cpufreq: User/admin documentation update and consolidation

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Power Management

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

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   :maxdepth: 2

   cpufreq

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   Indices

   =======

   * :ref:`genindex`

Processor boosting control

	- information for users -

Quick guide for the impatient:

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

/sys/devices/system/cpu/cpufreq/boost

controls the boost setting for the whole system. You can read and write

that file with either "0" (boosting disabled) or "1" (boosting allowed).

Reading or writing 1 does not mean that the system is boosting at this

very moment, but only that the CPU _may_ raise the frequency at it's

discretion.

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

Introduction

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

Some CPUs support a functionality to raise the operating frequency of

some cores in a multi-core package if certain conditions apply, mostly

if the whole chip is not fully utilized and below it's intended thermal

budget. The decision about boost disable/enable is made either at hardware

(e.g. x86) or software (e.g ARM).

On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core",

in technical documentation "Core performance boost". In Linux we use

the term "boost" for convenience.

Rationale for disable switch

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

Though the idea is to just give better performance without any user

intervention, sometimes the need arises to disable this functionality.

Most systems offer a switch in the (BIOS) firmware to disable the

functionality at all, but a more fine-grained and dynamic control would

be desirable:

1. While running benchmarks, reproducible results are important. Since

   the boosting functionality depends on the load of the whole package,

   single thread performance can vary. By explicitly disabling the boost

   functionality at least for the benchmark's run-time the system will run

   at a fixed frequency and results are reproducible again.

2. To examine the impact of the boosting functionality it is helpful

   to do tests with and without boosting.

3. Boosting means overclocking the processor, though under controlled

   conditions. By raising the frequency and the voltage the processor

   will consume more power than without the boosting, which may be

   undesirable for instance for mobile users. Disabling boosting may

   save power here, though this depends on the workload.

User controlled switch

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

To allow the user to toggle the boosting functionality, the cpufreq core

driver exports a sysfs knob to enable or disable it. There is a file:

/sys/devices/system/cpu/cpufreq/boost

which can either read "0" (boosting disabled) or "1" (boosting enabled).

The file is exported only when cpufreq driver supports boosting.

Explicitly changing the permissions and writing to that file anyway will

return EINVAL.

On supported CPUs one can write either a "0" or a "1" into this file.

This will either disable the boost functionality on all cores in the

whole system (0) or will allow the software or hardware to boost at will

(1).

Writing a "1" does not explicitly boost the system, but just allows the

CPU to boost at their discretion. Some implementations take external

factors like the chip's temperature into account, so boosting once does

not necessarily mean that it will occur every time even using the exact

same software setup.

AMD legacy cpb switch

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

The AMD powernow-k8 driver used to support a very similar switch to

disable or enable the "Core Performance Boost" feature of some AMD CPUs.

This switch was instantiated in each CPU's cpufreq directory

(/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb".

Though the per CPU existence hints at a more fine grained control, the

actual implementation only supported a system-global switch semantics,

which was simply reflected into each CPU's file. Writing a 0 or 1 into it

would pull the other CPUs to the same state.

For compatibility reasons this file and its behavior is still supported

on AMD CPUs, though it is now protected by a config switch

(X86_ACPI_CPUFREQ_CPB). On Intel CPUs this file will never be created,

even with the config option set.

This functionality is considered legacy and will be removed in some future

kernel version.

More fine grained boosting control

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

Technically it is possible to switch the boosting functionality at least

on a per package basis, for some CPUs even per core. Currently the driver

does not support it, but this may be implemented in the future.

     CPU frequency and voltage scaling code in the Linux(TM) kernel

		         L i n u x    C P U F r e q

		      C P U F r e q   G o v e r n o r s

		   - information for users and developers -

		    Dominik Brodowski  <linux@brodo.de>

            some additions and corrections by Nico Golde <nico@ngolde.de>

		Rafael J. Wysocki <rafael.j.wysocki@intel.com>

		   Viresh Kumar <viresh.kumar@linaro.org>

   Clock scaling allows you to change the clock speed of the CPUs on the

    fly. This is a nice method to save battery power, because the lower

            the clock speed, the less power the CPU consumes.

