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