intel_pstate
CPU Performance Scaling Driver¶
Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
General Information¶
intel_pstate
is a part of the
CPU performance scaling subsystem in the Linux kernel
(CPUFreq
). It is a scaling driver for the Sandy Bridge and later
generations of Intel processors. Note, however, that some of those processors
may not be supported. [To understand intel_pstate
it is necessary to know
how CPUFreq
works in general, so this is the time to read CPU Performance Scaling if
you have not done that yet.]
For the processors supported by intel_pstate
, the P-state concept is broader
than just an operating frequency or an operating performance point (see the
LinuxCon Europe 2015 presentation by Kristen Accardi for more
information about that). For this reason, the representation of P-states used
by intel_pstate
internally follows the hardware specification (for details
refer to Intel® 64 and IA-32 Architectures Software Developer’s Manual
Volume 3: System Programming Guide). However, the CPUFreq
core
uses frequencies for identifying operating performance points of CPUs and
frequencies are involved in the user space interface exposed by it, so
intel_pstate
maps its internal representation of P-states to frequencies too
(fortunately, that mapping is unambiguous). At the same time, it would not be
practical for intel_pstate
to supply the CPUFreq
core with a table of
available frequencies due to the possible size of it, so the driver does not do
that. Some functionality of the core is limited by that.
Since the hardware P-state selection interface used by intel_pstate
is
available at the logical CPU level, the driver always works with individual
CPUs. Consequently, if intel_pstate
is in use, every CPUFreq
policy
object corresponds to one logical CPU and CPUFreq
policies are effectively
equivalent to CPUs. In particular, this means that they become “inactive” every
time the corresponding CPU is taken offline and need to be re-initialized when
it goes back online.
intel_pstate
is not modular, so it cannot be unloaded, which means that the
only way to pass early-configuration-time parameters to it is via the kernel
command line. However, its configuration can be adjusted via sysfs
to a
great extent. In some configurations it even is possible to unregister it via
sysfs
which allows another CPUFreq
scaling driver to be loaded and
registered (see below).
Operation Modes¶
intel_pstate
can operate in three different modes: in the active mode with
or without hardware-managed P-states support and in the passive mode. Which of
them will be in effect depends on what kernel command line options are used and
on the capabilities of the processor.
Active Mode¶
This is the default operation mode of intel_pstate
. If it works in this
mode, the scaling_driver
policy attribute in sysfs
for all CPUFreq
policies contains the string “intel_pstate”.
In this mode the driver bypasses the scaling governors layer of CPUFreq
and
provides its own scaling algorithms for P-state selection. Those algorithms
can be applied to CPUFreq
policies in the same way as generic scaling
governors (that is, through the scaling_governor
policy attribute in
sysfs
). [Note that different P-state selection algorithms may be chosen for
different policies, but that is not recommended.]
They are not generic scaling governors, but their names are the same as the
names of some of those governors. Moreover, confusingly enough, they generally
do not work in the same way as the generic governors they share the names with.
For example, the powersave
P-state selection algorithm provided by
intel_pstate
is not a counterpart of the generic powersave
governor
(roughly, it corresponds to the schedutil
and ondemand
governors).
There are two P-state selection algorithms provided by intel_pstate
in the
active mode: powersave
and performance
. The way they both operate
depends on whether or not the hardware-managed P-states (HWP) feature has been
enabled in the processor and possibly on the processor model.
Which of the P-state selection algorithms is used by default depends on the
CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE
kernel configuration option.
Namely, if that option is set, the performance
algorithm will be used by
default, and the other one will be used by default if it is not set.
Active Mode With HWP¶
If the processor supports the HWP feature, it will be enabled during the
processor initialization and cannot be disabled after that. It is possible
to avoid enabling it by passing the intel_pstate=no_hwp
argument to the
kernel in the command line.
If the HWP feature has been enabled, intel_pstate
relies on the processor to
select P-states by itself, but still it can give hints to the processor’s
internal P-state selection logic. What those hints are depends on which P-state
selection algorithm has been applied to the given policy (or to the CPU it
corresponds to).
Even though the P-state selection is carried out by the processor automatically,
intel_pstate
registers utilization update callbacks with the CPU scheduler
in this mode. However, they are not used for running a P-state selection
algorithm, but for periodic updates of the current CPU frequency information to
be made available from the scaling_cur_freq
policy attribute in sysfs
.
