time
+	represented by it to be 750 times as high as the transition latency::
+
+	# echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate
+
+``up_threshold``
+	If the estimated CPU load is above this value (in percent), the governor
+	will set the frequency to the maximum value allowed for the policy.
+	Otherwise, the selected frequency will be proportional to the estimated
+	CPU load.
+
+``ignore_nice_load``
+	If set to 1 (default 0), it will cause the CPU load estimation code to
+	treat the CPU time spent on executing tasks with "nice" levels greater
+	than 0 as CPU idle time.
+
+	This may be useful if there are tasks in the system that should not be
+	taken into account when deciding what frequency to run the CPUs at.
+	Then, to make that happen it is sufficient to increase the "nice" level
+	of those tasks above 0 and set this attribute to 1.
+
+``sampling_down_factor``
+	Temporary multiplier, between 1 (default) and 100 inclusive, to apply to
+	the ``sampling_rate`` value if the CPU load goes above ``up_threshold``.
+
+	This causes the next execution of the governor's worker routine (after
+	setting the frequency to the allowed maximum) to be delayed, so the
+	frequency stays at the maximum level for a longer time.
+
+	Frequency fluctuations in some bursty workloads may be avoided this way
+	at the cost of additional energy spent on maintaining the maximum CPU
+	capacity.
+
+``powersave_bias``
+	Reduction factor to apply to the original frequency target of the
+	governor (including the maximum value used when the ``up_threshold``
+	value is exceeded by the estimated CPU load) or sensitivity threshold
+	for the AMD frequency sensitivity powersave bias driver
+	(:file:`drivers/cpufreq/amd_freq_sensitivity.c`), between 0 and 1000
+	inclusive.
+
+	If the AMD frequency sensitivity powersave bias driver is not loaded,
+	the effective frequency to apply is given by
+
+		f * (1 - ``powersave_bias`` / 1000)
+
+	where f is the governor's original frequency target.  The default value
+	of this attribute is 0 in that case.
+
+	If the AMD frequency sensitivity powersave bias driver is loaded, the
+	value of this attribute is 400 by default and it is used in a different
+	way.
+
+	On Family 16h (and later) AMD processors there is a mechanism to get a
+	measured workload sensitivity, between 0 and 100% inclusive, from the
+	hardware.  That value can be used to estimate how the performance of the
+	workload running on a CPU will change in response to frequency changes.
+
+	The performance of a workload with the sensitivity of 0 (memory-bound or
+	IO-bound) is not expected to increase at all as a result of increasing
+	the CPU frequency, whereas workloads with the sensitivity of 100%
+	(CPU-bound) are expected to perform much better if the CPU frequency is
+	increased.
+
+	If the workload sensitivity is less than the threshold represented by
+	the ``powersave_bias`` value, the sensitivity powersave bias driver
+	will cause the governor to select a frequency lower than its original
+	target, so as to avoid over-provisioning workloads that will not benefit
+	from running at higher CPU frequencies.
+
+``conservative``
+----------------
+
+This governor uses CPU load as a CPU frequency selection metric.
+
+It estimates the CPU load in the same way as the `ondemand`_ governor described
+above, but the CPU frequency selection algorithm implemented by it is different.
+
+Namely, it avoids changing the frequency significantly over short time intervals
+which may not be suitable for systems with limited power supply capacity (e.g.
+battery-powered).  To achieve that, it changes the frequency in relatively
+small steps, one step at a time, up or down - depending on whether or not a
+(configurable) threshold has been exceeded by the estimated CPU load.
+
+This governor exposes the following tunables:
+
+``freq_step``
+	Frequency step in percent of the maximum frequency the governor is
+	allowed to set (the ``scaling_max_freq`` policy limit), between 0 and
+	100 (5 by default).
+
+	This is how much the frequency is allowed to change in one go.  Setting
+	it to 0 will cause the default frequency step (5 percent) to be used
+	and setting it to 100 effectively causes the governor to periodically
+	switch the frequency between the ``scaling_min_freq`` and
+	``scaling_max_freq`` policy limits.
+
+``down_threshold``
+	Threshold value (in percent, 20 by default) used to determine the
+	frequency change direction.
+
+	If the estimated CPU load is greater than this value, the frequency will
+	go up (by ``freq_step``).  If the load is less than this value (and the
+	``sampling_down_factor`` mechanism is not in effect), the frequency will
+	go down.  Otherwise, the frequency will not be changed.
+
+``sampling_down_factor``
+	Frequency decrease deferral factor, between 1 (default) and 10
+	inclusive.
+
+	It effectively causes the frequency to go down ``sampling_down_factor``
+	times slower than it ramps up.
+
+
+Frequency Boost Support
+=======================
+
+Background
+----------
+
+Some processors support a mechanism to raise the operating frequency of some
+cores in a multicore package temporarily (and above the sustainable frequency
+threshold for the whole package) under certain conditions, for example if the
+whole chip is not fully utilized and below its intended thermal or power budget.
