blob: 77ec21574fb1873c37dfdd9791c2e82a97198de7 [file] [log] [blame]
Linus Torvalds1da177e2005-04-16 15:20:36 -07001 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>
Nico Golde594dd2c2005-06-25 14:58:33 -070012 some additions and corrections by Nico Golde <nico@ngolde.de>
Linus Torvalds1da177e2005-04-16 15:20:36 -070013
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
21Contents:
22---------
231. What is a CPUFreq Governor?
24
252. Governors In the Linux Kernel
262.1 Performance
272.2 Powersave
282.3 Userspace
Nico Golde594dd2c2005-06-25 14:58:33 -0700292.4 Ondemand
Alexander Clouter537208c2005-12-01 01:09:23 -0800302.5 Conservative
Linus Torvalds1da177e2005-04-16 15:20:36 -070031
323. The Governor Interface in the CPUfreq Core
33
34
35
361. What Is A CPUFreq Governor?
37==============================
38
39Most cpufreq drivers (in fact, all except one, longrun) or even most
40cpu frequency scaling algorithms only offer the CPU to be set to one
41frequency. In order to offer dynamic frequency scaling, the cpufreq
42core must be able to tell these drivers of a "target frequency". So
Viresh Kumar9c0ebcf2013-10-25 19:45:48 +053043these specific drivers will be transformed to offer a "->target/target_index"
Linus Torvalds1da177e2005-04-16 15:20:36 -070044call instead of the existing "->setpolicy" call. For "longrun", all
45stays the same, though.
46
47How to decide what frequency within the CPUfreq policy should be used?
48That's done using "cpufreq governors". Two are already in this patch
49-- they're the already existing "powersave" and "performance" which
50set the frequency statically to the lowest or highest frequency,
51respectively. At least two more such governors will be ready for
52addition in the near future, but likely many more as there are various
53different theories and models about dynamic frequency scaling
54around. Using such a generic interface as cpufreq offers to scaling
55governors, these can be tested extensively, and the best one can be
56selected for each specific use.
57
58Basically, it's the following flow graph:
59
Matt LaPlante2fe0ae72006-10-03 22:50:39 +020060CPU can be set to switch independently | CPU can only be set
Linus Torvalds1da177e2005-04-16 15:20:36 -070061 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 / \
Viresh Kumar9c0ebcf2013-10-25 19:45:48 +053074 Using the ->setpolicy call, Using the ->target/target_index call,
Linus Torvalds1da177e2005-04-16 15:20:36 -070075 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
812. Governors In the Linux Kernel
82================================
83
842.1 Performance
85---------------
86
87The CPUfreq governor "performance" sets the CPU statically to the
88highest frequency within the borders of scaling_min_freq and
89scaling_max_freq.
90
91
Nico Golde594dd2c2005-06-25 14:58:33 -0700922.2 Powersave
Linus Torvalds1da177e2005-04-16 15:20:36 -070093-------------
94
95The CPUfreq governor "powersave" sets the CPU statically to the
96lowest frequency within the borders of scaling_min_freq and
97scaling_max_freq.
98
99
Nico Golde594dd2c2005-06-25 14:58:33 -07001002.3 Userspace
Linus Torvalds1da177e2005-04-16 15:20:36 -0700101-------------
102
103The CPUfreq governor "userspace" allows the user, or any userspace
104program running with UID "root", to set the CPU to a specific frequency
105by making a sysfs file "scaling_setspeed" available in the CPU-device
106directory.
107
108
Nico Golde594dd2c2005-06-25 14:58:33 -07001092.4 Ondemand
110------------
111
Matt LaPlantea2ffd272006-10-03 22:49:15 +0200112The CPUfreq governor "ondemand" sets the CPU depending on the
Nico Golde594dd2c2005-06-25 14:58:33 -0700113current usage. To do this the CPU must have the capability to
Alexander Clouter537208c2005-12-01 01:09:23 -0800114switch the frequency very quickly. There are a number of sysfs file
115accessible parameters:
116
117sampling_rate: measured in uS (10^-6 seconds), this is how often you
118want the kernel to look at the CPU usage and to make decisions on
119what to do about the frequency. Typically this is set to values of
Thomas Renninger112124a2009-02-04 11:55:12 +0100120around '10000' or more. It's default value is (cmp. with users-guide.txt):
121transition_latency * 1000
Thomas Renninger112124a2009-02-04 11:55:12 +0100122Be aware that transition latency is in ns and sampling_rate is in us, so you
123get the same sysfs value by default.
124Sampling rate should always get adjusted considering the transition latency
125To set the sampling rate 750 times as high as the transition latency
126in the bash (as said, 1000 is default), do:
127echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
128 >ondemand/sampling_rate
Alexander Clouter537208c2005-12-01 01:09:23 -0800129
Paul Bollee7cbb5b2011-11-08 10:16:03 +0100130sampling_rate_min:
Thomas Renninger4f4d1ad2009-04-22 13:48:31 +0200131The sampling rate is limited by the HW transition latency:
132transition_latency * 100
133Or by kernel restrictions:
Frederic Weisbecker3451d022011-08-10 23:21:01 +0200134If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
135If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is used, the
Thomas Renninger4f4d1ad2009-04-22 13:48:31 +0200136limits depend on the CONFIG_HZ option:
137HZ=1000: min=20000us (20ms)
138HZ=250: min=80000us (80ms)
139HZ=100: min=200000us (200ms)
140The highest value of kernel and HW latency restrictions is shown and
141used as the minimum sampling rate.