Contents:

---------

1.   What is a CPUFreq Governor?

2.   Governors In the Linux Kernel

2.1  Performance

2.2  Powersave

2.3  Userspace

2.4  Ondemand

2.5  Conservative

2.6  Schedutil

3.   The Governor Interface in the CPUfreq Core

4.   References

1. What Is A CPUFreq Governor?

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

Most cpufreq drivers (except the intel_pstate and longrun) or even most

cpu frequency scaling algorithms only allow the CPU frequency to be set

to predefined fixed values.  In order to offer dynamic frequency

scaling, the cpufreq core must be able to tell these drivers of a

"target frequency". So these specific drivers will be transformed to

offer a "->target/target_index/fast_switch()" call instead of the

"->setpolicy()" call. For set_policy drivers, all stays the same,

though.

How to decide what frequency within the CPUfreq policy should be used?

That's done using "cpufreq governors".

Basically, it's the following flow graph:

CPU can be set to switch independently	 |	   CPU can only be set

      within specific "limits"		 |       to specific frequencies

                                 "CPUfreq policy"

		consists of frequency limits (policy->{min,max})

  		     and CPUfreq governor to be used

			 /		      \

			/		       \

		       /		       the cpufreq governor decides

		      /			       (dynamically or statically)

		     /			       what target_freq to set within

		    /			       the limits of policy->{min,max}

		   /			            \

		  /				     \

	Using the ->setpolicy call,		 Using the ->target/target_index/fast_switch call,

	    the limits and the			  the frequency closest

	     "policy" is set.			  to target_freq is set.

						  It is assured that it

						  is within policy->{min,max}

2. Governors In the Linux Kernel

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

2.1 Performance

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

The CPUfreq governor "performance" sets the CPU statically to the

highest frequency within the borders of scaling_min_freq and

scaling_max_freq.

2.2 Powersave

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

The CPUfreq governor "powersave" sets the CPU statically to the

lowest frequency within the borders of scaling_min_freq and

scaling_max_freq.

2.3 Userspace

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

The CPUfreq governor "userspace" allows the user, or any userspace

program running with UID "root", to set the CPU to a specific frequency

by making a sysfs file "scaling_setspeed" available in the CPU-device

directory.

2.4 Ondemand

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

The CPUfreq governor "ondemand" sets the CPU frequency depending on the

current system load. Load estimation is triggered by the scheduler

through the update_util_data->func hook; when triggered, cpufreq checks

the CPU-usage statistics over the last period and the governor sets the

CPU accordingly.  The CPU must have the capability to switch the

frequency very quickly.

Sysfs files:

* sampling_rate:

  Measured in uS (10^-6 seconds), this is how often you want the kernel

  to look at the CPU usage and to make decisions on what to do about the

  frequency.  Typically this is set to values of around '10000' or more.

  It's default value is (cmp. with users-guide.txt): transition_latency

  * 1000.  Be aware that transition latency is in ns and sampling_rate

  is in us, so you get the same sysfs value by default.  Sampling rate

  should always get adjusted considering the transition latency to set

  the sampling rate 750 times as high as the transition latency in the

  bash (as said, 1000 is default), do:

  $ echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate

* sampling_rate_min:

  The sampling rate is limited by the HW transition latency:

  transition_latency * 100

  Or by kernel restrictions:

  - If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.

  - If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is

    used, the limits depend on the CONFIG_HZ option:

    HZ=1000: min=20000us  (20ms)

    HZ=250:  min=80000us  (80ms)

    HZ=100:  min=200000us (200ms)

  The highest value of kernel and HW latency restrictions is shown and

  used as the minimum sampling rate.

* up_threshold:

  This defines what the average CPU usage between the samplings of

  'sampling_rate' needs to be for the kernel to make a decision on

  whether it should increase the frequency.  For example when it is set

  to its default value of '95' it means that between the checking

  intervals the CPU needs to be on average more than 95% in use to then

  decide that the CPU frequency needs to be increased.

* ignore_nice_load:

  This parameter takes a value of '0' or '1'. When set to '0' (its

  default), all processes are counted towards the 'cpu utilisation'

  value.  When set to '1', the processes that are run with a 'nice'

  value will not count (and thus be ignored) in the overall usage

  calculation.  This is useful if you are running a CPU intensive

  calculation on your laptop that you do not care how long it takes to

  complete as you can 'nice' it and prevent it from taking part in the

  deciding process of whether to increase your CPU frequency.

* sampling_down_factor:

  This parameter controls the rate at which the kernel makes a decision

  on when to decrease the frequency while running at top speed. When set

  to 1 (the default) decisions to reevaluate load are made at the same

  interval regardless of current clock speed. But when set to greater

  than 1 (e.g. 100) it acts as a multiplier for the scheduling interval

  for reevaluating load when the CPU is at its top speed due to high

  load. This improves performance by reducing the overhead of load

  evaluation and helping the CPU stay at its top speed when truly busy,

  rather than shifting back and forth in speed. This tunable has no

  effect on behavior at lower speeds/lower CPU loads.

