1 ===============================================
2 ``intel_pstate`` CPU Performance Scaling Driver
3 ===============================================
7 Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
13 ``intel_pstate`` is a part of the
14 :doc:`CPU performance scaling subsystem <cpufreq>` in the Linux kernel
15 (``CPUFreq``). It is a scaling driver for the Sandy Bridge and later
16 generations of Intel processors. Note, however, that some of those processors
17 may not be supported. [To understand ``intel_pstate`` it is necessary to know
18 how ``CPUFreq`` works in general, so this is the time to read :doc:`cpufreq` if
19 you have not done that yet.]
21 For the processors supported by ``intel_pstate``, the P-state concept is broader
22 than just an operating frequency or an operating performance point (see the
23 `LinuxCon Europe 2015 presentation by Kristen Accardi <LCEU2015_>`_ for more
24 information about that). For this reason, the representation of P-states used
25 by ``intel_pstate`` internally follows the hardware specification (for details
26 refer to `Intel® 64 and IA-32 Architectures Software Developer’s Manual
27 Volume 3: System Programming Guide <SDM_>`_). However, the ``CPUFreq`` core
28 uses frequencies for identifying operating performance points of CPUs and
29 frequencies are involved in the user space interface exposed by it, so
30 ``intel_pstate`` maps its internal representation of P-states to frequencies too
31 (fortunately, that mapping is unambiguous). At the same time, it would not be
32 practical for ``intel_pstate`` to supply the ``CPUFreq`` core with a table of
33 available frequencies due to the possible size of it, so the driver does not do
34 that. Some functionality of the core is limited by that.
36 Since the hardware P-state selection interface used by ``intel_pstate`` is
37 available at the logical CPU level, the driver always works with individual
38 CPUs. Consequently, if ``intel_pstate`` is in use, every ``CPUFreq`` policy
39 object corresponds to one logical CPU and ``CPUFreq`` policies are effectively
40 equivalent to CPUs. In particular, this means that they become "inactive" every
41 time the corresponding CPU is taken offline and need to be re-initialized when
44 ``intel_pstate`` is not modular, so it cannot be unloaded, which means that the
45 only way to pass early-configuration-time parameters to it is via the kernel
46 command line. However, its configuration can be adjusted via ``sysfs`` to a
47 great extent. In some configurations it even is possible to unregister it via
48 ``sysfs`` which allows another ``CPUFreq`` scaling driver to be loaded and
49 registered (see `below <status_attr_>`_).
55 ``intel_pstate`` can operate in three different modes: in the active mode with
56 or without hardware-managed P-states support and in the passive mode. Which of
57 them will be in effect depends on what kernel command line options are used and
58 on the capabilities of the processor.
63 This is the default operation mode of ``intel_pstate``. If it works in this
64 mode, the ``scaling_driver`` policy attribute in ``sysfs`` for all ``CPUFreq``
65 policies contains the string "intel_pstate".
67 In this mode the driver bypasses the scaling governors layer of ``CPUFreq`` and
68 provides its own scaling algorithms for P-state selection. Those algorithms
69 can be applied to ``CPUFreq`` policies in the same way as generic scaling
70 governors (that is, through the ``scaling_governor`` policy attribute in
71 ``sysfs``). [Note that different P-state selection algorithms may be chosen for
72 different policies, but that is not recommended.]
74 They are not generic scaling governors, but their names are the same as the
75 names of some of those governors. Moreover, confusingly enough, they generally
76 do not work in the same way as the generic governors they share the names with.
77 For example, the ``powersave`` P-state selection algorithm provided by
78 ``intel_pstate`` is not a counterpart of the generic ``powersave`` governor
79 (roughly, it corresponds to the ``schedutil`` and ``ondemand`` governors).
81 There are two P-state selection algorithms provided by ``intel_pstate`` in the
82 active mode: ``powersave`` and ``performance``. The way they both operate
83 depends on whether or not the hardware-managed P-states (HWP) feature has been
84 enabled in the processor and possibly on the processor model.
86 Which of the P-state selection algorithms is used by default depends on the
87 :c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option.
88 Namely, if that option is set, the ``performance`` algorithm will be used by
89 default, and the other one will be used by default if it is not set.
