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


             L i n u x    c p u f r e q - s t a t s   d r i v e r

                       - information for users -


             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 
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 is in descending order with increasing rows and 
Freq_j is in descending order with increasing columns. The output here also 
contains the actual freq values for each row and column for better readability.

--------------------------------------------------------------------------------
<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 translation statistics details


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

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

"CPU frequency translation statistics details" (CONFIG_CPU_FREQ_STAT_DETAILS)
provides fine grained cpufreq stats by trans_table. The reason for having a
separate config option for trans_table is:
- trans_table goes against the traditional /sysfs rule of one value per
  interface. It provides a whole bunch of value in a 2 dimensional matrix
  form.

Once these two options are enabled and your CPU supports cpufrequency, you
will be able to see the CPU frequency statistics in /sysfs.




