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diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst new file mode 100644 index 000000000..47153e64d --- /dev/null +++ b/Documentation/admin-guide/pm/cpufreq.rst @@ -0,0 +1,701 @@ +.. |struct cpufreq_policy| replace:: :c:type:`struct cpufreq_policy <cpufreq_policy>` +.. |intel_pstate| replace:: :doc:`intel_pstate <intel_pstate>` + +======================= +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_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 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. + + +.. _Per-entity load tracking: https://lwn.net/Articles/531853/ diff --git a/Documentation/admin-guide/pm/index.rst b/Documentation/admin-guide/pm/index.rst new file mode 100644 index 000000000..49237ac73 --- /dev/null +++ b/Documentation/admin-guide/pm/index.rst @@ -0,0 +1,10 @@ +================ +Power Management +================ + +.. toctree:: + :maxdepth: 2 + + strategies + system-wide + working-state diff --git a/Documentation/admin-guide/pm/intel_pstate.rst b/Documentation/admin-guide/pm/intel_pstate.rst new file mode 100644 index 000000000..8f1d3de44 --- /dev/null +++ b/Documentation/admin-guide/pm/intel_pstate.rst @@ -0,0 +1,718 @@ +=============================================== +``intel_pstate`` CPU Performance Scaling Driver +=============================================== + +:: + + Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> + + +General Information +=================== + +``intel_pstate`` is a part of the +:doc:`CPU performance scaling subsystem <cpufreq>` 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 :doc:`cpufreq` 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 <LCEU2015_>`_ 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 <SDM_>`_). 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 <status_attr_>`_). + + +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 +:c:macro:`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`` +..................... + +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`` +................... + +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_governor`` 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`` +............... + +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 driver configuration for the given CPU is updated +(e.g. via ``sysfs``). + +This is the default P-state selection algorithm if the +:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option +is set. + +``powersave`` +............. + +Without HWP, this P-state selection algorithm 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. + +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. 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 +:c:macro:`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: + +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 <acpi-cpufreq_>`_ +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 <no_turbo_attr_>`_) 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 <turbo_>`_. + + * Whether or not turbo P-states are supported at all. + + * The maximum supported `one-core turbo P-state <turbo_>`_ (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 <Active Mode_>`_. [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`` +================================= + +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/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. + +``max_perf_pct`` + Maximum P-state the driver is allowed to set in percent of the + maximum supported performance level (the highest supported `turbo + P-state <turbo_>`_). + + This attribute will not be exposed if the + ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel + command line. + +``min_perf_pct`` + Minimum P-state the driver is allowed to set in percent of the + maximum supported performance level (the highest supported `turbo + P-state <turbo_>`_). + + This attribute will not be exposed if the + ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel + command line. + +``num_pstates`` + 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 <no_turbo_attr_>`_. + + This attribute is read-only. + +``turbo_pct`` + Ratio of the `turbo range <turbo_>`_ size to the size of the entire + range of supported P-states, in percent. + + This attribute is read-only. + +.. _no_turbo_attr: + +``no_turbo`` + 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). + +``hwp_dynamic_boost`` + This attribute is only present if ``intel_pstate`` works in the + `active mode with the HWP feature enabled <Active Mode With HWP_>`_ in + the processor. If set (equal to 1), it causes the minimum P-state limit + to be increased dynamically for a short time whenever a task previously + waiting on I/O is selected to run on a given logical CPU (the purpose + of this mechanism is to improve performance). + + This setting has no effect on logical CPUs whose minimum P-state limit + is directly set to the highest non-turbo P-state or above it. + +.. _status_attr: + +``status`` + Operation mode of the driver: "active", "passive" or "off". + + "active" + The driver is functional and in the `active mode + <Active Mode_>`_. + + "passive" + The driver is functional and in the `passive mode + <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 <Active Mode With HWP_>`_, + the operation mode of the driver cannot be changed), and if it is not + supported in the current configuration, writes to this attribute will + fail with an appropriate