HWP + performance
¶
In this configuration intel_pstate
will write 0 to the processor’s
Energy-Performance Preference (EPP) knob (if supported) or its
Energy-Performance Bias (EPB) knob (otherwise), which means that the processor’s
internal P-state selection logic is expected to focus entirely on performance.
This will override the EPP/EPB setting coming from the sysfs
interface
(see Energy vs Performance Hints below).
Also, in this configuration the range of P-states available to the processor’s internal P-state selection logic is always restricted to the upper boundary (that is, the maximum P-state that the driver is allowed to use).
HWP + powersave
¶
In this configuration intel_pstate
will set the processor’s
Energy-Performance Preference (EPP) knob (if supported) or its
Energy-Performance Bias (EPB) knob (otherwise) to whatever value it was
previously set to via sysfs
(or whatever default value it was
set to by the platform firmware). This usually causes the processor’s
internal P-state selection logic to be less performance-focused.
Active Mode Without HWP¶
This is the default operation mode for processors that do not support the HWP
feature. It also is used by default with the intel_pstate=no_hwp
argument
in the kernel command line. However, in this mode intel_pstate
may refuse
to work with the given processor if it does not recognize it. [Note that
intel_pstate
will never refuse to work with any processor with the HWP
feature enabled.]
In this mode intel_pstate
registers utilization update callbacks with the
CPU scheduler in order to run a P-state selection algorithm, either
powersave
or performance
, depending on the scaling_cur_freq
policy
setting in sysfs
. The current CPU frequency information to be made
available from the scaling_cur_freq
policy attribute in sysfs
is
periodically updated by those utilization update callbacks too.
performance
¶
Without HWP, this P-state selection algorithm is always the same regardless of the processor model and platform configuration.
It selects the maximum P-state it is allowed to use, subject to limits set via
sysfs
, every time the driver configuration for the given CPU is updated
(e.g. via sysfs
).
This is the default P-state selection algorithm if the
CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE
kernel configuration option
is set.
powersave
¶
Without HWP, this P-state selection algorithm generally depends on the processor model and/or the system profile setting in the ACPI tables and there are two variants of it.
One of them is used with processors from the Atom line and (regardless of the
processor model) on platforms with the system profile in the ACPI tables set to
“mobile” (laptops mostly), “tablet”, “appliance PC”, “desktop”, or
“workstation”. It is also used with processors supporting the HWP feature if
that feature has not been enabled (that is, with the intel_pstate=no_hwp
argument in the kernel command line). It is similar to the algorithm
implemented by the generic schedutil
scaling governor except that the
utilization metric used by it is based on numbers coming from feedback
registers of the CPU. It generally selects P-states proportional to the
current CPU utilization, so it is referred to as the “proportional” algorithm.
The second variant of the powersave
P-state selection algorithm, used in all
of the other cases (generally, on processors from the Core line, so it is
referred to as the “Core” algorithm), is based on the values read from the APERF
and MPERF feedback registers and the previously requested target P-state.
It does not really take CPU utilization into account explicitly, but as a rule
it causes the CPU P-state to ramp up very quickly in response to increased
utilization which is generally desirable in server environments.
Regardless of the variant, this algorithm is run by the driver’s utilization
update callback for the given CPU when it is invoked by the CPU scheduler, but
not more often than every 10 ms (that can be tweaked via debugfs
in this
particular case). Like in the performance
case, the hardware configuration is not touched if the new P-state turns out to
be the same as the current one.
This is the default P-state selection algorithm if the
CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE
kernel configuration option
is not set.
Passive Mode¶
This mode is used if the intel_pstate=passive
argument is passed to the
kernel in the command line (it implies the intel_pstate=no_hwp
setting too).
Like in the active mode without HWP support, in this mode intel_pstate
may
refuse to work with the given processor if it does not recognize it.
If the driver works in this mode, the scaling_driver
policy attribute in
sysfs
for all CPUFreq
policies contains the string “intel_cpufreq”.
Then, the driver behaves like a regular CPUFreq
scaling driver. That is,
it is invoked by generic scaling governors when necessary to talk to the
hardware in order to change the P-state of a CPU (in particular, the
schedutil
governor can invoke it directly from scheduler context).
While in this mode, intel_pstate
can be used with all of the (generic)
scaling governors listed by the scaling_available_governors
policy attribute
in sysfs
(and the P-state selection algorithms described above are not
used). Then, it is responsible for the configuration of policy objects
corresponding to CPUs and provides the CPUFreq
core (and the scaling
governors attached to the policy objects) with accurate information on the
maximum and minimum operating frequencies supported by the hardware (including
the so-called “turbo” frequency ranges). In other words, in the passive mode
the entire range of available P-states is exposed by intel_pstate
to the
CPUFreq
core. However, in this mode the driver does not register
utilization update callbacks with the CPU scheduler and the scaling_cur_freq
information comes from the CPUFreq
core (and is the last frequency selected
by the current scaling governor for the given policy).
