cpufreq: intel_pstate: Document the current behavior and user interface

.. |struct cpufreq_policy| replace:: :c:type:`struct cpufreq_policy <cpufreq_policy>`

.. |intel_pstate| replace:: :doc:`intel_pstate <intel_pstate>`

=======================

CPU Performance Scaling

interface it comes from and may not be easily represented in an abstract,

platform-independent way.  For this reason, ``CPUFreq`` allows scaling drivers

to bypass the governor layer and implement their own performance scaling

algorithms.  That is done by the ``intel_pstate`` scaling driver.

algorithms.  That is done by the |intel_pstate| scaling driver.

``CPUFreq`` Policy Objects

into account.  That is achieved by invoking the governor's ``->stop`` and

``->start()`` callbacks, in this order, for the entire policy.

As mentioned before, the ``intel_pstate`` scaling driver bypasses the scaling

As mentioned before, the |intel_pstate| scaling driver bypasses the scaling

governor layer of ``CPUFreq`` and provides its own P-state selection algorithms.

Consequently, if ``intel_pstate`` is used, scaling governors are not attached to

Consequently, if |intel_pstate| is used, scaling governors are not attached to

new policy objects.  Instead, the driver's ``->setpolicy()`` callback is invoked

to register per-CPU utilization update callbacks for each policy.  These

callbacks are invoked by the CPU scheduler in the same way as for scaling

governors, but in the ``intel_pstate`` case they both determine the P-state to

governors, but in the |intel_pstate| case they both determine the P-state to

use and change the hardware configuration accordingly in one go from scheduler

context.

``scaling_available_governors``

	List of ``CPUFreq`` scaling governors present in the kernel that can

	be attached to this policy or (if the ``intel_pstate`` scaling driver is

	be attached to this policy or (if the |intel_pstate| scaling driver is

	in use) list of scaling algorithms provided by the driver that can be

	applied to this policy.

	the CPU is actually running at (due to hardware design and other

	limitations).

	Some scaling drivers (e.g. ``intel_pstate``) attempt to provide

	Some scaling drivers (e.g. |intel_pstate|) attempt to provide

	information more precisely reflecting the current CPU frequency through

	this attribute, but that still may not be the exact current CPU

	frequency as seen by the hardware at the moment.

``scaling_governor``

	The scaling governor currently attached to this policy or (if the

	``intel_pstate`` scaling driver is in use) the scaling algorithm

	|intel_pstate| scaling driver is in use) the scaling algorithm

	provided by the driver that is currently applied to this policy.

	This attribute is read-write and writing to it will cause a new scaling

	governor to be attached to this policy or a new scaling algorithm

	provided by the scaling driver to be applied to it (in the

	``intel_pstate`` case), as indicated by the string written to this

	|intel_pstate| case), as indicated by the string written to this

	attribute (which must be one of the names listed by the

	``scaling_available_governors`` attribute described above).

the "boost" setting for the whole system.  It is not present if the underlying

scaling driver does not support the frequency boost mechanism (or supports it,

but provides a driver-specific interface for controlling it, like

``intel_pstate``).

|intel_pstate|).

If the value in this file is 1, the frequency boost mechanism is enabled.  This

means that either the hardware can be put into states in which it is able to

   :maxdepth: 2

   cpufreq

   intel_pstate

.. only::  subproject and html

Intel P-State driver

--------------------

This driver provides an interface to control the P-State selection for the

SandyBridge+ Intel processors.

The following document explains P-States:

http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf

As stated in the document, P-State doesn’t exactly mean a frequency. However, for

the sake of the relationship with cpufreq, P-State and frequency are used

interchangeably.

Understanding the cpufreq core governors and policies are important before

discussing more details about the Intel P-State driver. Based on what callbacks

a cpufreq driver provides to the cpufreq core, it can support two types of

drivers:

- with target_index() callback: In this mode, the drivers using cpufreq core

simply provide the minimum and maximum frequency limits and an additional

interface target_index() to set the current frequency. The cpufreq subsystem

has a number of scaling governors ("performance", "powersave", "ondemand",

etc.). Depending on which governor is in use, cpufreq core will call for

transitions to a specific frequency using target_index() callback.

- setpolicy() callback: In this mode, drivers do not provide target_index()

callback, so cpufreq core can't request a transition to a specific frequency.

The driver provides minimum and maximum frequency limits and callbacks to set a

policy. The policy in cpufreq sysfs is referred to as the "scaling governor".

