| blob | c7a2eb8450c226b50e55b28f8c6669c6041cc0e3 |
1 CPU frequency and voltage scaling code in the Linux(TM) kernel
4 L i n u x C P U F r e q
6 C P U F r e q G o v e r n o r s
8 - information for users and developers -
11 Dominik Brodowski <linux@brodo.de>
12 some additions and corrections by Nico Golde <nico@ngolde.de>
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.
21 Contents:
22 ---------
23 1. What is a CPUFreq Governor?
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
32 3. The Governor Interface in the CPUfreq Core
36 1. What Is A CPUFreq Governor?
37 ==============================
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.
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.
58 Basically, it's the following flow graph:
60 CPU can be set to switch independently | CPU can only be set
61 within specific "limits" | to specific frequencies
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}
81 2. Governors In the Linux Kernel
82 ================================
84 2.1 Performance
85 ---------------
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.
92 2.2 Powersave
93 -------------
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.
100 2.3 Userspace
101 -------------
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.
109 2.4 Ondemand
110 ------------
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:
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. It's default value is (cmp. with users-guide.txt):
121 transition_latency * 1000
122 Be aware that transition latency is in ns and sampling_rate is in us, so you
123 get the same sysfs value by default.
124 Sampling rate should always get adjusted considering the transition latency
125 To set the sampling rate 750 times as high as the transition latency
126 in the bash (as said, 1000 is default), do:
127 echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
128 >ondemand/sampling_rate
130 sampling_rate_min:
131 The sampling rate is limited by the HW transition latency:
132 transition_latency * 100
133 Or by kernel restrictions:
134 If CONFIG_NO_HZ is set, the limit is 10ms fixed.
135 If CONFIG_NO_HZ is not set or nohz=off boot parameter is used, the
136 limits depend on the CONFIG_HZ option:
137 HZ=1000: min=20000us (20ms)
138 HZ=250: min=80000us (80ms)
139 HZ=100: min=200000us (200ms)
140 The highest value of kernel and HW latency restrictions is shown and
141 used as the minimum sampling rate.
143 up_threshold: defines what the average CPU usage between the samplings
144 of 'sampling_rate' needs to be for the kernel to make a decision on
145 whether it should increase the frequency. For example when it is set
146 to its default value of '95' it means that between the checking
147 intervals the CPU needs to be on average more than 95% in use to then
148 decide that the CPU frequency needs to be increased.
150 ignore_nice_load: this parameter takes a value of '0' or '1'. When
151 set to '0' (its default), all processes are counted towards the
152 'cpu utilisation' value. When set to '1', the processes that are
153 run with a 'nice' value will not count (and thus be ignored) in the
154 overall usage calculation. This is useful if you are running a CPU
155 intensive calculation on your laptop that you do not care how long it
156 takes to complete as you can 'nice' it and prevent it from taking part
157 in the deciding process of whether to increase your CPU frequency.
159 sampling_down_factor: this parameter controls the rate at which the
160 kernel makes a decision on when to decrease the frequency while running
161 at top speed. When set to 1 (the default) decisions to reevaluate load
162 are made at the same interval regardless of current clock speed. But
163 when set to greater than 1 (e.g. 100) it acts as a multiplier for the
164 scheduling interval for reevaluating load when the CPU is at its top
165 speed due to high load. This improves performance by reducing the overhead
166 of load evaluation and helping the CPU stay at its top speed when truly
167 busy, rather than shifting back and forth in speed. This tunable has no
168 effect on behavior at lower speeds/lower CPU loads.
171 2.5 Conservative
172 ----------------
174 The CPUfreq governor "conservative", much like the "ondemand"
175 governor, sets the CPU depending on the current usage. It differs in
176 behaviour in that it gracefully increases and decreases the CPU speed
177 rather than jumping to max speed the moment there is any load on the
178 CPU. This behaviour more suitable in a battery powered environment.
179 The governor is tweaked in the same manner as the "ondemand" governor
180 through sysfs with the addition of:
182 freq_step: this describes what percentage steps the cpu freq should be
183 increased and decreased smoothly by. By default the cpu frequency will
184 increase in 5% chunks of your maximum cpu frequency. You can change this
185 value to anywhere between 0 and 100 where '0' will effectively lock your
186 CPU at a speed regardless of its load whilst '100' will, in theory, make
187 it behave identically to the "ondemand" governor.
189 down_threshold: same as the 'up_threshold' found for the "ondemand"
190 governor but for the opposite direction. For example when set to its
191 default value of '20' it means that if the CPU usage needs to be below
192 20% between samples to have the frequency decreased.
194 3. The Governor Interface in the CPUfreq Core
195 =============================================
197 A new governor must register itself with the CPUfreq core using
198 "cpufreq_register_governor". The struct cpufreq_governor, which has to
199 be passed to that function, must contain the following values:
201 governor->name - A unique name for this governor
202 governor->governor - The governor callback function
203 governor->owner - .THIS_MODULE for the governor module (if
204 appropriate)
206 The governor->governor callback is called with the current (or to-be-set)
207 cpufreq_policy struct for that CPU, and an unsigned int event. The
208 following events are currently defined:
210 CPUFREQ_GOV_START: This governor shall start its duty for the CPU
211 policy->cpu
212 CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
213 policy->cpu
214 CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
215 policy->min and policy->max.
217 If you need other "events" externally of your driver, _only_ use the
218 cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
219 CPUfreq core to ensure proper locking.
222 The CPUfreq governor may call the CPU processor driver using one of
223 these two functions:
225 int cpufreq_driver_target(struct cpufreq_policy *policy,
226 unsigned int target_freq,
227 unsigned int relation);
229 int __cpufreq_driver_target(struct cpufreq_policy *policy,
230 unsigned int target_freq,
231 unsigned int relation);
233 target_freq must be within policy->min and policy->max, of course.
234 What's the difference between these two functions? When your governor
235 still is in a direct code path of a call to governor->governor, the
236 per-CPU cpufreq lock is still held in the cpufreq core, and there's
237 no need to lock it again (in fact, this would cause a deadlock). So
238 use __cpufreq_driver_target only in these cases. In all other cases
239 (for example, when there's a "daemonized" function that wakes up
240 every second), use cpufreq_driver_target to lock the cpufreq per-CPU
241 lock before the command is passed to the cpufreq processor driver.