1.. SPDX-License-Identifier: GPL-2.0 2.. include:: <isonum.txt> 3 4=============================================== 5``intel_pstate`` CPU Performance Scaling Driver 6=============================================== 7 8:Copyright: |copy| 2017 Intel Corporation 9 10:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com> 11 12 13General Information 14=================== 15 16``intel_pstate`` is a part of the 17:doc:`CPU performance scaling subsystem <cpufreq>` in the Linux kernel 18(``CPUFreq``). It is a scaling driver for the Sandy Bridge and later 19generations of Intel processors. Note, however, that some of those processors 20may not be supported. [To understand ``intel_pstate`` it is necessary to know 21how ``CPUFreq`` works in general, so this is the time to read :doc:`cpufreq` if 22you have not done that yet.] 23 24For the processors supported by ``intel_pstate``, the P-state concept is broader 25than just an operating frequency or an operating performance point (see the 26LinuxCon Europe 2015 presentation by Kristen Accardi [1]_ for more 27information about that). For this reason, the representation of P-states used 28by ``intel_pstate`` internally follows the hardware specification (for details 29refer to Intel Software Developer’s Manual [2]_). However, the ``CPUFreq`` core 30uses frequencies for identifying operating performance points of CPUs and 31frequencies are involved in the user space interface exposed by it, so 32``intel_pstate`` maps its internal representation of P-states to frequencies too 33(fortunately, that mapping is unambiguous). At the same time, it would not be 34practical for ``intel_pstate`` to supply the ``CPUFreq`` core with a table of 35available frequencies due to the possible size of it, so the driver does not do 36that. Some functionality of the core is limited by that. 37 38Since the hardware P-state selection interface used by ``intel_pstate`` is 39available at the logical CPU level, the driver always works with individual 40CPUs. Consequently, if ``intel_pstate`` is in use, every ``CPUFreq`` policy 41object corresponds to one logical CPU and ``CPUFreq`` policies are effectively 42equivalent to CPUs. In particular, this means that they become "inactive" every 43time the corresponding CPU is taken offline and need to be re-initialized when 44it goes back online. 45 46``intel_pstate`` is not modular, so it cannot be unloaded, which means that the 47only way to pass early-configuration-time parameters to it is via the kernel 48command line. However, its configuration can be adjusted via ``sysfs`` to a 49great extent. In some configurations it even is possible to unregister it via 50``sysfs`` which allows another ``CPUFreq`` scaling driver to be loaded and 51registered (see `below <status_attr_>`_). 52 53 54Operation Modes 55=============== 56 57``intel_pstate`` can operate in two different modes, active or passive. In the 58active mode, it uses its own internal performance scaling governor algorithm or 59allows the hardware to do preformance scaling by itself, while in the passive 60mode it responds to requests made by a generic ``CPUFreq`` governor implementing 61a certain performance scaling algorithm. Which of them will be in effect 62depends on what kernel command line options are used and on the capabilities of 63the processor. 64 65Active Mode 66----------- 67 68This is the default operation mode of ``intel_pstate`` for processors with 69hardware-managed P-states (HWP) support. If it works in this mode, the 70``scaling_driver`` policy attribute in ``sysfs`` for all ``CPUFreq`` policies 71contains the string "intel_pstate". 72 73In this mode the driver bypasses the scaling governors layer of ``CPUFreq`` and 74provides its own scaling algorithms for P-state selection. Those algorithms 75can be applied to ``CPUFreq`` policies in the same way as generic scaling 76governors (that is, through the ``scaling_governor`` policy attribute in 77``sysfs``). [Note that different P-state selection algorithms may be chosen for 78different policies, but that is not recommended.] 79 80They are not generic scaling governors, but their names are the same as the 81names of some of those governors. Moreover, confusingly enough, they generally 82do not work in the same way as the generic governors they share the names with. 83For example, the ``powersave`` P-state selection algorithm provided by 84``intel_pstate`` is not a counterpart of the generic ``powersave`` governor 85(roughly, it corresponds to the ``schedutil`` and ``ondemand`` governors). 86 87There are two P-state selection algorithms provided by ``intel_pstate`` in the 88active mode: ``powersave`` and ``performance``. The way they both operate 89depends on whether or not the hardware-managed P-states (HWP) feature has been 90enabled in the processor and possibly on the processor model. 