error. + +Interpretation of Policy Attributes +----------------------------------- + +The interpretation of some ``CPUFreq`` policy attributes described in +:doc:`cpufreq` is special with ``intel_pstate`` as the current scaling driver +and it generally depends on the driver's `operation mode <Operation Modes_>`_. + +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 <no_turbo_attr_>`_ 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 <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 <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 <Active Mode_>`_) or "intel_cpufreq" (in the +`passive mode <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 +<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 <Active Mode With HWP_>`_, 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 <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 +<Active Mode With HWP_>`_ 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: + +``energy_performance_preference`` + 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. + +``energy_performance_available_preferences`` + 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.] + +.. _acpi-cpufreq: + +``intel_pstate`` vs ``acpi-cpufreq`` +==================================== + +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 <turbo_>`_ 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 <turbo_>`_ 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 <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`` +================================================ + +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 <Passive Mode_>`_ to + start with. + + This option implies the ``no_hwp`` one described below. + +``force`` + 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 + <Active Mode With HWP_>`_ 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 <Active Mode With HWP_>`_ 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 <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):: + + # cd /sys/kernel/debug/tracing/ + # echo 1 > events/power/pstate_sample/enable + # echo 1 > events/power/cpu_frequency/enable + # cat trace + gnome-terminal--4510 [001] ..s. 1177.680733: pstate_sample: core_busy=107 scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618 freq=2474476 + cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2 + +If ``intel_pstate`` works in the `passive mode <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`` +---------- + +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 +:c:func:`intel_pstate_set_pstate`:: + + # cd /sys/kernel/debug/tracing/ + # cat available_filter_functions | grep -i pstate + intel_pstate_set_pstate + intel_pstate_cpu_init + ... + # echo intel_pstate_set_pstate > set_ftrace_filter + # echo function > current_tracer + # cat trace | head -15 + # tracer: function + # + # entries-in-buffer/entries-written: 80/80 #P:4 + # + # _-----=> irqs-off + # / _----=> need-resched + # | / _---=> hardirq/softirq + # || / _--=> preempt-depth + # ||| / delay + # TASK-PID CPU# |||| TIMESTAMP FUNCTION + # | | | |||| | | + Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func + gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func + gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func + <idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func + + +.. _LCEU2015: http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf +.. _SDM: http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html +.. _ACPI specification: http://www.uefi.org/sites/default/files/resources/ACPI_6_1.pdf diff --git a/Documentation/admin-guide/pm/sleep-states.rst b/Documentation/admin-guide/pm/sleep-states.rst new file mode 100644 index 000000000..dbf5acd49 --- /dev/null +++ b/Documentation/admin-guide/pm/sleep-states.rst @@ -0,0 +1,245 @@ +=================== +System Sleep States +=================== + +:: + + Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> + +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, including +hibernation and up to three variants of system suspend. The sleep states that +can be supported by the kernel are listed below. + +.. _s2idle: + +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 :ref:`standby <standby>` +or :ref:`suspend-to-RAM <s2ram>`, 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 :c:macro:`CONFIG_SUSPEND` kernel configuration option is set. + +.. _standby: + +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 :ref:`suspend-to-idle +<s2idle>` 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 :ref:`suspend-to-idle <s2idle>`, 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 :ref:`suspend-to-idle <s2idle>` and it may be necessary to +rely on the platform for setting up the wakeup functionality as appropriate. + +This state is supported if the :c:macro:`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. + +.. _s2ram: + +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 :ref:`standby <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 :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it +may be necessary to rely on the platform for setting up the wakeup functionality +as appropriate. + +S2RAM is supported if the :c:macro:`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: + +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 :c:macro:`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 :file:`/sys/power/` directory can be used by +user space for sleep states control. + +``state`` + 