Documentation/cpu-freq/governors.txt

   1      CPU frequency and voltage scaling code in the Linux(TM) kernel
   2
   3
   4                          L i n u x    C P U F r e q
   5
   6                       C P U F r e q   G o v e r n o r s
   7
   8                    - information for users and developers -
   9
  10
  11                     Dominik Brodowski  <linux@brodo.de>
  12             some additions and corrections by Nico Golde <nico@ngolde.de>
  13
  14
  15
  16    Clock scaling allows you to change the clock speed of the CPUs on the
  17     fly. This is a nice method to save battery power, because the lower
  18             the clock speed, the less power the CPU consumes.
  19
  20
  21 Contents:
  22 ---------
  23 1.   What is a CPUFreq Governor?
  24
  25 2.   Governors In the Linux Kernel
  26 2.1  Performance
  27 2.2  Powersave
  28 2.3  Userspace
  29 2.4  Ondemand
  30 2.5  Conservative
  31
  32 3.   The Governor Interface in the CPUfreq Core
  33
  34
  35
  36 1. What Is A CPUFreq Governor?
  37 ==============================
  38
  39 Most cpufreq drivers (in fact, all except one, longrun) or even most
  40 cpu frequency scaling algorithms only offer the CPU to be set to one
  41 frequency. In order to offer dynamic frequency scaling, the cpufreq
  42 core must be able to tell these drivers of a "target frequency". So
  43 these specific drivers will be transformed to offer a "->target"
  44 call instead of the existing "->setpolicy" call. For "longrun", all
  45 stays the same, though.
  46
  47 How to decide what frequency within the CPUfreq policy should be used?
  48 That's done using "cpufreq governors". Two are already in this patch
  49 -- they're the already existing "powersave" and "performance" which
  50 set the frequency statically to the lowest or highest frequency,
  51 respectively. At least two more such governors will be ready for
  52 addition in the near future, but likely many more as there are various
  53 different theories and models about dynamic frequency scaling
  54 around. Using such a generic interface as cpufreq offers to scaling
  55 governors, these can be tested extensively, and the best one can be
  56 selected for each specific use.
  57
  58 Basically, it's the following flow graph:
  59
  60 CPU can be set to switch independently   |         CPU can only be set
  61       within specific "limits"           |       to specific frequencies
  62
  63                                  "CPUfreq policy"
  64                 consists of frequency limits (policy->{min,max})
  65                      and CPUfreq governor to be used
  66                          /                    \
  67                         /                      \
  68                        /                       the cpufreq governor decides
  69                       /                        (dynamically or statically)
  70                      /                         what target_freq to set within
  71                     /                          the limits of policy->{min,max}
  72                    /                                \
  73                   /                                  \
  74         Using the ->setpolicy call,              Using the ->target call,
  75             the limits and the                    the frequency closest
  76              "policy" is set.                     to target_freq is set.
  77                                                   It is assured that it
  78                                                   is within policy->{min,max}
  79
  80
  81 2. Governors In the Linux Kernel
  82 ================================
  83
  84 2.1 Performance
  85 ---------------
  86
  87 The CPUfreq governor "performance" sets the CPU statically to the
  88 highest frequency within the borders of scaling_min_freq and
  89 scaling_max_freq.
  90
  91
  92 2.2 Powersave
  93 -------------
  94
  95 The CPUfreq governor "powersave" sets the CPU statically to the
  96 lowest frequency within the borders of scaling_min_freq and
  97 scaling_max_freq.
  98
  99
 100 2.3 Userspace
 101 -------------
 102
 103 The CPUfreq governor "userspace" allows the user, or any userspace
 104 program running with UID "root", to set the CPU to a specific frequency
 105 by making a sysfs file "scaling_setspeed" available in the CPU-device
 106 directory.
 107
 108
 109 2.4 Ondemand
 110 ------------
 111
 112 The CPUfreq governor "ondemand" sets the CPU depending on the
 113 current usage. To do this the CPU must have the capability to
 114 switch the frequency very quickly.  There are a number of sysfs file
 115 accessible parameters:
 116
 117 sampling_rate: measured in uS (10^-6 seconds), this is how often you
 118 want the kernel to look at the CPU usage and to make decisions on
 119 what to do about the frequency.  Typically this is set to values of
 120 around '10000' or more.
 121
 122 show_sampling_rate_(min|max): THIS INTERFACE IS DEPRECATED, DON'T USE IT.