91 92Which of the P-state selection algorithms is used by default depends on the 93:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option. 94Namely, if that option is set, the ``performance`` algorithm will be used by 95default, and the other one will be used by default if it is not set. 96 97Active Mode With HWP 98~~~~~~~~~~~~~~~~~~~~ 99 100If the processor supports the HWP feature, it will be enabled during the 101processor initialization and cannot be disabled after that. It is possible 102to avoid enabling it by passing the ``intel_pstate=no_hwp`` argument to the 103kernel in the command line. 104 105If the HWP feature has been enabled, ``intel_pstate`` relies on the processor to 106select P-states by itself, but still it can give hints to the processor's 107internal P-state selection logic. What those hints are depends on which P-state 108selection algorithm has been applied to the given policy (or to the CPU it 109corresponds to). 110 111Even though the P-state selection is carried out by the processor automatically, 112``intel_pstate`` registers utilization update callbacks with the CPU scheduler 113in this mode. However, they are not used for running a P-state selection 114algorithm, but for periodic updates of the current CPU frequency information to 115be made available from the ``scaling_cur_freq`` policy attribute in ``sysfs``. 116 117HWP + ``performance`` 118..................... 119 120In this configuration ``intel_pstate`` will write 0 to the processor's 121Energy-Performance Preference (EPP) knob (if supported) or its 122Energy-Performance Bias (EPB) knob (otherwise), which means that the processor's 123internal P-state selection logic is expected to focus entirely on performance. 124 125This will override the EPP/EPB setting coming from the ``sysfs`` interface 126(see `Energy vs Performance Hints`_ below). Moreover, any attempts to change 127the EPP/EPB to a value different from 0 ("performance") via ``sysfs`` in this 128configuration will be rejected. 129 130Also, in this configuration the range of P-states available to the processor's 131internal P-state selection logic is always restricted to the upper boundary 132(that is, the maximum P-state that the driver is allowed to use). 133 134HWP + ``powersave`` 135................... 136 137In this configuration ``intel_pstate`` will set the processor's 138Energy-Performance Preference (EPP) knob (if supported) or its 139Energy-Performance Bias (EPB) knob (otherwise) to whatever value it was 140previously set to via ``sysfs`` (or whatever default value it was 141set to by the platform firmware). This usually causes the processor's 142internal P-state selection logic to be less performance-focused. 143 144Active Mode Without HWP 145~~~~~~~~~~~~~~~~~~~~~~~ 146 147This operation mode is optional for processors that do not support the HWP 148feature or when the ``intel_pstate=no_hwp`` argument is passed to the kernel in 149the command line. The active mode is used in those cases if the 150``intel_pstate=active`` argument is passed to the kernel in the command line. 151In this mode ``intel_pstate`` may refuse to work with processors that are not 152recognized by it. [Note that ``intel_pstate`` will never refuse to work with 153any processor with the HWP feature enabled.] 154 155In this mode ``intel_pstate`` registers utilization update callbacks with the 156CPU scheduler in order to run a P-state selection algorithm, either 157``powersave`` or ``performance``, depending on the ``scaling_governor`` policy 158setting in ``sysfs``. The current CPU frequency information to be made 159available from the ``scaling_cur_freq`` policy attribute in ``sysfs`` is 160periodically updated by those utilization update callbacks too. 161 162``performance`` 163............... 164 165Without HWP, this P-state selection algorithm is always the same regardless of 166the processor model and platform configuration. 167 168It selects the maximum P-state it is allowed to use, subject to limits set via 169``sysfs``, every time the driver configuration for the given CPU is updated 170(e.g. via ``sysfs``). 171 172This is the default P-state selection algorithm if the 173:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option 174is set. 175 176``powersave`` 177............. 178 179Without HWP, this P-state selection algorithm is similar to the algorithm 180implemented by the generic ``schedutil`` scaling governor except that the 181utilization metric used by it is based on numbers coming from feedback 182registers of the CPU. It generally selects P-states proportional to the 183current CPU utilization. 