cpufreq: User/admin documentation update and consolidation
The user/admin documentation of cpufreq is badly outdated. It
conains stale and/or inaccurate information along with things
that are not particularly useful. Also, some of the important
pieces are missing from it.
For this reason, add a new user/admin document for cpufreq
containing current information to admin-guide and drop the old
outdated .txt documents it is replacing.
Since there will be more PM documents in admin-guide going forward,
create a separate directory for them and put the cpufreq document
in there right away.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst
new file mode 100644
index 0000000..289c80f
--- /dev/null
+++ b/Documentation/admin-guide/pm/cpufreq.rst
@@ -0,0 +1,700 @@
+.. |struct cpufreq_policy| replace:: :c:type:`struct cpufreq_policy <cpufreq_policy>`
+
+=======================
+CPU Performance Scaling
+=======================
+
+::
+
+ Copyright (c) 2017 Intel Corp., 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 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 it 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_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).
+
+ For the majority of scaling drivers, this is the frequency of the last
+ P-state requested by the 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 scaling drivers (e.g. ``intel_pstate``) 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 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
+
+
+``min_sampling_rate``
+ The minimum value of ``sampling_rate``.
+
+ Equal to 10000 (10 ms) if :c:macro:`CONFIG_NO_HZ_COMMON` and
+ :c:data:`tick_nohz_active` are both set or to 20 times the value of
+ :c:data:`jiffies` in microseconds otherwise.
+
+``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.
+
+
+.. _Per-entity load tracking: https://lwn.net/Articles/531853/