Turbo P-states Support¶
In the majority of cases, the entire range of P-states available to
intel_pstate
can be divided into two sub-ranges that correspond to
different types of processor behavior, above and below a boundary that
will be referred to as the “turbo threshold” in what follows.
The P-states above the turbo threshold are referred to as “turbo P-states” and the whole sub-range of P-states they belong to is referred to as the “turbo range”. These names are related to the Turbo Boost technology allowing a multicore processor to opportunistically increase the P-state of one or more cores if there is enough power to do that and if that is not going to cause the thermal envelope of the processor package to be exceeded.
Specifically, if software sets the P-state of a CPU core within the turbo range (that is, above the turbo threshold), the processor is permitted to take over performance scaling control for that core and put it into turbo P-states of its choice going forward. However, that permission is interpreted differently by different processor generations. Namely, the Sandy Bridge generation of processors will never use any P-states above the last one set by software for the given core, even if it is within the turbo range, whereas all of the later processor generations will take it as a license to use any P-states from the turbo range, even above the one set by software. In other words, on those processors setting any P-state from the turbo range will enable the processor to put the given core into all turbo P-states up to and including the maximum supported one as it sees fit.
One important property of turbo P-states is that they are not sustainable. More precisely, there is no guarantee that any CPUs will be able to stay in any of those states indefinitely, because the power distribution within the processor package may change over time or the thermal envelope it was designed for might be exceeded if a turbo P-state was used for too long.
In turn, the P-states below the turbo threshold generally are sustainable. In fact, if one of them is set by software, the processor is not expected to change it to a lower one unless in a thermal stress or a power limit violation situation (a higher P-state may still be used if it is set for another CPU in the same package at the same time, for example).
Some processors allow multiple cores to be in turbo P-states at the same time, but the maximum P-state that can be set for them generally depends on the number of cores running concurrently. The maximum turbo P-state that can be set for 3 cores at the same time usually is lower than the analogous maximum P-state for 2 cores, which in turn usually is lower than the maximum turbo P-state that can be set for 1 core. The one-core maximum turbo P-state is thus the maximum supported one overall.
The maximum supported turbo P-state, the turbo threshold (the maximum supported non-turbo P-state) and the minimum supported P-state are specific to the processor model and can be determined by reading the processor’s model-specific registers (MSRs). Moreover, some processors support the Configurable TDP (Thermal Design Power) feature and, when that feature is enabled, the turbo threshold effectively becomes a configurable value that can be set by the platform firmware.
Unlike _PSS
objects in the ACPI tables, intel_pstate
always exposes
the entire range of available P-states, including the whole turbo range, to the
CPUFreq
core and (in the passive mode) to generic scaling governors. This
generally causes turbo P-states to be set more often when intel_pstate
is
used relative to ACPI-based CPU performance scaling (see below
for more information).
Moreover, since intel_pstate
always knows what the real turbo threshold is
(even if the Configurable TDP feature is enabled in the processor), its
no_turbo
attribute in sysfs
(described below) should
work as expected in all cases (that is, if set to disable turbo P-states, it
always should prevent intel_pstate
from using them).
Processor Support¶
To handle a given processor intel_pstate
requires a number of different
pieces of information on it to be known, including:
- The minimum supported P-state.
- The maximum supported non-turbo P-state.
- Whether or not turbo P-states are supported at all.
- The maximum supported one-core turbo P-state (if turbo P-states are supported).
- The scaling formula to translate the driver’s internal representation of P-states into frequencies and the other way around.
Generally, ways to obtain that information are specific to the processor model or family. Although it often is possible to obtain all of it from the processor itself (using model-specific registers), there are cases in which hardware manuals need to be consulted to get to it too.
For this reason, there is a list of supported processors in intel_pstate
and
the driver initialization will fail if the detected processor is not in that
list, unless it supports the HWP feature. [The interface to
obtain all of the information listed above is the same for all of the processors
supporting the HWP feature, which is why they all are supported by
intel_pstate
.]
User Space Interface in sysfs
¶
Global Attributes¶
intel_pstate
exposes several global attributes (files) in sysfs
to
control its functionality at the system level. They are located in the
/sys/devices/system/cpu/cpufreq/intel_pstate/
directory and affect all
CPUs.