184 185This algorithm is run by the driver's utilization update callback for the 186given CPU when it is invoked by the CPU scheduler, but not more often than 187every 10 ms. Like in the ``performance`` case, the hardware configuration 188is not touched if the new P-state turns out to be the same as the current 189one. 190 191This is the default P-state selection algorithm if the 192:c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option 193is not set. 194 195Passive Mode 196------------ 197 198This is the default operation mode of ``intel_pstate`` for processors without 199hardware-managed P-states (HWP) support. It is always used if the 200``intel_pstate=passive`` argument is passed to the kernel in the command line 201regardless of whether or not the given processor supports HWP. [Note that the 202``intel_pstate=no_hwp`` setting causes the driver to start in the passive mode 203if it is not combined with ``intel_pstate=active``.] Like in the active mode 204without HWP support, in this mode ``intel_pstate`` may refuse to work with 205processors that are not recognized by it if HWP is prevented from being enabled 206through the kernel command line. 207 208If the driver works in this mode, the ``scaling_driver`` policy attribute in 209``sysfs`` for all ``CPUFreq`` policies contains the string "intel_cpufreq". 210Then, the driver behaves like a regular ``CPUFreq`` scaling driver. That is, 211it is invoked by generic scaling governors when necessary to talk to the 212hardware in order to change the P-state of a CPU (in particular, the 213``schedutil`` governor can invoke it directly from scheduler context). 214 215While in this mode, ``intel_pstate`` can be used with all of the (generic) 216scaling governors listed by the ``scaling_available_governors`` policy attribute 217in ``sysfs`` (and the P-state selection algorithms described above are not 218used). Then, it is responsible for the configuration of policy objects 219corresponding to CPUs and provides the ``CPUFreq`` core (and the scaling 220governors attached to the policy objects) with accurate information on the 221maximum and minimum operating frequencies supported by the hardware (including 222the so-called "turbo" frequency ranges). In other words, in the passive mode 223the entire range of available P-states is exposed by ``intel_pstate`` to the 224``CPUFreq`` core. However, in this mode the driver does not register 225utilization update callbacks with the CPU scheduler and the ``scaling_cur_freq`` 226information comes from the ``CPUFreq`` core (and is the last frequency selected 227by the current scaling governor for the given policy). 228 229 230.. _turbo: 231 232Turbo P-states Support 233====================== 234 235In the majority of cases, the entire range of P-states available to 236``intel_pstate`` can be divided into two sub-ranges that correspond to 237different types of processor behavior, above and below a boundary that 238will be referred to as the "turbo threshold" in what follows. 239 240The P-states above the turbo threshold are referred to as "turbo P-states" and 241the whole sub-range of P-states they belong to is referred to as the "turbo 242range". These names are related to the Turbo Boost technology allowing a 243multicore processor to opportunistically increase the P-state of one or more 244cores if there is enough power to do that and if that is not going to cause the 245thermal envelope of the processor package to be exceeded. 246 247Specifically, if software sets the P-state of a CPU core within the turbo range 248(that is, above the turbo threshold), the processor is permitted to take over 249performance scaling control for that core and put it into turbo P-states of its 250choice going forward. However, that permission is interpreted differently by 251different processor generations. Namely, the Sandy Bridge generation of 252processors will never use any P-states above the last one set by software for 253the given core, even if it is within the turbo range, whereas all of the later 254processor generations will take it as a license to use any P-states from the 255turbo range, even above the one set by software. In other words, on those 256processors setting any P-state from the turbo range will enable the processor 257to put the given core into all turbo P-states up to and including the maximum 258supported one as it sees fit. 259 260One important property of turbo P-states is that they are not sustainable. More 261precisely, there is no guarantee that any CPUs will be able to stay in any of 262those states indefinitely, because the power distribution within the processor 263package may change over time or the thermal envelope it was designed for might 264be exceeded if a turbo P-state was used for too long. 265 266In turn, the P-states below the turbo threshold generally are sustainable. In 267fact, if one of them is set by software, the processor is not expected to change 268it to a lower one unless in a thermal stress or a power limit violation 269situation (a higher P-state may still be used if it is set for another CPU in 270the same package at the same time, for example). 