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 + :ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and + :ref:`standby <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. + +``mem_sleep`` + 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 :ref:`suspend-to-idle + <s2idle>` and, by convention, "shallow" and "deep" represent + :ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`, + 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. + +``disk`` + 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 :file:`/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 +:ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze" +directly to :file:`/sys/power/state`. The second one is to write "s2idle" to +:file:`/sys/power/mem_sleep` and then to write "mem" to +:file:`/sys/power/state`. Likewise, there are two ways to make the system go +into the :ref:`standby <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 :ref:`suspend-to-RAM <s2ram>` state (write "deep" into +:file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`). + +The default suspend variant (ie. the one to be used without writing anything +into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems +supporting :ref:`suspend-to-RAM <s2ram>`) 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 :ref:`suspend-to-RAM <s2ram>` is supported. diff --git a/Documentation/admin-guide/pm/strategies.rst b/Documentation/admin-guide/pm/strategies.rst new file mode 100644 index 000000000..afe4d3f83 --- /dev/null +++ b/Documentation/admin-guide/pm/strategies.rst @@ -0,0 +1,52 @@ +=========================== +Power Management Strategies +=========================== + +:: + + Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> + +The Linux kernel supports two major high-level power management strategies. + +One of them is based on using global low-power states of the whole system in +which user space code cannot be executed and the overall system activity is +significantly reduced, referred to as :doc:`sleep states <sleep-states>`. The +kernel puts the system into one of these states when requested by user space +and the system stays in it until a special signal is received from one of +designated devices, triggering a transition to the ``working state`` in which +user space code can run. Because sleep states are global and the whole system +is affected by the state changes, this strategy is referred to as the +:doc:`system-wide power management <system-wide>`. + +The other strategy, referred to as the :doc:`working-state power management +<working-state>`, is based on adjusting the power states of individual hardware +components of the system, as needed, in the working state. In consequence, if +this strategy is in use, the working state of the system usually does not +correspond to any particular physical configuration of it, but can be treated as +a metastate covering a range of different power states of the system in which +the individual components of it can be either ``active`` (in use) or +``inactive`` (idle). If they are active, they have to be in power states +allowing them to process data and to be accessed by software. In turn, if they +are inactive, ideally, they should be in low-power states in which they may not +be accessible. + +If all of the system components are active, the system as a whole is regarded as +"runtime active" and that situation typically corresponds to the maximum power +draw (or maximum energy usage) of it. If all of them are inactive, the system +as a whole is regarded as "runtime idle" which may be very close to a sleep +state from the physical system configuration and power draw perspective, but +then it takes much less time and effort to start executing user space code than +for the same system in a sleep state. However, transitions from sleep states +back to the working state can only be started by a limited set of devices, so +typically the system can spend much more time in a sleep state than it can be +runtime idle in one go. For this reason, systems usually use less energy in +sleep states than when they are runtime idle most of the time. + +Moreover, the two power management strategies address different usage scenarios. +Namely, if the user indicates that the system will not be in use going forward, +for example by closing its lid (if the system is a laptop), it probably should +go into a sleep state at that point. On the other hand, if the user simply goes +away from the laptop keyboard, it probably should stay in the working state and +use the working-state power management in case it becomes idle, because the user +may come back to it at any time and then may want the system to be immediately +accessible. diff --git a/Documentation/admin-guide/pm/system-wide.rst b/Documentation/admin-guide/pm/system-wide.rst new file mode 100644 index 000000000..0c81e4c5d --- /dev/null +++ b/Documentation/admin-guide/pm/system-wide.rst @@ -0,0 +1,8 @@ +============================ +System-Wide Power Management +============================ + +.. toctree:: + :maxdepth: 2 + + sleep-states diff --git a/Documentation/admin-guide/pm/working-state.rst b/Documentation/admin-guide/pm/working-state.rst new file mode 100644 index 000000000..fa01bf083 --- /dev/null +++ b/Documentation/admin-guide/pm/working-state.rst @@ -0,0 +1,9 @@ +============================== +Working-State Power Management +============================== + +.. toctree:: + :maxdepth: 2 + + cpufreq + intel_pstate |