Some of them are not present if the intel_pstate=per_cpu_perf_limits
argument is passed to the kernel in the command line.
max_perf_pct
Maximum P-state the driver is allowed to set in percent of the maximum supported performance level (the highest supported turbo P-state).
This attribute will not be exposed if the
intel_pstate=per_cpu_perf_limits
argument is present in the kernel command line.min_perf_pct
Minimum P-state the driver is allowed to set in percent of the maximum supported performance level (the highest supported turbo P-state).
This attribute will not be exposed if the
intel_pstate=per_cpu_perf_limits
argument is present in the kernel command line.num_pstates
Number of P-states supported by the processor (between 0 and 255 inclusive) including both turbo and non-turbo P-states (see Turbo P-states Support).
The value of this attribute is not affected by the
no_turbo
setting described below.This attribute is read-only.
turbo_pct
Ratio of the turbo range size to the size of the entire range of supported P-states, in percent.
This attribute is read-only.
no_turbo
If set (equal to 1), the driver is not allowed to set any turbo P-states (see Turbo P-states Support). If unset (equalt to 0, which is the default), turbo P-states can be set by the driver. [Note that
intel_pstate
does not support the generalboost
attribute (supported by some other scaling drivers) which is replaced by this one.]This attrubute does not affect the maximum supported frequency value supplied to the
CPUFreq
core and exposed via the policy interface, but it affects the maximum possible value of per-policy P-state limits (see Interpretation of Policy Attributes below for details).
status
Operation mode of the driver: “active”, “passive” or “off”.
- “active”
- The driver is functional and in the active mode.
- “passive”
- The driver is functional and in the passive mode.
- “off”
- The driver is not functional (it is not registered as a scaling
driver with the
CPUFreq
core).
This attribute can be written to in order to change the driver’s operation mode or to unregister it. The string written to it must be one of the possible values of it and, if successful, the write will cause the driver to switch over to the operation mode represented by that string - or to be unregistered in the “off” case. [Actually, switching over from the active mode to the passive mode or the other way around causes the driver to be unregistered and registered again with a different set of callbacks, so all of its settings (the global as well as the per-policy ones) are then reset to their default values, possibly depending on the target operation mode.]
That only is supported in some configurations, though (for example, if the HWP feature is enabled in the processor, the operation mode of the driver cannot be changed), and if it is not supported in the current configuration, writes to this attribute with fail with an appropriate error.
Interpretation of Policy Attributes¶
The interpretation of some CPUFreq
policy attributes described in
CPU Performance Scaling is special with intel_pstate
as the current scaling driver
and it generally depends on the driver’s operation mode.
First of all, the values of the cpuinfo_max_freq
, cpuinfo_min_freq
and
scaling_cur_freq
attributes are produced by applying a processor-specific
multiplier to the internal P-state representation used by intel_pstate
.
Also, the values of the scaling_max_freq
and scaling_min_freq
attributes are capped by the frequency corresponding to the maximum P-state that
the driver is allowed to set.
If the no_turbo
global attribute is set, the driver is
not allowed to use turbo P-states, so the maximum value of scaling_max_freq
and scaling_min_freq
is limited to the maximum non-turbo P-state frequency.
Accordingly, setting no_turbo
causes scaling_max_freq
and
scaling_min_freq
to go down to that value if they were above it before.
However, the old values of scaling_max_freq
and scaling_min_freq
will be
restored after unsetting no_turbo
, unless these attributes have been written
to after no_turbo
was set.
If no_turbo
is not set, the maximum possible value of scaling_max_freq
and scaling_min_freq
corresponds to the maximum supported turbo P-state,
which also is the value of cpuinfo_max_freq
in either case.
Next, the following policy attributes have special meaning if
intel_pstate
works in the active mode:
scaling_available_governors
- List of P-state selection algorithms provided by
intel_pstate
. scaling_governor
- P-state selection algorithm provided by
intel_pstate
currently in use with the given policy. scaling_cur_freq
- Frequency of the average P-state of the CPU represented by the given policy for the time interval between the last two invocations of the driver’s utilization update callback by the CPU scheduler for that CPU.
The meaning of these attributes in the passive mode is the same as for other scaling drivers.
Additionally, the value of the scaling_driver
attribute for intel_pstate
depends on the operation mode of the driver. Namely, it is either
“intel_pstate” (in the active mode) or “intel_cpufreq” (in the
passive mode).