271 272Some processors allow multiple cores to be in turbo P-states at the same time, 273but the maximum P-state that can be set for them generally depends on the number 274of cores running concurrently. The maximum turbo P-state that can be set for 3 275cores at the same time usually is lower than the analogous maximum P-state for 2762 cores, which in turn usually is lower than the maximum turbo P-state that can 277be set for 1 core. The one-core maximum turbo P-state is thus the maximum 278supported one overall. 279 280The maximum supported turbo P-state, the turbo threshold (the maximum supported 281non-turbo P-state) and the minimum supported P-state are specific to the 282processor model and can be determined by reading the processor's model-specific 283registers (MSRs). Moreover, some processors support the Configurable TDP 284(Thermal Design Power) feature and, when that feature is enabled, the turbo 285threshold effectively becomes a configurable value that can be set by the 286platform firmware. 287 288Unlike ``_PSS`` objects in the ACPI tables, ``intel_pstate`` always exposes 289the entire range of available P-states, including the whole turbo range, to the 290``CPUFreq`` core and (in the passive mode) to generic scaling governors. This 291generally causes turbo P-states to be set more often when ``intel_pstate`` is 292used relative to ACPI-based CPU performance scaling (see `below <acpi-cpufreq_>`_ 293for more information). 294 295Moreover, since ``intel_pstate`` always knows what the real turbo threshold is 296(even if the Configurable TDP feature is enabled in the processor), its 297``no_turbo`` attribute in ``sysfs`` (described `below <no_turbo_attr_>`_) should 298work as expected in all cases (that is, if set to disable turbo P-states, it 299always should prevent ``intel_pstate`` from using them). 300 301 302Processor Support 303================= 304 305To handle a given processor ``intel_pstate`` requires a number of different 306pieces of information on it to be known, including: 307 308 * The minimum supported P-state. 309 310 * The maximum supported `non-turbo P-state <turbo_>`_. 311 312 * Whether or not turbo P-states are supported at all. 313 314 * The maximum supported `one-core turbo P-state <turbo_>`_ (if turbo P-states 315 are supported). 316 317 * The scaling formula to translate the driver's internal representation 318 of P-states into frequencies and the other way around. 319 320Generally, ways to obtain that information are specific to the processor model 321or family. Although it often is possible to obtain all of it from the processor 322itself (using model-specific registers), there are cases in which hardware 323manuals need to be consulted to get to it too. 324 325For this reason, there is a list of supported processors in ``intel_pstate`` and 326the driver initialization will fail if the detected processor is not in that 327list, unless it supports the HWP feature. [The interface to obtain all of the 328information listed above is the same for all of the processors supporting the 329HWP feature, which is why ``intel_pstate`` works with all of them.] 330 331 332User Space Interface in ``sysfs`` 333================================= 334 335Global Attributes 336----------------- 337 338``intel_pstate`` exposes several global attributes (files) in ``sysfs`` to 339control its functionality at the system level. They are located in the 340``/sys/devices/system/cpu/intel_pstate/`` directory and affect all CPUs. 341 342Some of them are not present if the ``intel_pstate=per_cpu_perf_limits`` 343argument is passed to the kernel in the command line. 344 345``max_perf_pct`` 346 Maximum P-state the driver is allowed to set in percent of the 347 maximum supported performance level (the highest supported `turbo 348 P-state <turbo_>`_). 349 350 This attribute will not be exposed if the 351 ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel 352 command line. 353 354``min_perf_pct`` 355 Minimum P-state the driver is allowed to set in percent of the 356 maximum supported performance level (the highest supported `turbo 357 P-state <turbo_>`_). 358 359 This attribute will not be exposed if the 360 ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel 361 command line. 362 363``num_pstates`` 364 Number of P-states supported by the processor (between 0 and 255 365 inclusive) including both turbo and non-turbo P-states (see 366 `Turbo P-states Support`_). 