94 If the processor supports the HWP feature, it will be enabled during the
95 processor initialization and cannot be disabled after that. It is possible
96 to avoid enabling it by passing the ``intel_pstate=no_hwp`` argument to the
97 kernel in the command line.
99 If the HWP feature has been enabled, ``intel_pstate`` relies on the processor to
100 select P-states by itself, but still it can give hints to the processor's
101 internal P-state selection logic. What those hints are depends on which P-state
102 selection algorithm has been applied to the given policy (or to the CPU it
105 Even though the P-state selection is carried out by the processor automatically,
106 ``intel_pstate`` registers utilization update callbacks with the CPU scheduler
107 in this mode. However, they are not used for running a P-state selection
108 algorithm, but for periodic updates of the current CPU frequency information to
109 be made available from the ``scaling_cur_freq`` policy attribute in ``sysfs``.
111 HWP + ``performance``
112 .....................
114 In this configuration ``intel_pstate`` will write 0 to the processor's
115 Energy-Performance Preference (EPP) knob (if supported) or its
116 Energy-Performance Bias (EPB) knob (otherwise), which means that the processor's
117 internal P-state selection logic is expected to focus entirely on performance.
119 This will override the EPP/EPB setting coming from the ``sysfs`` interface
120 (see `Energy vs Performance Hints`_ below).
122 Also, in this configuration the range of P-states available to the processor's
123 internal P-state selection logic is always restricted to the upper boundary
124 (that is, the maximum P-state that the driver is allowed to use).
129 In this configuration ``intel_pstate`` will set the processor's
130 Energy-Performance Preference (EPP) knob (if supported) or its
131 Energy-Performance Bias (EPB) knob (otherwise) to whatever value it was
132 previously set to via ``sysfs`` (or whatever default value it was
133 set to by the platform firmware). This usually causes the processor's
134 internal P-state selection logic to be less performance-focused.
136 Active Mode Without HWP
137 ~~~~~~~~~~~~~~~~~~~~~~~
139 This is the default operation mode for processors that do not support the HWP
140 feature. It also is used by default with the ``intel_pstate=no_hwp`` argument
141 in the kernel command line. However, in this mode ``intel_pstate`` may refuse
142 to work with the given processor if it does not recognize it. [Note that
143 ``intel_pstate`` will never refuse to work with any processor with the HWP
146 In this mode ``intel_pstate`` registers utilization update callbacks with the
147 CPU scheduler in order to run a P-state selection algorithm, either
148 ``powersave`` or ``performance``, depending on the ``scaling_governor`` policy
149 setting in ``sysfs``. The current CPU frequency information to be made
150 available from the ``scaling_cur_freq`` policy attribute in ``sysfs`` is
151 periodically updated by those utilization update callbacks too.
156 Without HWP, this P-state selection algorithm is always the same regardless of
157 the processor model and platform configuration.
159 It selects the maximum P-state it is allowed to use, subject to limits set via
160 ``sysfs``, every time the driver configuration for the given CPU is updated
161 (e.g. via ``sysfs``).
163 This is the default P-state selection algorithm if the
164 :c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option
170 Without HWP, this P-state selection algorithm is similar to the algorithm
171 implemented by the generic ``schedutil`` scaling governor except that the
172 utilization metric used by it is based on numbers coming from feedback
173 registers of the CPU. It generally selects P-states proportional to the
174 current CPU utilization.
176 This algorithm is run by the driver's utilization update callback for the
177 given CPU when it is invoked by the CPU scheduler, but not more often than
178 every 10 ms. Like in the ``performance`` case, the hardware configuration
179 is not touched if the new P-state turns out to be the same as the current
182 This is the default P-state selection algorithm if the
183 :c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option
189 This mode is used if the ``intel_pstate=passive`` argument is passed to the
190 kernel in the command line (it implies the ``intel_pstate=no_hwp`` setting too).
191 Like in the active mode without HWP support, in this mode ``intel_pstate`` may
192 refuse to work with the given processor if it does not recognize it.
194 If the driver works in this mode, the ``scaling_driver`` policy attribute in
195 ``sysfs`` for all ``CPUFreq`` policies contains the string "intel_cpufreq".
