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Diffstat (limited to 'Documentation/admin-guide/pm/intel_pstate.rst')
| -rw-r--r-- | Documentation/admin-guide/pm/intel_pstate.rst | 218 |
1 files changed, 171 insertions, 47 deletions
diff --git a/Documentation/admin-guide/pm/intel_pstate.rst b/Documentation/admin-guide/pm/intel_pstate.rst index 39d80bc29ccd..26e702c7016e 100644 --- a/Documentation/admin-guide/pm/intel_pstate.rst +++ b/Documentation/admin-guide/pm/intel_pstate.rst @@ -18,8 +18,8 @@ General Information (``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.] +how ``CPUFreq`` works in general, so this is the time to read +Documentation/admin-guide/pm/cpufreq.rst 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 @@ -54,10 +54,13 @@ 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. +``intel_pstate`` can operate in two different modes, active or passive. In the +active mode, it uses its own internal performance scaling governor algorithm or +allows the hardware to do performance scaling by itself, while in the passive +mode it responds to requests made by a generic ``CPUFreq`` governor implementing +a certain performance scaling algorithm. 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 ----------- @@ -120,7 +123,9 @@ 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). +(see `Energy vs Performance Hints`_ below). Moreover, any attempts to change +the EPP/EPB to a value different from 0 ("performance") via ``sysfs`` in this +configuration will be rejected. 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 @@ -194,10 +199,11 @@ This is the default operation mode of ``intel_pstate`` for processors without hardware-managed P-states (HWP) support. It is always used if the ``intel_pstate=passive`` argument is passed to the kernel in the command line regardless of whether or not the given processor supports HWP. [Note that the -``intel_pstate=no_hwp`` setting implies ``intel_pstate=passive`` if it is used -without ``intel_pstate=active``.] Like in the active mode without HWP support, -in this mode ``intel_pstate`` may refuse to work with processors that are not -recognized by it. +``intel_pstate=no_hwp`` setting causes the driver to start in the passive mode +if it is not combined with ``intel_pstate=active``.] Like in the active mode +without HWP support, in this mode ``intel_pstate`` may refuse to work with +processors that are not recognized by it if HWP is prevented from being enabled +through the kernel command line. If the driver works in this mode, the ``scaling_driver`` policy attribute in ``sysfs`` for all ``CPUFreq`` policies contains the string "intel_cpufreq". @@ -318,10 +324,109 @@ 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``.] +list, unless it supports the HWP feature. [The interface to obtain all of the +information listed above is the same for all of the processors supporting the +HWP feature, which is why ``intel_pstate`` works with all of them.] + + +Support for Hybrid Processors +============================= + +Some processors supported by ``intel_pstate`` contain two or more types of CPU +cores differing by the maximum turbo P-state, performance vs power characteristics, +cache sizes, and possibly other properties. They are commonly referred to as +hybrid processors. To support them, ``intel_pstate`` requires HWP to be enabled +and it assumes the HWP performance units to be the same for all CPUs in the +system, so a given HWP performance level always represents approximately the +same physical performance regardless of the core (CPU) type. + +Hybrid Processors with SMT +-------------------------- + +On systems where SMT (Simultaneous Multithreading), also referred to as +HyperThreading (HT) in the context of Intel processors, is enabled on at least +one core, ``intel_pstate`` assigns performance-based priorities to CPUs. Namely, +the priority of a given CPU reflects its highest HWP performance level which +causes the CPU scheduler to generally prefer more performant CPUs, so the less +performant CPUs are used when the other ones are fully loaded. However, SMT +siblings (that is, logical CPUs sharing one physical core) are treated in a +special way such that if one of them is in use, the effective priority of the +other ones is lowered below the priorities of the CPUs located in the other +physical cores. + +This approach maximizes performance in the majority of cases, but unfortunately +it also leads to excessive energy usage in some important scenarios, like video +playback, which is not generally desirable. While there is no other viable +choice with SMT enabled because the effective capacity and utilization of SMT +siblings are hard to determine, hybrid processors without SMT