367 368 The value of this attribute is not affected by the ``no_turbo`` 369 setting described `below <no_turbo_attr_>`_. 370 371 This attribute is read-only. 372 373``turbo_pct`` 374 Ratio of the `turbo range <turbo_>`_ size to the size of the entire 375 range of supported P-states, in percent. 376 377 This attribute is read-only. 378 379.. _no_turbo_attr: 380 381``no_turbo`` 382 If set (equal to 1), the driver is not allowed to set any turbo P-states 383 (see `Turbo P-states Support`_). If unset (equalt to 0, which is the 384 default), turbo P-states can be set by the driver. 385 [Note that ``intel_pstate`` does not support the general ``boost`` 386 attribute (supported by some other scaling drivers) which is replaced 387 by this one.] 388 389 This attrubute does not affect the maximum supported frequency value 390 supplied to the ``CPUFreq`` core and exposed via the policy interface, 391 but it affects the maximum possible value of per-policy P-state limits 392 (see `Interpretation of Policy Attributes`_ below for details). 393 394``hwp_dynamic_boost`` 395 This attribute is only present if ``intel_pstate`` works in the 396 `active mode with the HWP feature enabled <Active Mode With HWP_>`_ in 397 the processor. If set (equal to 1), it causes the minimum P-state limit 398 to be increased dynamically for a short time whenever a task previously 399 waiting on I/O is selected to run on a given logical CPU (the purpose 400 of this mechanism is to improve performance). 401 402 This setting has no effect on logical CPUs whose minimum P-state limit 403 is directly set to the highest non-turbo P-state or above it. 404 405.. _status_attr: 406 407``status`` 408 Operation mode of the driver: "active", "passive" or "off". 409 410 "active" 411 The driver is functional and in the `active mode 412 <Active Mode_>`_. 413 414 "passive" 415 The driver is functional and in the `passive mode 416 <Passive Mode_>`_. 417 418 "off" 419 The driver is not functional (it is not registered as a scaling 420 driver with the ``CPUFreq`` core). 421 422 This attribute can be written to in order to change the driver's 423 operation mode or to unregister it. The string written to it must be 424 one of the possible values of it and, if successful, the write will 425 cause the driver to switch over to the operation mode represented by 426 that string - or to be unregistered in the "off" case. [Actually, 427 switching over from the active mode to the passive mode or the other 428 way around causes the driver to be unregistered and registered again 429 with a different set of callbacks, so all of its settings (the global 430 as well as the per-policy ones) are then reset to their default 431 values, possibly depending on the target operation mode.] 432 433``energy_efficiency`` 434 This attribute is only present on platforms with CPUs matching the Kaby 435 Lake or Coffee Lake desktop CPU model. By default, energy-efficiency 436 optimizations are disabled on these CPU models if HWP is enabled. 437 Enabling energy-efficiency optimizations may limit maximum operating 438 frequency with or without the HWP feature. With HWP enabled, the 439 optimizations are done only in the turbo frequency range. Without it, 440 they are done in the entire available frequency range. Setting this 441 attribute to "1" enables the energy-efficiency optimizations and setting 442 to "0" disables them. 443 444Interpretation of Policy Attributes 445----------------------------------- 446 447The interpretation of some ``CPUFreq`` policy attributes described in 448:doc:`cpufreq` is special with ``intel_pstate`` as the current scaling driver 449and it generally depends on the driver's `operation mode <Operation Modes_>`_. 450 451First of all, the values of the ``cpuinfo_max_freq``, ``cpuinfo_min_freq`` and 452``scaling_cur_freq`` attributes are produced by applying a processor-specific 453multiplier to the internal P-state representation used by ``intel_pstate``. 454Also, the values of the ``scaling_max_freq`` and ``scaling_min_freq`` 455attributes are capped by the frequency corresponding to the maximum P-state that 456the driver is allowed to set. 457 458If the ``no_turbo`` `global attribute <no_turbo_attr_>`_ is set, the driver is 459not allowed to use turbo P-states, so the maximum value of ``scaling_max_freq`` 460and ``scaling_min_freq`` is limited to the maximum non-turbo P-state frequency. 461Accordingly, setting ``no_turbo`` causes ``scaling_max_freq`` and 462``scaling_min_freq`` to go down to that value if they were above it before. 