Coordination of P-State Limits¶
intel_pstate
allows P-state limits to be set in two ways: with the help of
the max_perf_pct
and min_perf_pct
global attributes or via the scaling_max_freq
and scaling_min_freq
CPUFreq
policy attributes. The coordination between those limits is based
on the following rules, regardless of the current operation mode of the driver:
- All CPUs are affected by the global limits (that is, none of them can be requested to run faster than the global maximum and none of them can be requested to run slower than the global minimum).
- Each individual CPU is affected by its own per-policy limits (that is, it cannot be requested to run faster than its own per-policy maximum and it cannot be requested to run slower than its own per-policy minimum).
- The global and per-policy limits can be set independently.
If the HWP feature is enabled in the processor, the resulting effective values are written into its registers whenever the limits change in order to request its internal P-state selection logic to always set P-states within these limits. Otherwise, the limits are taken into account by scaling governors (in the passive mode) and by the driver every time before setting a new P-state for a CPU.
Additionally, if the intel_pstate=per_cpu_perf_limits
command line argument
is passed to the kernel, max_perf_pct
and min_perf_pct
are not exposed
at all and the only way to set the limits is by using the policy attributes.
Energy vs Performance Hints¶
If intel_pstate
works in the active mode with the HWP feature enabled in the processor, additional attributes are present
in every CPUFreq
policy directory in sysfs
. They are intended to allow
user space to help intel_pstate
to adjust the processor’s internal P-state
selection logic by focusing it on performance or on energy-efficiency, or
somewhere between the two extremes:
energy_performance_preference
Current value of the energy vs performance hint for the given policy (or the CPU represented by it).
The hint can be changed by writing to this attribute.
energy_performance_available_preferences
List of strings that can be written to the
energy_performance_preference
attribute.They represent different energy vs performance hints and should be self-explanatory, except that
default
represents whatever hint value was set by the platform firmware.
Strings written to the energy_performance_preference
attribute are
internally translated to integer values written to the processor’s
Energy-Performance Preference (EPP) knob (if supported) or its
Energy-Performance Bias (EPB) knob.
[Note that tasks may by migrated from one CPU to another by the scheduler’s load-balancing algorithm and if different energy vs performance hints are set for those CPUs, that may lead to undesirable outcomes. To avoid such issues it is better to set the same energy vs performance hint for all CPUs or to pin every task potentially sensitive to them to a specific CPU.]
intel_pstate
vs acpi-cpufreq
¶
On the majority of systems supported by intel_pstate
, the ACPI tables
provided by the platform firmware contain _PSS
objects returning information
that can be used for CPU performance scaling (refer to the ACPI specification
for details on the _PSS
objects and the format of the information returned
by them).
The information returned by the ACPI _PSS
objects is used by the
acpi-cpufreq
scaling driver. On systems supported by intel_pstate
the acpi-cpufreq
driver uses the same hardware CPU performance scaling
interface, but the set of P-states it can use is limited by the _PSS
output.
On those systems each _PSS
object returns a list of P-states supported by
the corresponding CPU which basically is a subset of the P-states range that can
be used by intel_pstate
on the same system, with one exception: the whole
turbo range is represented by one item in it (the topmost one). By
convention, the frequency returned by _PSS
for that item is greater by 1 MHz
than the frequency of the highest non-turbo P-state listed by it, but the
corresponding P-state representation (following the hardware specification)
returned for it matches the maximum supported turbo P-state (or is the
special value 255 meaning essentially “go as high as you can get”).
The list of P-states returned by _PSS
is reflected by the table of
available frequencies supplied by acpi-cpufreq
to the CPUFreq
core and
scaling governors and the minimum and maximum supported frequencies reported by
it come from that list as well. In particular, given the special representation
of the turbo range described above, this means that the maximum supported
frequency reported by acpi-cpufreq
is higher by 1 MHz than the frequency
of the highest supported non-turbo P-state listed by _PSS
which, of course,
affects decisions made by the scaling governors, except for powersave
and
performance
.
For example, if a given governor attempts to select a frequency proportional to
estimated CPU load and maps the load of 100% to the maximum supported frequency
(possibly multiplied by a constant), then it will tend to choose P-states below
the turbo threshold if acpi-cpufreq
is used as the scaling driver, because
in that case the turbo range corresponds to a small fraction of the frequency
band it can use (1 MHz vs 1 GHz or more). In consequence, it will only go to
the turbo range for the highest loads and the other loads above 50% that might
benefit from running at turbo frequencies will be given non-turbo P-states
instead.