196 Then, the driver behaves like a regular ``CPUFreq`` scaling driver. That is,
197 it is invoked by generic scaling governors when necessary to talk to the
198 hardware in order to change the P-state of a CPU (in particular, the
199 ``schedutil`` governor can invoke it directly from scheduler context).
201 While in this mode, ``intel_pstate`` can be used with all of the (generic)
202 scaling governors listed by the ``scaling_available_governors`` policy attribute
203 in ``sysfs`` (and the P-state selection algorithms described above are not
204 used). Then, it is responsible for the configuration of policy objects
205 corresponding to CPUs and provides the ``CPUFreq`` core (and the scaling
206 governors attached to the policy objects) with accurate information on the
207 maximum and minimum operating frequencies supported by the hardware (including
208 the so-called "turbo" frequency ranges). In other words, in the passive mode
209 the entire range of available P-states is exposed by ``intel_pstate`` to the
210 ``CPUFreq`` core. However, in this mode the driver does not register
211 utilization update callbacks with the CPU scheduler and the ``scaling_cur_freq``
212 information comes from the ``CPUFreq`` core (and is the last frequency selected
213 by the current scaling governor for the given policy).
218 Turbo P-states Support
219 ======================
221 In the majority of cases, the entire range of P-states available to
222 ``intel_pstate`` can be divided into two sub-ranges that correspond to
223 different types of processor behavior, above and below a boundary that
224 will be referred to as the "turbo threshold" in what follows.
226 The P-states above the turbo threshold are referred to as "turbo P-states" and
227 the whole sub-range of P-states they belong to is referred to as the "turbo
228 range". These names are related to the Turbo Boost technology allowing a
229 multicore processor to opportunistically increase the P-state of one or more
230 cores if there is enough power to do that and if that is not going to cause the
231 thermal envelope of the processor package to be exceeded.
233 Specifically, if software sets the P-state of a CPU core within the turbo range
234 (that is, above the turbo threshold), the processor is permitted to take over
235 performance scaling control for that core and put it into turbo P-states of its
236 choice going forward. However, that permission is interpreted differently by
237 different processor generations. Namely, the Sandy Bridge generation of
238 processors will never use any P-states above the last one set by software for
239 the given core, even if it is within the turbo range, whereas all of the later
240 processor generations will take it as a license to use any P-states from the
241 turbo range, even above the one set by software. In other words, on those
242 processors setting any P-state from the turbo range will enable the processor
243 to put the given core into all turbo P-states up to and including the maximum
244 supported one as it sees fit.
246 One important property of turbo P-states is that they are not sustainable. More
247 precisely, there is no guarantee that any CPUs will be able to stay in any of
248 those states indefinitely, because the power distribution within the processor
249 package may change over time or the thermal envelope it was designed for might
250 be exceeded if a turbo P-state was used for too long.
252 In turn, the P-states below the turbo threshold generally are sustainable. In
253 fact, if one of them is set by software, the processor is not expected to change
254 it to a lower one unless in a thermal stress or a power limit violation
255 situation (a higher P-state may still be used if it is set for another CPU in
256 the same package at the same time, for example).
258 Some processors allow multiple cores to be in turbo P-states at the same time,
259 but the maximum P-state that can be set for them generally depends on the number
260 of cores running concurrently. The maximum turbo P-state that can be set for 3
261 cores at the same time usually is lower than the analogous maximum P-state for
262 2 cores, which in turn usually is lower than the maximum turbo P-state that can
263 be set for 1 core. The one-core maximum turbo P-state is thus the maximum
264 supported one overall.
266 The maximum supported turbo P-state, the turbo threshold (the maximum supported
267 non-turbo P-state) and the minimum supported P-state are specific to the
268 processor model and can be determined by reading the processor's model-specific
269 registers (MSRs). Moreover, some processors support the Configurable TDP
270 (Thermal Design Power) feature and, when that feature is enabled, the turbo
271 threshold effectively becomes a configurable value that can be set by the
274 Unlike ``_PSS`` objects in the ACPI tables, ``intel_pstate`` always exposes
275 the entire range of available P-states, including the whole turbo range, to the
276 ``CPUFreq`` core and (in the passive mode) to generic scaling governors. This
277 generally causes turbo P-states to be set more often when ``intel_pstate`` is
278 used relative to ACPI-based CPU performance scaling (see `below <acpi-cpufreq_>`_
279 for more information).
