cpufreq: stats: Handle the case when trans_table goes beyond PAGE_SIZE

[linux-2.6-block.git] / Documentation / cpu-freq / cpufreq-stats.txt

     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
(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 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.

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.