can be handled in +more energy-efficient ways. + +.. _CAS: + +Capacity-Aware Scheduling Support +--------------------------------- + +The capacity-aware scheduling (CAS) support in the CPU scheduler is enabled by +``intel_pstate`` by default on hybrid processors without SMT. CAS generally +causes the scheduler to put tasks on a CPU so long as there is a sufficient +amount of spare capacity on it, and if the utilization of a given task is too +high for it, the task will need to go somewhere else. + +Since CAS takes CPU capacities into account, it does not require CPU +prioritization and it allows tasks to be distributed more symmetrically among +the more performant and less performant CPUs. Once placed on a CPU with enough +capacity to accommodate it, a task may just continue to run there regardless of +whether or not the other CPUs are fully loaded, so on average CAS reduces the +utilization of the more performant CPUs which causes the energy usage to be more +balanced because the more performant CPUs are generally less energy-efficient +than the less performant ones. + +In order to use CAS, the scheduler needs to know the capacity of each CPU in +the system and it needs to be able to compute scale-invariant utilization of +CPUs, so ``intel_pstate`` provides it with the requisite information. + +First of all, the capacity of each CPU is represented by the ratio of its highest +HWP performance level, multiplied by 1024, to the highest HWP performance level +of the most performant CPU in the system, which works because the HWP performance +units are the same for all CPUs. Second, the frequency-invariance computations, +carried out by the scheduler to always express CPU utilization in the same units +regardless of the frequency it is currently running at, are adjusted to take the +CPU capacity into account. All of this happens when ``intel_pstate`` has +registered itself with the ``CPUFreq`` core and it has figured out that it is +running on a hybrid processor without SMT. + +Energy-Aware Scheduling Support +------------------------------- + +If ``CONFIG_ENERGY_MODEL`` has been set during kernel configuration and +``intel_pstate`` runs on a hybrid processor without SMT, in addition to enabling +`CAS <CAS_>`_ it registers an Energy Model for the processor. This allows the +Energy-Aware Scheduling (EAS) support to be enabled in the CPU scheduler if +``schedutil`` is used as the ``CPUFreq`` governor which requires ``intel_pstate`` +to operate in the `passive mode <Passive Mode_>`_. + +The Energy Model registered by ``intel_pstate`` is artificial (that is, it is +based on abstract cost values and it does not include any real power numbers) +and it is relatively simple to avoid unnecessary computations in the scheduler. +There is a performance domain in it for every CPU in the system and the cost +values for these performance domains have been chosen so that running a task on +a less performant (small) CPU appears to be always cheaper than running that +task on a more performant (big) CPU. However, for two CPUs of the same type, +the cost difference depends on their current utilization, and the CPU whose +current utilization is higher generally appears to be a more expensive +destination for a given task. This helps to balance the load among CPUs of the +same type. + +Since EAS works on top of CAS, high-utilization tasks are always migrated to +CPUs with enough capacity to accommodate them, but thanks to EAS, low-utilization +tasks tend to be placed on the CPUs that look less expensive to the scheduler. +Effectively, this causes the less performant and less loaded CPUs to be +preferred as long as they have enough spare capacity to run the given task +which generally leads to reduced energy usage. + +The Energy Model created by ``intel_pstate`` can be inspected by looking at +the ``energy_model`` directory in ``debugfs`` (typlically mounted on +``/sys/kernel/debug/``). User Space Interface in ``sysfs`` @@ -360,6 +465,9 @@ argument is passed to the kernel in the command line. inclusive) including both turbo and non-turbo P-states (see `Turbo P-states Support`_). + This attribute is present only if the value exposed by it is the same + for all of the CPUs in the system. + The value of this attribute is not affected by the ``no_turbo`` setting described `below <no_turbo_attr_>`_. @@ -369,19 +477,22 @@ argument is passed to the kernel in the command line. Ratio of the `turbo range <turbo_>`_ size to the size of the entire range of supported P-states, in percent. + This attribute is present only if the value exposed by it is the same + for all of the CPUs in the system. + 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 + (see `Turbo P-states Support`_). If unset (equal 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 + This attribute 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). @@ -425,18 +536,24 @@ argument is passed to the kernel in the command line. 