 123 You can use wider ranges now and the general
 124 cpuinfo_transition_latency variable (cmp. with user-guide.txt) can be
 125 used to obtain exactly the same info:
 126 show_sampling_rate_min = transtition_latency * 500    / 1000
 127 show_sampling_rate_max = transtition_latency * 500000 / 1000
 128 (divided by 1000 is to illustrate that sampling rate is in us and
 129 transition latency is exported ns).
 130
 131 up_threshold: defines what the average CPU usage between the samplings
 132 of 'sampling_rate' needs to be for the kernel to make a decision on
 133 whether it should increase the frequency.  For example when it is set
 134 to its default value of '80' it means that between the checking
 135 intervals the CPU needs to be on average more than 80% in use to then
 136 decide that the CPU frequency needs to be increased.
 137
 138 ignore_nice_load: this parameter takes a value of '0' or '1'. When
 139 set to '0' (its default), all processes are counted towards the
 140 'cpu utilisation' value.  When set to '1', the processes that are
 141 run with a 'nice' value will not count (and thus be ignored) in the
 142 overall usage calculation.  This is useful if you are running a CPU
 143 intensive calculation on your laptop that you do not care how long it
 144 takes to complete as you can 'nice' it and prevent it from taking part
 145 in the deciding process of whether to increase your CPU frequency.
 146
 147
 148 2.5 Conservative
 149 ----------------
 150
 151 The CPUfreq governor "conservative", much like the "ondemand"
 152 governor, sets the CPU depending on the current usage.  It differs in
 153 behaviour in that it gracefully increases and decreases the CPU speed
 154 rather than jumping to max speed the moment there is any load on the
 155 CPU.  This behaviour more suitable in a battery powered environment.
 156 The governor is tweaked in the same manner as the "ondemand" governor
 157 through sysfs with the addition of:
 158
 159 freq_step: this describes what percentage steps the cpu freq should be
 160 increased and decreased smoothly by.  By default the cpu frequency will
 161 increase in 5% chunks of your maximum cpu frequency.  You can change this
 162 value to anywhere between 0 and 100 where '0' will effectively lock your
 163 CPU at a speed regardless of its load whilst '100' will, in theory, make
 164 it behave identically to the "ondemand" governor.
 165
 166 down_threshold: same as the 'up_threshold' found for the "ondemand"
 167 governor but for the opposite direction.  For example when set to its
 168 default value of '20' it means that if the CPU usage needs to be below
 169 20% between samples to have the frequency decreased.
 170
 171 3. The Governor Interface in the CPUfreq Core
 172 =============================================
 173
 174 A new governor must register itself with the CPUfreq core using
 175 "cpufreq_register_governor". The struct cpufreq_governor, which has to
 176 be passed to that function, must contain the following values:
 177
 178 governor->name -            A unique name for this governor
 179 governor->governor -        The governor callback function
 180 governor->owner -           .THIS_MODULE for the governor module (if
 181                             appropriate)
 182
 183 The governor->governor callback is called with the current (or to-be-set)
 184 cpufreq_policy struct for that CPU, and an unsigned int event. The
 185 following events are currently defined:
 186
 187 CPUFREQ_GOV_START:   This governor shall start its duty for the CPU
 188                      policy->cpu
 189 CPUFREQ_GOV_STOP:    This governor shall end its duty for the CPU
 190                      policy->cpu
 191 CPUFREQ_GOV_LIMITS:  The limits for CPU policy->cpu have changed to
 192                      policy->min and policy->max.
 193
 194 If you need other "events" externally of your driver, _only_ use the
 195 cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
 196 CPUfreq core to ensure proper locking.
 197
 198
 199 The CPUfreq governor may call the CPU processor driver using one of
 200 these two functions:
 201
 202 int cpufreq_driver_target(struct cpufreq_policy *policy,
 203                                  unsigned int target_freq,
 204                                  unsigned int relation);
 205
 206 int __cpufreq_driver_target(struct cpufreq_policy *policy,
 207                                    unsigned int target_freq,
 208                                    unsigned int relation);
 209
 210 target_freq must be within policy->min and policy->max, of course.
 211 What's the difference between these two functions? When your governor
 212 still is in a direct code path of a call to governor->governor, the
 213 per-CPU cpufreq lock is still held in the cpufreq core, and there's
 214 no need to lock it again (in fact, this would cause a deadlock). So
 215 use __cpufreq_driver_target only in these cases. In all other cases
 216 (for example, when there's a "daemonized" function that wakes up
 217 every second), use cpufreq_driver_target to lock the cpufreq per-CPU
 218 lock before the command is passed to the cpufreq processor driver.
 219