The cpufreq core can request the driver to operate in any of the two policies:

"performance" and "powersave". The driver decides which frequency to use based

on the above policy selection considering minimum and maximum frequency limits.

The Intel P-State driver falls under the latter category, which implements the

setpolicy() callback. This driver decides what P-State to use based on the

requested policy from the cpufreq core. If the processor is capable of

selecting its next P-State internally, then the driver will offload this

responsibility to the processor (aka HWP: Hardware P-States). If not, the

driver implements algorithms to select the next P-State.

Since these policies are implemented in the driver, they are not same as the

cpufreq scaling governors implementation, even if they have the same name in

the cpufreq sysfs (scaling_governors). For example the "performance" policy is

similar to cpufreq’s "performance" governor, but "powersave" is completely

different than the cpufreq "powersave" governor. The strategy here is similar

to cpufreq "ondemand", where the requested P-State is related to the system load.

Sysfs Interface

In addition to the frequency-controlling interfaces provided by the cpufreq

core, the driver provides its own sysfs files to control the P-State selection.

These files have been added to /sys/devices/system/cpu/intel_pstate/.

Any changes made to these files are applicable to all CPUs (even in a

multi-package system, Refer to later section on placing "Per-CPU limits").

      max_perf_pct: Limits the maximum P-State that will be requested by

      the driver. It states it as a percentage of the available performance. The

      available (P-State) performance may be reduced by the no_turbo

      setting described below.

      min_perf_pct: Limits the minimum P-State that will be requested by

      the driver. It states it as a percentage of the max (non-turbo)

      performance level.

      no_turbo: Limits the driver to selecting P-State below the turbo

      frequency range.

      turbo_pct: Displays the percentage of the total performance that

      is supported by hardware that is in the turbo range. This number

      is independent of whether turbo has been disabled or not.

      num_pstates: Displays the number of P-States that are supported

      by hardware. This number is independent of whether turbo has

      been disabled or not.

For example, if a system has these parameters:

	Max 1 core turbo ratio: 0x21 (Max 1 core ratio is the maximum P-State)

	Max non turbo ratio: 0x17

	Minimum ratio : 0x08 (Here the ratio is called max efficiency ratio)

Sysfs will show :

	max_perf_pct:100, which corresponds to 1 core ratio

	min_perf_pct:24, max_efficiency_ratio / max 1 Core ratio

	no_turbo:0, turbo is not disabled

	num_pstates:26 = (max 1 Core ratio - Max Efficiency Ratio + 1)

	turbo_pct:39 = (max 1 core ratio - max non turbo ratio) / num_pstates

Refer to "Intel® 64 and IA-32 Architectures Software Developer’s Manual

Volume 3: System Programming Guide" to understand ratios.

There is one more sysfs attribute in /sys/devices/system/cpu/intel_pstate/

that can be used for controlling the operation mode of the driver:

      status: Three settings are possible:

      "off"     - The driver is not in use at this time.

      "active"  - The driver works as a P-state governor (default).

      "passive" - The driver works as a regular cpufreq one and collaborates

                  with the generic cpufreq governors (it sets P-states as

                  requested by those governors).

      The current setting is returned by reads from this attribute.  Writing one

      of the above strings to it changes the operation mode as indicated by that

      string, if possible.  If HW-managed P-states (HWP) are enabled, it is not

      possible to change the driver's operation mode and attempts to write to

      this attribute will fail.

cpufreq sysfs for Intel P-State

Since this driver registers with cpufreq, cpufreq sysfs is also presented.

There are some important differences, which need to be considered.

scaling_cur_freq: This displays the real frequency which was used during

the last sample period instead of what is requested. Some other cpufreq driver,

like acpi-cpufreq, displays what is requested (Some changes are on the

way to fix this for acpi-cpufreq driver). The same is true for frequencies

displayed at /proc/cpuinfo.

scaling_governor: This displays current active policy. Since each CPU has a

cpufreq sysfs, it is possible to set a scaling governor to each CPU. But this

is not possible with Intel P-States, as there is one common policy for all

CPUs. Here, the last requested policy will be applicable to all CPUs. It is

suggested that one use the cpupower utility to change policy to all CPUs at the

same time.

scaling_setspeed: This attribute can never be used with Intel P-State.

scaling_max_freq/scaling_min_freq: This interface can be used similarly to

the max_perf_pct/min_perf_pct of Intel P-State sysfs. However since frequencies

are converted to nearest possible P-State, this is prone to rounding errors.

