docs: cpu-freq: convert cpufreq-stats.txt to ReST

[people/ms/linux.git] / Documentation / cpu-freq / cpufreq-stats.rst

.. SPDX-License-Identifier: GPL-2.0

==========================================
General Description of sysfs CPUFreq Stats
==========================================

information for users


Author: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>

.. Contents

   1. Introduction
   2. Statistics Provided (with example)
   3. Configuring cpufreq-stats


1. Introduction
===============

cpufreq-stats is a driver that provides CPU frequency statistics for each CPU.
These statistics are provided in /sysfs as a bunch of read_only interfaces. This
interface (when configured) will appear in a separate directory under cpufreq
in /sysfs (<sysfs root>/devices/system/cpu/cpuX/cpufreq/stats/) for each CPU.
Various statistics will form read_only files under this directory.

This driver is designed to be independent of any particular cpufreq_driver
that may be running on your CPU. So, it will work with any cpufreq_driver.


2. Statistics Provided (with example)
=====================================

cpufreq stats provides following statistics (explained in detail below).

-  time_in_state
-  total_trans
-  trans_table

All the statistics will be from the time the stats driver has been inserted
(or the time the stats were reset) to the time when a read of a particular
statistic is done. Obviously, stats driver will not have any information
about the frequency transitions before the stats driver insertion.

::

    <mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # ls -l
    total 0
    drwxr-xr-x  2 root root    0 May 14 16:06 .
    drwxr-xr-x  3 root root    0 May 14 15:58 ..
    --w-------  1 root root 4096 May 14 16:06 reset
    -r--r--r--  1 root root 4096 May 14 16:06 time_in_state
    -r--r--r--  1 root root 4096 May 14 16:06 total_trans
    -r--r--r--  1 root root 4096 May 14 16:06 trans_table

- **reset**

Write-only attribute that can be used to reset the stat counters. This can be
useful for evaluating system behaviour under different governors without the
need for a reboot.

- **time_in_state**

This gives the amount of time spent in each of the frequencies supported by
this CPU. The cat output will have "<frequency> <time>" pair in each line, which
will mean this CPU spent <time> usertime units of time at <frequency>. Output
will have one line for each of the supported frequencies. usertime units here
is 10mS (similar to other time exported in /proc).

::

    <mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat time_in_state
    3600000 2089
    3400000 136
    3200000 34
    3000000 67
    2800000 172488


- **total_trans**

This gives the total number of frequency transitions on this CPU. The cat
output will have a single count which is the total number of frequency
transitions.

::

    <mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat total_trans
    20

- **trans_table**

This will give a fine grained information about all the CPU frequency
transitions. The cat output here is a two dimensional matrix, where an entry
<i,j> (row i, column j) represents the count of number of transitions from
Freq_i to Freq_j. Freq_i rows and Freq_j columns follow the sorting order in
which the driver has provided the frequency table initially to the cpufreq core
and so can be sorted (ascending or descending) or unsorted.  The output here
also contains the actual freq values for each row and column for better
readability.

If the transition table is bigger than PAGE_SIZE, reading this will
return an -EFBIG error.

::

    <mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat trans_table
    From  :    To
	    :   3600000   3400000   3200000   3000000   2800000
    3600000:         0         5         0         0         0
    3400000:         4         0         2         0         0
    3200000:         0         1         0         2         0
    3000000:         0         0         1         0         3
    2800000:         0         0         0         2         0

3. Configuring cpufreq-stats
============================

To configure cpufreq-stats in your kernel::

	Config Main Menu
		Power management options (ACPI, APM)  --->
			CPU Frequency scaling  --->
				[*] CPU Frequency scaling
				[*]   CPU frequency translation statistics


"CPU Frequency scaling" (CONFIG_CPU_FREQ) should be enabled to configure
cpufreq-stats.

"CPU frequency translation statistics" (CONFIG_CPU_FREQ_STAT) provides the
statistics which includes time_in_state, total_trans and trans_table.

Once this option is enabled and your CPU supports cpufrequency, you
will be able to see the CPU frequency statistics in /sysfs.