281 Moreover, since ``intel_pstate`` always knows what the real turbo threshold is
282 (even if the Configurable TDP feature is enabled in the processor), its
283 ``no_turbo`` attribute in ``sysfs`` (described `below <no_turbo_attr_>`_) should
284 work as expected in all cases (that is, if set to disable turbo P-states, it
285 always should prevent ``intel_pstate`` from using them).
291 To handle a given processor ``intel_pstate`` requires a number of different
292 pieces of information on it to be known, including:
294 * The minimum supported P-state.
296 * The maximum supported `non-turbo P-state <turbo_>`_.
298 * Whether or not turbo P-states are supported at all.
300 * The maximum supported `one-core turbo P-state <turbo_>`_ (if turbo P-states
303 * The scaling formula to translate the driver's internal representation
304 of P-states into frequencies and the other way around.
306 Generally, ways to obtain that information are specific to the processor model
307 or family. Although it often is possible to obtain all of it from the processor
308 itself (using model-specific registers), there are cases in which hardware
309 manuals need to be consulted to get to it too.
311 For this reason, there is a list of supported processors in ``intel_pstate`` and
312 the driver initialization will fail if the detected processor is not in that
313 list, unless it supports the `HWP feature <Active Mode_>`_. [The interface to
314 obtain all of the information listed above is the same for all of the processors
315 supporting the HWP feature, which is why they all are supported by
319 User Space Interface in ``sysfs``
320 =================================
325 ``intel_pstate`` exposes several global attributes (files) in ``sysfs`` to
326 control its functionality at the system level. They are located in the
327 ``/sys/devices/system/cpu/cpufreq/intel_pstate/`` directory and affect all
330 Some of them are not present if the ``intel_pstate=per_cpu_perf_limits``
331 argument is passed to the kernel in the command line.
334 Maximum P-state the driver is allowed to set in percent of the
335 maximum supported performance level (the highest supported `turbo
338 This attribute will not be exposed if the
339 ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel
343 Minimum P-state the driver is allowed to set in percent of the
344 maximum supported performance level (the highest supported `turbo
347 This attribute will not be exposed if the
348 ``intel_pstate=per_cpu_perf_limits`` argument is present in the kernel
352 Number of P-states supported by the processor (between 0 and 255
353 inclusive) including both turbo and non-turbo P-states (see
354 `Turbo P-states Support`_).
356 The value of this attribute is not affected by the ``no_turbo``
357 setting described `below <no_turbo_attr_>`_.
359 This attribute is read-only.
362 Ratio of the `turbo range <turbo_>`_ size to the size of the entire
363 range of supported P-states, in percent.
365 This attribute is read-only.
370 If set (equal to 1), the driver is not allowed to set any turbo P-states
371 (see `Turbo P-states Support`_). If unset (equalt to 0, which is the
372 default), turbo P-states can be set by the driver.
373 [Note that ``intel_pstate`` does not support the general ``boost``
374 attribute (supported by some other scaling drivers) which is replaced
377 This attrubute does not affect the maximum supported frequency value
378 supplied to the ``CPUFreq`` core and exposed via the policy interface,
379 but it affects the maximum possible value of per-policy P-state limits
380 (see `Interpretation of Policy Attributes`_ below for details).
385 Operation mode of the driver: "active", "passive" or "off".
388 The driver is functional and in the `active mode
392 The driver is functional and in the `passive mode
396 The driver is not functional (it is not registered as a scaling
397 driver with the ``CPUFreq`` core).
399 This attribute can be written to in order to change the driver's
400 operation mode or to unregister it. The string written to it must be
401 one of the possible values of it and, if successful, the write will
402 cause the driver to switch over to the operation mode represented by
403 that string - or to be unregistered in the "off" case. [Actually,
404 switching over from the active mode to the passive mode or the other
405 way around causes the driver to be unregistered and registered again
406 with a different set of callbacks, so all of its settings (the global
407 as well as the per-policy ones) are then reset to their default
408 values, possibly depending on the target operation mode.]
410 That only is supported in some configurations, though (for example, if
411 the `HWP feature is enabled in the processor <Active Mode With HWP_>`_,
412 the operation mode of the driver cannot be changed), and if it is not
413 supported in the current configuration, writes to this attribute with
414 fail with an appropriate error.