+
+Different names are used by different vendors to refer to this functionality.
+For Intel processors it is referred to as "Turbo Boost", AMD calls it
+"Turbo-Core" or (in technical documentation) "Core Performance Boost" and so on.
+As a rule, it also is implemented differently by different vendors.  The simple
+term "frequency boost" is used here for brevity to refer to all of those
+implementations.
+
+The frequency boost mechanism may be either hardware-based or software-based.
+If it is hardware-based (e.g. on x86), the decision to trigger the boosting is
+made by the hardware (although in general it requires the hardware to be put
+into a special state in which it can control the CPU frequency within certain
+limits).  If it is software-based (e.g. on ARM), the scaling driver decides
+whether or not to trigger boosting and when to do that.
+
+The ``boost`` File in ``sysfs``
+-------------------------------
+
+This file is located under :file:`/sys/devices/system/cpu/cpufreq/` and controls
+the "boost" setting for the whole system.  It is not present if the underlying
+scaling driver does not support the frequency boost mechanism (or supports it,
+but provides a driver-specific interface for controlling it, like
+|intel_pstate|).
+
+If the value in this file is 1, the frequency boost mechanism is enabled.  This
+means that either the hardware can be put into states in which it is able to
+trigger boosting (in the hardware-based case), or the software is allowed to
+trigger boosting (in the software-based case).  It does not mean that boosting
+is actually in use at the moment on any CPUs in the system.  It only means a
+permission to use the frequency boost mechanism (which still may never be used
+for other reasons).
+
+If the value in this file is 0, the frequency boost mechanism is disabled and
+cannot be used at all.
+
+The only values that can be written to this file are 0 and 1.
+
+Rationale for Boost Control Knob
+--------------------------------
+
+The frequency boost mechanism is generally intended to help to achieve optimum
+CPU performance on time scales below software resolution (e.g. below the
+scheduler tick interval) and it is demonstrably suitable for many workloads, but
+it may lead to problems in certain situations.
+
+For this reason, many systems make it possible to disable the frequency boost
+mechanism in the platform firmware (BIOS) setup, but that requires the system to
+be restarted for the setting to be adjusted as desired, which may not be
+practical at least in some cases.  For example:
+
+  1. Boosting means overclocking the processor, although under controlled
+     conditions.  Generally, the processor's energy consumption increases
+     as a result of increasing its frequency and voltage, even temporarily.
+     That may not be desirable on systems that switch to power sources of
+     limited capacity, such as batteries, so the ability to disable the boost
+     mechanism while the system is running may help there (but that depends on
+     the workload too).
+
+  2. In some situations deterministic behavior is more important than
+     performance or energy consumption (or both) and the ability to disable
+     boosting while the system is running may be useful then.
+
+  3. To examine the impact of the frequency boost mechanism itself, it is useful
+     to be able to run tests with and without boosting, preferably without
+     restarting the system in the meantime.
+
+  4. Reproducible results are important when running benchmarks.  Since
+     the boosting functionality depends on the load of the whole package,
+     single-thread performance may vary because of it which may lead to
+     unreproducible results sometimes.  That can be avoided by disabling the
+     frequency boost mechanism before running benchmarks sensitive to that
+     issue.
+
+Legacy AMD ``cpb`` Knob
+-----------------------
+
+The AMD powernow-k8 scaling driver supports a ``sysfs`` knob very similar to
+the global ``boost`` one.  It is used for disabling/enabling the "Core
+Performance Boost" feature of some AMD processors.
+
+If present, that knob is located in every ``CPUFreq`` policy directory in
+``sysfs`` (:file:`/sys/devices/system/cpu/cpufreq/policyX/`) and is called
+``cpb``, which indicates a more fine grained control interface.  The actual
+implementation, however, works on the system-wide basis and setting that knob
+for one policy causes the same value of it to be set for all of the other
+policies at the same time.
+
+That knob is still supported on AMD processors that support its underlying
+hardware feature, but it may be configured out of the kernel (via the
+:c:macro:`CONFIG_X86_ACPI_CPUFREQ_CPB` configuration option) and the global
+``boost`` knob is present regardless.  Thus it is always possible use the
+``boost`` knob instead of the ``cpb`` one which is highly recommended, as that
+is more consistent with what all of the other systems do (and the ``cpb`` knob
+may not be supported any more in the future).
+
+The ``cpb`` knob is never present for any processors without the underlying
+hardware feature (e.g. all Intel ones), even if the
+:c:macro:`CONFIG_X86_ACPI_CPUFREQ_CPB` configuration option is set.
+
+
+References
+==========
+
+.. [1] Jonathan Corbet, *Per-entity load tracking*,
+       https://lwn.net/Articles/531853/