463However, the old values of ``scaling_max_freq`` and ``scaling_min_freq`` will be 464restored after unsetting ``no_turbo``, unless these attributes have been written 465to after ``no_turbo`` was set. 466 467If ``no_turbo`` is not set, the maximum possible value of ``scaling_max_freq`` 468and ``scaling_min_freq`` corresponds to the maximum supported turbo P-state, 469which also is the value of ``cpuinfo_max_freq`` in either case. 470 471Next, the following policy attributes have special meaning if 472``intel_pstate`` works in the `active mode <Active Mode_>`_: 473 474``scaling_available_governors`` 475 List of P-state selection algorithms provided by ``intel_pstate``. 476 477``scaling_governor`` 478 P-state selection algorithm provided by ``intel_pstate`` currently in 479 use with the given policy. 480 481``scaling_cur_freq`` 482 Frequency of the average P-state of the CPU represented by the given 483 policy for the time interval between the last two invocations of the 484 driver's utilization update callback by the CPU scheduler for that CPU. 485 486One more policy attribute is present if the HWP feature is enabled in the 487processor: 488 489``base_frequency`` 490 Shows the base frequency of the CPU. Any frequency above this will be 491 in the turbo frequency range. 492 493The meaning of these attributes in the `passive mode <Passive Mode_>`_ is the 494same as for other scaling drivers. 495 496Additionally, the value of the ``scaling_driver`` attribute for ``intel_pstate`` 497depends on the operation mode of the driver. Namely, it is either 498"intel_pstate" (in the `active mode <Active Mode_>`_) or "intel_cpufreq" (in the 499`passive mode <Passive Mode_>`_). 500 501Coordination of P-State Limits 502------------------------------ 503 504``intel_pstate`` allows P-state limits to be set in two ways: with the help of 505the ``max_perf_pct`` and ``min_perf_pct`` `global attributes 506<Global Attributes_>`_ or via the ``scaling_max_freq`` and ``scaling_min_freq`` 507``CPUFreq`` policy attributes. The coordination between those limits is based 508on the following rules, regardless of the current operation mode of the driver: 509 510 1. All CPUs are affected by the global limits (that is, none of them can be 511 requested to run faster than the global maximum and none of them can be 512 requested to run slower than the global minimum). 513 514 2. Each individual CPU is affected by its own per-policy limits (that is, it 515 cannot be requested to run faster than its own per-policy maximum and it 516 cannot be requested to run slower than its own per-policy minimum). The 517 effective performance depends on whether the platform supports per core 518 P-states, hyper-threading is enabled and on current performance requests 519 from other CPUs. When platform doesn't support per core P-states, the 520 effective performance can be more than the policy limits set on a CPU, if 521 other CPUs are requesting higher performance at that moment. Even with per 522 core P-states support, when hyper-threading is enabled, if the sibling CPU 523 is requesting higher performance, the other siblings will get higher 524 performance than their policy limits. 525 526 3. The global and per-policy limits can be set independently. 527 528In the `active mode with the HWP feature enabled <Active Mode With HWP_>`_, the 529resulting effective values are written into hardware registers whenever the 530limits change in order to request its internal P-state selection logic to always 531set P-states within these limits. Otherwise, the limits are taken into account 532by scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver 533every time before setting a new P-state for a CPU. 534 535Additionally, if the ``intel_pstate=per_cpu_perf_limits`` command line argument 536is passed to the kernel, ``max_perf_pct`` and ``min_perf_pct`` are not exposed 537at all and the only way to set the limits is by using the policy attributes. 538 539 540Energy vs Performance Hints 541--------------------------- 542 543If the hardware-managed P-states (HWP) is enabled in the processor, additional 544attributes, intended to allow user space to help ``intel_pstate`` to adjust the 545processor's internal P-state selection logic by focusing it on performance or on 546energy-efficiency, or somewhere between the two extremes, are present in every 547``CPUFreq`` policy directory in ``sysfs``. They are : 548 549``energy_performance_preference`` 550 Current value of the energy vs performance hint for the given policy 551 (or the CPU represented by it). 552 553 The hint can be changed by writing to this attribute. 554 555``energy_performance_available_preferences`` 556 List of strings that can be written to the 557 ``energy_performance_preference`` attribute. 558 559 They represent different energy vs performance hints and should be 560 self-explanatory, except that ``default`` represents whatever hint 561 value was set by the platform firmware. 