416 Interpretation of Policy Attributes
417 -----------------------------------
419 The interpretation of some ``CPUFreq`` policy attributes described in
420 :doc:`cpufreq` is special with ``intel_pstate`` as the current scaling driver
421 and it generally depends on the driver's `operation mode <Operation Modes_>`_.
423 First of all, the values of the ``cpuinfo_max_freq``, ``cpuinfo_min_freq`` and
424 ``scaling_cur_freq`` attributes are produced by applying a processor-specific
425 multiplier to the internal P-state representation used by ``intel_pstate``.
426 Also, the values of the ``scaling_max_freq`` and ``scaling_min_freq``
427 attributes are capped by the frequency corresponding to the maximum P-state that
428 the driver is allowed to set.
430 If the ``no_turbo`` `global attribute <no_turbo_attr_>`_ is set, the driver is
431 not allowed to use turbo P-states, so the maximum value of ``scaling_max_freq``
432 and ``scaling_min_freq`` is limited to the maximum non-turbo P-state frequency.
433 Accordingly, setting ``no_turbo`` causes ``scaling_max_freq`` and
434 ``scaling_min_freq`` to go down to that value if they were above it before.
435 However, the old values of ``scaling_max_freq`` and ``scaling_min_freq`` will be
436 restored after unsetting ``no_turbo``, unless these attributes have been written
437 to after ``no_turbo`` was set.
439 If ``no_turbo`` is not set, the maximum possible value of ``scaling_max_freq``
440 and ``scaling_min_freq`` corresponds to the maximum supported turbo P-state,
441 which also is the value of ``cpuinfo_max_freq`` in either case.
443 Next, the following policy attributes have special meaning if
444 ``intel_pstate`` works in the `active mode <Active Mode_>`_:
446 ``scaling_available_governors``
447 List of P-state selection algorithms provided by ``intel_pstate``.
450 P-state selection algorithm provided by ``intel_pstate`` currently in
451 use with the given policy.
454 Frequency of the average P-state of the CPU represented by the given
455 policy for the time interval between the last two invocations of the
456 driver's utilization update callback by the CPU scheduler for that CPU.
458 The meaning of these attributes in the `passive mode <Passive Mode_>`_ is the
459 same as for other scaling drivers.
461 Additionally, the value of the ``scaling_driver`` attribute for ``intel_pstate``
462 depends on the operation mode of the driver. Namely, it is either
463 "intel_pstate" (in the `active mode <Active Mode_>`_) or "intel_cpufreq" (in the
464 `passive mode <Passive Mode_>`_).
466 Coordination of P-State Limits
467 ------------------------------
469 ``intel_pstate`` allows P-state limits to be set in two ways: with the help of
470 the ``max_perf_pct`` and ``min_perf_pct`` `global attributes
471 <Global Attributes_>`_ or via the ``scaling_max_freq`` and ``scaling_min_freq``
472 ``CPUFreq`` policy attributes. The coordination between those limits is based
473 on the following rules, regardless of the current operation mode of the driver:
475 1. All CPUs are affected by the global limits (that is, none of them can be
476 requested to run faster than the global maximum and none of them can be
477 requested to run slower than the global minimum).
479 2. Each individual CPU is affected by its own per-policy limits (that is, it
480 cannot be requested to run faster than its own per-policy maximum and it
481 cannot be requested to run slower than its own per-policy minimum).
483 3. The global and per-policy limits can be set independently.
485 If the `HWP feature is enabled in the processor <Active Mode With HWP_>`_, the
486 resulting effective values are written into its registers whenever the limits
487 change in order to request its internal P-state selection logic to always set
488 P-states within these limits. Otherwise, the limits are taken into account by
489 scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver
490 every time before setting a new P-state for a CPU.
492 Additionally, if the ``intel_pstate=per_cpu_perf_limits`` command line argument
493 is passed to the kernel, ``max_perf_pct`` and ``min_perf_pct`` are not exposed
494 at all and the only way to set the limits is by using the policy attributes.