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. +``energy_efficiency`` + This attribute is only present on platforms with CPUs matching the Kaby + Lake or Coffee Lake desktop CPU model. By default, energy-efficiency + optimizations are disabled on these CPU models if HWP is enabled. + Enabling energy-efficiency optimizations may limit maximum operating + frequency with or without the HWP feature. With HWP enabled, the + optimizations are done only in the turbo frequency range. Without it, + they are done in the entire available frequency range. Setting this + attribute to "1" enables the energy-efficiency optimizations and setting + to "0" disables them. 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_>`_. +Documentation/admin-guide/pm/cpufreq.rst 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 @@ -473,8 +590,8 @@ Next, the following policy attributes have special meaning if policy for the time interval between the last two invocations of the driver's utilization update callback by the CPU scheduler for that CPU. -One more policy attribute is present if the `HWP feature is enabled in the -processor <Active Mode With HWP_>`_: +One more policy attribute is present if the HWP feature is enabled in the +processor: ``base_frequency`` Shows the base frequency of the CPU. Any frequency above this will be @@ -515,11 +632,11 @@ on the following rules, regardless of the current operation mode of the driver: 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 +In the `active mode with the HWP feature enabled <Active Mode With HWP_>`_, the +resulting effective values are written into hardware 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 @@ -530,12 +647,11 @@ 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: +If the hardware-managed P-states (HWP) is enabled in the processor, additional +attributes, 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, are present in every +``CPUFreq`` policy directory in ``sysfs``. They are : ``energy_performance_preference`` Current value of the energy vs performance hint for the given policy @@ -554,7 +670,11 @@ somewhere between the two extremes: 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. +Energy-Performance Bias (EPB) knob. It is also possible to write a positive +integer value between 0 to 255, if the EPP feature is present. If the EPP +feature is not present, writing integer value to this attribute is not +supported. In this case, user can use the +"/sys/devices/system/cpu/cpu*/power/energy_perf_bias" interface. [Note that tasks may by migrated from one CPU to another by the scheduler's load-balancing algorithm and if different energy vs performance hints are @@ -635,12 +755,14 @@ of them have to be prepended with the ``intel_pstate=`` prefix. Do not register ``intel_pstate`` as the scaling driver even if the processor is supported by it. +``active`` + Register ``intel_pstate`` in the `active mode <Active Mode_>`_ to start + with. + ``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. @@ -655,13 +777,12 @@ of them have to be prepended with the ``intel_pstate=`` prefix. 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. + Do not enable the hardware-managed P-states (HWP) feature even if it is + supported by the processor. ``hwp_only`` Register ``intel_pstate`` as the scaling driver only if the - `hardware-managed P-states (HWP) feature <Active Mode With HWP_>`_ is - supported by the processor. + hardware-managed P-states (HWP) feature is supported by the processor. ``support_acpi_ppc`` Take ACPI ``_PPC`` performance limits into account. @@ -675,6 +796,9 @@ of them have to be prepended with the ``intel_pstate=`` prefix. Use per-logical-CPU P-State limits (see `Coordination of P-state Limits`_ for details). +``no_cas`` + Do not enable `capacity-aware scheduling <CAS_>`_ which is enabled by + default on hybrid systems without SMT. Diagnostics and Tuning ====================== @@ -691,7 +815,7 @@ 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/ + # cd /sys/kernel/tracing/ # echo 1 > events/power/pstate_sample/enable # echo 1 > events/power/cpu_frequency/enable # cat trace @@ -708,10 +832,10 @@ core (for the policies with other scaling governors). 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 +P-state is called, the ``ftrace`` filter can be set to :c:func:`intel_pstate_set_pstate`:: - # cd /sys/kernel/debug/tracing/ + # cd /sys/kernel/tracing/ # cat available_filter_functions | grep -i pstate intel_pstate_set_pstate intel_pstate_cpu_init |