562 563Strings written to the ``energy_performance_preference`` attribute are 564internally translated to integer values written to the processor's 565Energy-Performance Preference (EPP) knob (if supported) or its 566Energy-Performance Bias (EPB) knob. It is also possible to write a positive 567integer value between 0 to 255, if the EPP feature is present. If the EPP 568feature is not present, writing integer value to this attribute is not 569supported. In this case, user can use the 570"/sys/devices/system/cpu/cpu*/power/energy_perf_bias" interface. 571 572[Note that tasks may by migrated from one CPU to another by the scheduler's 573load-balancing algorithm and if different energy vs performance hints are 574set for those CPUs, that may lead to undesirable outcomes. To avoid such 575issues it is better to set the same energy vs performance hint for all CPUs 576or to pin every task potentially sensitive to them to a specific CPU.] 577 578.. _acpi-cpufreq: 579 580``intel_pstate`` vs ``acpi-cpufreq`` 581==================================== 582 583On the majority of systems supported by ``intel_pstate``, the ACPI tables 584provided by the platform firmware contain ``_PSS`` objects returning information 585that can be used for CPU performance scaling (refer to the ACPI specification 586[3]_ for details on the ``_PSS`` objects and the format of the information 587returned by them). 588 589The information returned by the ACPI ``_PSS`` objects is used by the 590``acpi-cpufreq`` scaling driver. On systems supported by ``intel_pstate`` 591the ``acpi-cpufreq`` driver uses the same hardware CPU performance scaling 592interface, but the set of P-states it can use is limited by the ``_PSS`` 593output. 594 595On those systems each ``_PSS`` object returns a list of P-states supported by 596the corresponding CPU which basically is a subset of the P-states range that can 597be used by ``intel_pstate`` on the same system, with one exception: the whole 598`turbo range <turbo_>`_ is represented by one item in it (the topmost one). By 599convention, the frequency returned by ``_PSS`` for that item is greater by 1 MHz 600than the frequency of the highest non-turbo P-state listed by it, but the 601corresponding P-state representation (following the hardware specification) 602returned for it matches the maximum supported turbo P-state (or is the 603special value 255 meaning essentially "go as high as you can get"). 604 605The list of P-states returned by ``_PSS`` is reflected by the table of 606available frequencies supplied by ``acpi-cpufreq`` to the ``CPUFreq`` core and 607scaling governors and the minimum and maximum supported frequencies reported by 608it come from that list as well. In particular, given the special representation 609of the turbo range described above, this means that the maximum supported 610frequency reported by ``acpi-cpufreq`` is higher by 1 MHz than the frequency 611of the highest supported non-turbo P-state listed by ``_PSS`` which, of course, 612affects decisions made by the scaling governors, except for ``powersave`` and 613``performance``. 614 615For example, if a given governor attempts to select a frequency proportional to 616estimated CPU load and maps the load of 100% to the maximum supported frequency 617(possibly multiplied by a constant), then it will tend to choose P-states below 618the turbo threshold if ``acpi-cpufreq`` is used as the scaling driver, because 619in that case the turbo range corresponds to a small fraction of the frequency 620band it can use (1 MHz vs 1 GHz or more). In consequence, it will only go to 621the turbo range for the highest loads and the other loads above 50% that might 622benefit from running at turbo frequencies will be given non-turbo P-states 623instead. 624 625One more issue related to that may appear on systems supporting the 626`Configurable TDP feature <turbo_>`_ allowing the platform firmware to set the 627turbo threshold. Namely, if that is not coordinated with the lists of P-states 628returned by ``_PSS`` properly, there may be more than one item corresponding to 629a turbo P-state in those lists and there may be a problem with avoiding the 630turbo range (if desirable or necessary). Usually, to avoid using turbo 631P-states overall, ``acpi-cpufreq`` simply avoids using the topmost state listed 632by ``_PSS``, but that is not sufficient when there are other turbo P-states in 633the list returned by it. 634 635Apart from the above, ``acpi-cpufreq`` works like ``intel_pstate`` in the 636`passive mode <Passive Mode_>`_, except that the number of P-states it can set 637is limited to the ones listed by the ACPI ``_PSS`` objects. 