This method is not preferred to limit performance.

affected_cpus: Not used

related_cpus: Not used

For contemporary Intel processors, the frequency is controlled by the

processor itself and the P-State exposed to software is related to

performance levels.  The idea that frequency can be set to a single

frequency is fictional for Intel Core processors. Even if the scaling

driver selects a single P-State, the actual frequency the processor

will run at is selected by the processor itself.

Per-CPU limits

The kernel command line option "intel_pstate=per_cpu_perf_limits" forces

the intel_pstate driver to use per-CPU performance limits.  When it is set,

the sysfs control interface described above is subject to limitations.

- The following controls are not available for both read and write

	/sys/devices/system/cpu/intel_pstate/max_perf_pct

	/sys/devices/system/cpu/intel_pstate/min_perf_pct

- The following controls can be used to set performance limits, as far as the

architecture of the processor permits:

	/sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq

	/sys/devices/system/cpu/cpu*/cpufreq/scaling_min_freq

	/sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

- User can still observe turbo percent and number of P-States from

	/sys/devices/system/cpu/intel_pstate/turbo_pct

	/sys/devices/system/cpu/intel_pstate/num_pstates

- User can read write system wide turbo status

	/sys/devices/system/cpu/no_turbo

Support of energy performance hints

It is possible to provide hints to the HWP algorithms in the processor

to be more performance centric to more energy centric. When the driver

is using HWP, two additional cpufreq sysfs attributes are presented for

each logical CPU.

These attributes are:

	- energy_performance_available_preferences

	- energy_performance_preference

To get list of supported hints:

$ cat energy_performance_available_preferences

    default performance balance_performance balance_power power

The current preference can be read or changed via cpufreq sysfs

attribute "energy_performance_preference". Reading from this attribute

will display current effective setting. User can write any of the valid

preference string to this attribute. User can always restore to power-on

default by writing "default".

Since threads can migrate to different CPUs, this is possible that the

new CPU may have different energy performance preference than the previous

one. To avoid such issues, either threads can be pinned to specific CPUs

or set the same energy performance preference value to all CPUs.

Tuning Intel P-State driver

When the performance can be tuned using PID (Proportional Integral

Derivative) controller, debugfs files are provided for adjusting performance.

They are presented under:

/sys/kernel/debug/pstate_snb/

The PID tunable parameters are:

      deadband

      d_gain_pct

      i_gain_pct

      p_gain_pct

      sample_rate_ms

      setpoint

To adjust these parameters, some understanding of driver implementation is

necessary. There are some tweeks described here, but be very careful. Adjusting

them requires expert level understanding of power and performance relationship.

These limits are only useful when the "powersave" policy is active.

-To make the system more responsive to load changes, sample_rate_ms can

be adjusted  (current default is 10ms).

-To make the system use higher performance, even if the load is lower, setpoint

can be adjusted to a lower number. This will also lead to faster ramp up time

to reach the maximum P-State.

If there are no derivative and integral coefficients, The next P-State will be

equal to:

	current P-State - ((setpoint - current cpu load) * p_gain_pct)

For example, if the current PID parameters are (Which are defaults for the core

processors like SandyBridge):

      deadband = 0

      d_gain_pct = 0

      i_gain_pct = 0

      p_gain_pct = 20

      sample_rate_ms = 10

      setpoint = 97

If the current P-State = 0x08 and current load = 100, this will result in the

next P-State = 0x08 - ((97 - 100) * 0.2) = 8.6 (rounded to 9). Here the P-State

goes up by only 1. If during next sample interval the current load doesn't

change and still 100, then P-State goes up by one again. This process will

continue as long as the load is more than the setpoint until the maximum P-State

is reached.

For the same load at setpoint = 60, this will result in the next P-State

= 0x08 - ((60 - 100) * 0.2) = 16

So by changing the setpoint from 97 to 60, there is an increase of the

next P-State from 9 to 16. So this will make processor execute at higher

P-State for the same CPU load. If the load continues to be more than the

setpoint during next sample intervals, then P-State will go up again till the

maximum P-State is reached. But the ramp up time to reach the maximum P-State

will be much faster when the setpoint is 60 compared to 97.

Debugging Intel P-State driver

Event tracing

To debug P-State transition, the Linux event tracing interface can be used.

There are two specific events, which can be enabled (Provided the kernel

configs related to event tracing are enabled).

# 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

Using ftrace

If function level tracing is required, the Linux ftrace interface can be used.

For example if we want to check how often a function to set a P-State is

called, we can set ftrace filter 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