One more issue related to that may appear on systems supporting the
Configurable TDP feature allowing the platform firmware to set the
turbo threshold. Namely, if that is not coordinated with the lists of P-states
returned by _PSS
properly, there may be more than one item corresponding to
a turbo P-state in those lists and there may be a problem with avoiding the
turbo range (if desirable or necessary). Usually, to avoid using turbo
P-states overall, acpi-cpufreq
simply avoids using the topmost state listed
by _PSS
, but that is not sufficient when there are other turbo P-states in
the list returned by it.
Apart from the above, acpi-cpufreq
works like intel_pstate
in the
passive mode, except that the number of P-states it can set
is limited to the ones listed by the ACPI _PSS
objects.
Kernel Command Line Options for intel_pstate
¶
Several kernel command line options can be used to pass early-configuration-time
parameters to intel_pstate
in order to enforce specific behavior of it. All
of them have to be prepended with the intel_pstate=
prefix.
disable
- Do not register
intel_pstate
as the scaling driver even if the processor is supported by it. passive
Register
intel_pstate
in the passive mode to start with.This option implies the
no_hwp
one described below.force
Register
intel_pstate
as the scaling driver instead ofacpi-cpufreq
even if the latter is preferred on the given system.This may prevent some platform features (such as thermal controls and power capping) that rely on the availability of ACPI P-states information from functioning as expected, so it should be used with caution.
This option does not work with processors that are not supported by
intel_pstate
and on platforms where thepcc-cpufreq
scaling driver is used instead ofacpi-cpufreq
.no_hwp
- Do not enable the hardware-managed P-states (HWP) feature even if it is supported by the processor.
hwp_only
- Register
intel_pstate
as the scaling driver only if the hardware-managed P-states (HWP) feature is supported by the processor. support_acpi_ppc
Take ACPI
_PPC
performance limits into account.If the preferred power management profile in the FADT (Fixed ACPI Description Table) is set to “Enterprise Server” or “Performance Server”, the ACPI
_PPC
limits are taken into account by default and this option has no effect.per_cpu_perf_limits
- Use per-logical-CPU P-State limits (see Coordination of P-state Limits for details).
Diagnostics and Tuning¶
Trace Events¶
There are two static trace events that can be used for intel_pstate
diagnostics. One of them is the cpu_frequency
trace event generally used
by CPUFreq
, and the other one is the pstate_sample
trace event specific
to intel_pstate
. Both of them are triggered by intel_pstate
only if
it works in the active mode.
The following sequence of shell commands can be used to enable them and see their output (if the kernel is generally configured to support event tracing):
# cd /sys/kernel/debug/tracing/
# echo 1 > events/power/pstate_sample/enable
# echo 1 > events/power/cpu_frequency/enable
# cat trace
gnome-terminal--4510 [001] ..s. 1177.680733: pstate_sample: core_busy=107 scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618 freq=2474476
cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2
If intel_pstate
works in the passive mode, the
cpu_frequency
trace event will be triggered either by the schedutil
scaling governor (for the policies it is attached to), or by the CPUFreq
core (for the policies with other scaling governors).
ftrace
¶
The ftrace
interface can be used for low-level diagnostics of
intel_pstate
. For example, to check how often the function to set a
P-state is called, the ftrace
filter can be set to to
intel_pstate_set_pstate()
:
# cd /sys/kernel/debug/tracing/
# cat available_filter_functions | grep -i pstate
intel_pstate_set_pstate
intel_pstate_cpu_init
...
# echo intel_pstate_set_pstate > set_ftrace_filter
# echo function > current_tracer
# cat trace | head -15
# tracer: function
#
# entries-in-buffer/entries-written: 80/80 #P:4
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
<idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func
Tuning Interface in debugfs
¶
The powersave
algorithm provided by intel_pstate
for the Core line of
processors in the active mode is based on a PID controller
whose parameters were chosen to address a number of different use cases at the
same time. However, it still is possible to fine-tune it to a specific workload
and the debugfs
interface under /sys/kernel/debug/pstate_snb/
is
provided for this purpose. [Note that the pstate_snb
directory will be
present only if the specific P-state selection algorithm matching the interface
in it actually is in use.]
The following files present in that directory can be used to modify the PID controller parameters at run time:
deadband
d_gain_pct
i_gain_pct
p_gain_pct
sample_rate_ms
setpoint
Note, however, that achieving desirable results this way generally requires expert-level understanding of the power vs performance tradeoff, so extra care is recommended when attempting to do that.