638 639 640Kernel Command Line Options for ``intel_pstate`` 641================================================ 642 643Several kernel command line options can be used to pass early-configuration-time 644parameters to ``intel_pstate`` in order to enforce specific behavior of it. All 645of them have to be prepended with the ``intel_pstate=`` prefix. 646 647``disable`` 648 Do not register ``intel_pstate`` as the scaling driver even if the 649 processor is supported by it. 650 651``active`` 652 Register ``intel_pstate`` in the `active mode <Active Mode_>`_ to start 653 with. 654 655``passive`` 656 Register ``intel_pstate`` in the `passive mode <Passive Mode_>`_ to 657 start with. 658 659``force`` 660 Register ``intel_pstate`` as the scaling driver instead of 661 ``acpi-cpufreq`` even if the latter is preferred on the given system. 662 663 This may prevent some platform features (such as thermal controls and 664 power capping) that rely on the availability of ACPI P-states 665 information from functioning as expected, so it should be used with 666 caution. 667 668 This option does not work with processors that are not supported by 669 ``intel_pstate`` and on platforms where the ``pcc-cpufreq`` scaling 670 driver is used instead of ``acpi-cpufreq``. 671 672``no_hwp`` 673 Do not enable the hardware-managed P-states (HWP) feature even if it is 674 supported by the processor. 675 676``hwp_only`` 677 Register ``intel_pstate`` as the scaling driver only if the 678 hardware-managed P-states (HWP) feature is supported by the processor. 679 680``support_acpi_ppc`` 681 Take ACPI ``_PPC`` performance limits into account. 682 683 If the preferred power management profile in the FADT (Fixed ACPI 684 Description Table) is set to "Enterprise Server" or "Performance 685 Server", the ACPI ``_PPC`` limits are taken into account by default 686 and this option has no effect. 687 688``per_cpu_perf_limits`` 689 Use per-logical-CPU P-State limits (see `Coordination of P-state 690 Limits`_ for details). 691 692 693Diagnostics and Tuning 694====================== 695 696Trace Events 697------------ 698 699There are two static trace events that can be used for ``intel_pstate`` 700diagnostics. One of them is the ``cpu_frequency`` trace event generally used 701by ``CPUFreq``, and the other one is the ``pstate_sample`` trace event specific 702to ``intel_pstate``. Both of them are triggered by ``intel_pstate`` only if 703it works in the `active mode <Active Mode_>`_. 704 705The following sequence of shell commands can be used to enable them and see 706their output (if the kernel is generally configured to support event tracing):: 707 708 # cd /sys/kernel/debug/tracing/ 709 # echo 1 > events/power/pstate_sample/enable 710 # echo 1 > events/power/cpu_frequency/enable 711 # cat trace 712 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 713 cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2 714 715If ``intel_pstate`` works in the `passive mode <Passive Mode_>`_, the 716``cpu_frequency`` trace event will be triggered either by the ``schedutil`` 717scaling governor (for the policies it is attached to), or by the ``CPUFreq`` 718core (for the policies with other scaling governors). 719 720``ftrace`` 721---------- 722 723The ``ftrace`` interface can be used for low-level diagnostics of 724``intel_pstate``. For example, to check how often the function to set a 725P-state is called, the ``ftrace`` filter can be set to 726:c:func:`intel_pstate_set_pstate`:: 727 728 # cd /sys/kernel/debug/tracing/ 729 # cat available_filter_functions | grep -i pstate 730 intel_pstate_set_pstate 731 intel_pstate_cpu_init 732 ... 733 # echo intel_pstate_set_pstate > set_ftrace_filter 734 # echo function > current_tracer 735 # cat trace | head -15 736 # tracer: function 737 # 738 # entries-in-buffer/entries-written: 80/80 #P:4 739 # 740 # _-----=> irqs-off 741 # / _----=> need-resched 742 # | / _---=> hardirq/softirq 743 # || / _--=> preempt-depth 744 # ||| / delay 745 # TASK-PID CPU# |||| TIMESTAMP FUNCTION 746 # | | | |||| | | 747 Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func 748 gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func 749 gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func 750 <idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func 751 752 753References 754========== 755 756.. [1] Kristen Accardi, *Balancing Power and Performance in the Linux Kernel*, 757 https://events.static.linuxfound.org/sites/events/files/slides/LinuxConEurope_2015.pdf 758 759.. [2] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3: System Programming Guide*, 760 https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html 761 762.. [3] *Advanced Configuration and Power Interface Specification*, 763 https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf 764