* powersave_bias:

  This parameter takes a value between 0 to 1000. It defines the

  percentage (times 10) value of the target frequency that will be

  shaved off of the target. For example, when set to 100 -- 10%, when

  ondemand governor would have targeted 1000 MHz, it will target

  1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0

  (disabled) by default.

  When AMD frequency sensitivity powersave bias driver --

  drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter

  defines the workload frequency sensitivity threshold in which a lower

  frequency is chosen instead of ondemand governor's original target.

  The frequency sensitivity is a hardware reported (on AMD Family 16h

  Processors and above) value between 0 to 100% that tells software how

  the performance of the workload running on a CPU will change when

  frequency changes. A workload with sensitivity of 0% (memory/IO-bound)

  will not perform any better on higher core frequency, whereas a

  workload with sensitivity of 100% (CPU-bound) will perform better

  higher the frequency. When the driver is loaded, this is set to 400 by

  default -- for CPUs running workloads with sensitivity value below

  40%, a lower frequency is chosen. Unloading the driver or writing 0

  will disable this feature.

2.5 Conservative

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

The CPUfreq governor "conservative", much like the "ondemand"

governor, sets the CPU frequency depending on the current usage.  It

differs in behaviour in that it gracefully increases and decreases the

CPU speed rather than jumping to max speed the moment there is any load

on the CPU. This behaviour is more suitable in a battery powered

environment.  The governor is tweaked in the same manner as the

"ondemand" governor through sysfs with the addition of:

* freq_step:

  This describes what percentage steps the cpu freq should be increased

  and decreased smoothly by.  By default the cpu frequency will increase

  in 5% chunks of your maximum cpu frequency.  You can change this value

  to anywhere between 0 and 100 where '0' will effectively lock your CPU

  at a speed regardless of its load whilst '100' will, in theory, make

  it behave identically to the "ondemand" governor.

* down_threshold:

  Same as the 'up_threshold' found for the "ondemand" governor but for

  the opposite direction.  For example when set to its default value of

  '20' it means that if the CPU usage needs to be below 20% between

  samples to have the frequency decreased.

* sampling_down_factor:

  Similar functionality as in "ondemand" governor.  But in

  "conservative", it controls the rate at which the kernel makes a

  decision on when to decrease the frequency while running in any speed.

  Load for frequency increase is still evaluated every sampling rate.

2.6 Schedutil

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

The "schedutil" governor aims at better integration with the Linux

kernel scheduler.  Load estimation is achieved through the scheduler's

Per-Entity Load Tracking (PELT) mechanism, which also provides

information about the recent load [1].  This governor currently does

load based DVFS only for tasks managed by CFS. RT and DL scheduler tasks

are always run at the highest frequency.  Unlike all the other

governors, the code is located under the kernel/sched/ directory.

Sysfs files:

* rate_limit_us:

  This contains a value in microseconds. The governor waits for

  rate_limit_us time before reevaluating the load again, after it has

  evaluated the load once.

For an in-depth comparison with the other governors refer to [2].

3. The Governor Interface in the CPUfreq Core

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

A new governor must register itself with the CPUfreq core using

"cpufreq_register_governor". The struct cpufreq_governor, which has to

be passed to that function, must contain the following values:

governor->name - A unique name for this governor.

governor->owner - .THIS_MODULE for the governor module (if appropriate).

plus a set of hooks to the functions implementing the governor's logic.

The CPUfreq governor may call the CPU processor driver using one of

these two functions:

int cpufreq_driver_target(struct cpufreq_policy *policy,

                                 unsigned int target_freq,

                                 unsigned int relation);

int __cpufreq_driver_target(struct cpufreq_policy *policy,

                                   unsigned int target_freq,

                                   unsigned int relation);

target_freq must be within policy->min and policy->max, of course.

What's the difference between these two functions? When your governor is

in a direct code path of a call to governor callbacks, like

governor->start(), the policy->rwsem is still held in the cpufreq core,

and there's no need to lock it again (in fact, this would cause a

deadlock). So use __cpufreq_driver_target only in these cases. In all

other cases (for example, when there's a "daemonized" function that

wakes up every second), use cpufreq_driver_target to take policy->rwsem

before the command is passed to the cpufreq driver.

4. References

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

[1] Per-entity load tracking: https://lwn.net/Articles/531853/

[2] Improvements in CPU frequency management: https://lwn.net/Articles/682391/