497 Energy vs Performance Hints
498 ---------------------------
500 If ``intel_pstate`` works in the `active mode with the HWP feature enabled
501 <Active Mode With HWP_>`_ in the processor, additional attributes are present
502 in every ``CPUFreq`` policy directory in ``sysfs``. They are intended to allow
503 user space to help ``intel_pstate`` to adjust the processor's internal P-state
504 selection logic by focusing it on performance or on energy-efficiency, or
505 somewhere between the two extremes:
507 ``energy_performance_preference``
508 Current value of the energy vs performance hint for the given policy
509 (or the CPU represented by it).
511 The hint can be changed by writing to this attribute.
513 ``energy_performance_available_preferences``
514 List of strings that can be written to the
515 ``energy_performance_preference`` attribute.
517 They represent different energy vs performance hints and should be
518 self-explanatory, except that ``default`` represents whatever hint
519 value was set by the platform firmware.
521 Strings written to the ``energy_performance_preference`` attribute are
522 internally translated to integer values written to the processor's
523 Energy-Performance Preference (EPP) knob (if supported) or its
524 Energy-Performance Bias (EPB) knob.
526 [Note that tasks may by migrated from one CPU to another by the scheduler's
527 load-balancing algorithm and if different energy vs performance hints are
528 set for those CPUs, that may lead to undesirable outcomes. To avoid such
529 issues it is better to set the same energy vs performance hint for all CPUs
530 or to pin every task potentially sensitive to them to a specific CPU.]
534 ``intel_pstate`` vs ``acpi-cpufreq``
535 ====================================
537 On the majority of systems supported by ``intel_pstate``, the ACPI tables
538 provided by the platform firmware contain ``_PSS`` objects returning information
539 that can be used for CPU performance scaling (refer to the `ACPI specification`_
540 for details on the ``_PSS`` objects and the format of the information returned
543 The information returned by the ACPI ``_PSS`` objects is used by the
544 ``acpi-cpufreq`` scaling driver. On systems supported by ``intel_pstate``
545 the ``acpi-cpufreq`` driver uses the same hardware CPU performance scaling
546 interface, but the set of P-states it can use is limited by the ``_PSS``
549 On those systems each ``_PSS`` object returns a list of P-states supported by
550 the corresponding CPU which basically is a subset of the P-states range that can
551 be used by ``intel_pstate`` on the same system, with one exception: the whole
552 `turbo range <turbo_>`_ is represented by one item in it (the topmost one). By
553 convention, the frequency returned by ``_PSS`` for that item is greater by 1 MHz
554 than the frequency of the highest non-turbo P-state listed by it, but the
555 corresponding P-state representation (following the hardware specification)
556 returned for it matches the maximum supported turbo P-state (or is the
557 special value 255 meaning essentially "go as high as you can get").
559 The list of P-states returned by ``_PSS`` is reflected by the table of
560 available frequencies supplied by ``acpi-cpufreq`` to the ``CPUFreq`` core and
561 scaling governors and the minimum and maximum supported frequencies reported by
562 it come from that list as well. In particular, given the special representation
563 of the turbo range described above, this means that the maximum supported
564 frequency reported by ``acpi-cpufreq`` is higher by 1 MHz than the frequency
565 of the highest supported non-turbo P-state listed by ``_PSS`` which, of course,
566 affects decisions made by the scaling governors, except for ``powersave`` and
569 For example, if a given governor attempts to select a frequency proportional to
570 estimated CPU load and maps the load of 100% to the maximum supported frequency
571 (possibly multiplied by a constant), then it will tend to choose P-states below
572 the turbo threshold if ``acpi-cpufreq`` is used as the scaling driver, because
573 in that case the turbo range corresponds to a small fraction of the frequency
574 band it can use (1 MHz vs 1 GHz or more). In consequence, it will only go to
575 the turbo range for the highest loads and the other loads above 50% that might
576 benefit from running at turbo frequencies will be given non-turbo P-states
579 One more issue related to that may appear on systems supporting the
580 `Configurable TDP feature <turbo_>`_ allowing the platform firmware to set the
581 turbo threshold. Namely, if that is not coordinated with the lists of P-states
582 returned by ``_PSS`` properly, there may be more than one item corresponding to
583 a turbo P-state in those lists and there may be a problem with avoiding the
584 turbo range (if desirable or necessary). Usually, to avoid using turbo
585 P-states overall, ``acpi-cpufreq`` simply avoids using the topmost state listed
586 by ``_PSS``, but that is not sufficient when there are other turbo P-states in
587 the list returned by it.
589 Apart from the above, ``acpi-cpufreq`` works like ``intel_pstate`` in the
590 `passive mode <Passive Mode_>`_, except that the number of P-states it can set
591 is limited to the ones listed by the ACPI ``_PSS`` objects.
594 Kernel Command Line Options for ``intel_pstate``
595 ================================================
597 Several kernel command line options can be used to pass early-configuration-time
598 parameters to ``intel_pstate`` in order to enforce specific behavior of it. All
599 of them have to be prepended with the ``intel_pstate=`` prefix.
602 Do not register ``intel_pstate`` as the scaling driver even if the
603 processor is supported by it.
606 Register ``intel_pstate`` in the `passive mode <Passive Mode_>`_ to
609 This option implies the ``no_hwp`` one described below.
612 Register ``intel_pstate`` as the scaling driver instead of
613 ``acpi-cpufreq`` even if the latter is preferred on the given system.
615 This may prevent some platform features (such as thermal controls and
616 power capping) that rely on the availability of ACPI P-states
617 information from functioning as expected, so it should be used with
620 This option does not work with processors that are not supported by
621 ``intel_pstate`` and on platforms where the ``pcc-cpufreq`` scaling
622 driver is used instead of ``acpi-cpufreq``.
625 Do not enable the `hardware-managed P-states (HWP) feature
626 <Active Mode With HWP_>`_ even if it is supported by the processor.
629 Register ``intel_pstate`` as the scaling driver only if the
630 `hardware-managed P-states (HWP) feature <Active Mode With HWP_>`_ is
631 supported by the processor.
634 Take ACPI ``_PPC`` performance limits into account.
636 If the preferred power management profile in the FADT (Fixed ACPI
637 Description Table) is set to "Enterprise Server" or "Performance
638 Server", the ACPI ``_PPC`` limits are taken into account by default
639 and this option has no effect.
641 ``per_cpu_perf_limits``
642 Use per-logical-CPU P-State limits (see `Coordination of P-state
643 Limits`_ for details).
646 Diagnostics and Tuning
647 ======================
652 There are two static trace events that can be used for ``intel_pstate``
653 diagnostics. One of them is the ``cpu_frequency`` trace event generally used
654 by ``CPUFreq``, and the other one is the ``pstate_sample`` trace event specific
655 to ``intel_pstate``. Both of them are triggered by ``intel_pstate`` only if
656 it works in the `active mode <Active Mode_>`_.
658 The following sequence of shell commands can be used to enable them and see
659 their output (if the kernel is generally configured to support event tracing)::
661 # cd /sys/kernel/debug/tracing/
662 # echo 1 > events/power/pstate_sample/enable
663 # echo 1 > events/power/cpu_frequency/enable
665 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
666 cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2
668 If ``intel_pstate`` works in the `passive mode <Passive Mode_>`_, the
669 ``cpu_frequency`` trace event will be triggered either by the ``schedutil``
670 scaling governor (for the policies it is attached to), or by the ``CPUFreq``
671 core (for the policies with other scaling governors).
676 The ``ftrace`` interface can be used for low-level diagnostics of
677 ``intel_pstate``. For example, to check how often the function to set a
678 P-state is called, the ``ftrace`` filter can be set to to
679 :c:func:`intel_pstate_set_pstate`::
681 # cd /sys/kernel/debug/tracing/
682 # cat available_filter_functions | grep -i pstate
683 intel_pstate_set_pstate
684 intel_pstate_cpu_init
686 # echo intel_pstate_set_pstate > set_ftrace_filter
687 # echo function > current_tracer
688 # cat trace | head -15
691 # entries-in-buffer/entries-written: 80/80 #P:4
694 # / _----=> need-resched
695 # | / _---=> hardirq/softirq
696 # || / _--=> preempt-depth
698 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
700 Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
701 gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
702 gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
703 <idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func
706 .. _LCEU2015: http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
707 .. _SDM: http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html
708 .. _ACPI specification: http://www.uefi.org/sites/default/files/resources/ACPI_6_1.pdf
git.samba.org / sfrench / cifs-2.6.git / blob