142
Matt LaPlanted9195882008-07-25 19:45:33 -0700143up_threshold: defines what the average CPU usage between the samplings
Alexander Clouter537208c2005-12-01 01:09:23 -0800144of 'sampling_rate' needs to be for the kernel to make a decision on
145whether it should increase the frequency. For example when it is set
Mike Frysinger292e0042009-12-09 06:56:40 -0500146to its default value of '95' it means that between the checking
147intervals the CPU needs to be on average more than 95% in use to then
Alexander Clouter537208c2005-12-01 01:09:23 -0800148decide that the CPU frequency needs to be increased.
149
Matt LaPlante992caac2006-10-03 22:52:05 +0200150ignore_nice_load: this parameter takes a value of '0' or '1'. When
151set to '0' (its default), all processes are counted towards the
152'cpu utilisation' value. When set to '1', the processes that are
Alexander Clouter537208c2005-12-01 01:09:23 -0800153run with a 'nice' value will not count (and thus be ignored) in the
Matt LaPlante992caac2006-10-03 22:52:05 +0200154overall usage calculation. This is useful if you are running a CPU
Alexander Clouter537208c2005-12-01 01:09:23 -0800155intensive calculation on your laptop that you do not care how long it
156takes to complete as you can 'nice' it and prevent it from taking part
157in the deciding process of whether to increase your CPU frequency.
Nico Golde594dd2c2005-06-25 14:58:33 -0700158
Vishwanath BS5b953642011-01-25 20:12:41 +0530159sampling_down_factor: this parameter controls the rate at which the
160kernel makes a decision on when to decrease the frequency while running
161at top speed. When set to 1 (the default) decisions to reevaluate load
162are made at the same interval regardless of current clock speed. But
163when set to greater than 1 (e.g. 100) it acts as a multiplier for the
164scheduling interval for reevaluating load when the CPU is at its top
165speed due to high load. This improves performance by reducing the overhead
166of load evaluation and helping the CPU stay at its top speed when truly
167busy, rather than shifting back and forth in speed. This tunable has no
168effect on behavior at lower speeds/lower CPU loads.
169
Jacob Shin9c5320c2013-04-04 16:19:04 +0000170powersave_bias: this parameter takes a value between 0 to 1000. It
171defines the percentage (times 10) value of the target frequency that
172will be shaved off of the target. For example, when set to 100 -- 10%,
173when ondemand governor would have targeted 1000 MHz, it will target
1741000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
175(disabled) by default.
176When AMD frequency sensitivity powersave bias driver --
177drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
178defines the workload frequency sensitivity threshold in which a lower
179frequency is chosen instead of ondemand governor's original target.
180The frequency sensitivity is a hardware reported (on AMD Family 16h
181Processors and above) value between 0 to 100% that tells software how
182the performance of the workload running on a CPU will change when
183frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
184will not perform any better on higher core frequency, whereas a
185workload with sensitivity of 100% (CPU-bound) will perform better
186higher the frequency. When the driver is loaded, this is set to 400
187by default -- for CPUs running workloads with sensitivity value below
18840%, a lower frequency is chosen. Unloading the driver or writing 0
189will disable this feature.
190
Nico Golde594dd2c2005-06-25 14:58:33 -0700191
Alexander Clouter537208c2005-12-01 01:09:23 -08001922.5 Conservative
193----------------
194
195The CPUfreq governor "conservative", much like the "ondemand"
196governor, sets the CPU depending on the current usage. It differs in
197behaviour in that it gracefully increases and decreases the CPU speed
198rather than jumping to max speed the moment there is any load on the
199CPU. This behaviour more suitable in a battery powered environment.
200The governor is tweaked in the same manner as the "ondemand" governor
201through sysfs with the addition of:
202
203freq_step: this describes what percentage steps the cpu freq should be
204increased and decreased smoothly by. By default the cpu frequency will
205increase in 5% chunks of your maximum cpu frequency. You can change this
206value to anywhere between 0 and 100 where '0' will effectively lock your
207CPU at a speed regardless of its load whilst '100' will, in theory, make
208it behave identically to the "ondemand" governor.
209
210down_threshold: same as the 'up_threshold' found for the "ondemand"
211governor but for the opposite direction. For example when set to its
212default value of '20' it means that if the CPU usage needs to be below
21320% between samples to have the frequency decreased.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700214
Stratos Karafotis7af1c052013-03-05 22:06:29 +0000215sampling_down_factor: similar functionality as in "ondemand" governor.
216But in "conservative", it controls the rate at which the kernel makes
217a decision on when to decrease the frequency while running in any
218speed. Load for frequency increase is still evaluated every
219sampling rate.
220
Linus Torvalds1da177e2005-04-16 15:20:36 -07002213. The Governor Interface in the CPUfreq Core
222=============================================
223
224A new governor must register itself with the CPUfreq core using
225"cpufreq_register_governor". The struct cpufreq_governor, which has to
226be passed to that function, must contain the following values:
227
228governor->name - A unique name for this governor
229governor->governor - The governor callback function
230governor->owner - .THIS_MODULE for the governor module (if
231 appropriate)
232
233The governor->governor callback is called with the current (or to-be-set)
234cpufreq_policy struct for that CPU, and an unsigned int event. The
235following events are currently defined:
236
237CPUFREQ_GOV_START: This governor shall start its duty for the CPU
238 policy->cpu
239CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
240 policy->cpu
241CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
242 policy->min and policy->max.
243
244If you need other "events" externally of your driver, _only_ use the
245cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
246CPUfreq core to ensure proper locking.
247
248
249The CPUfreq governor may call the CPU processor driver using one of
250these two functions:
251
252int cpufreq_driver_target(struct cpufreq_policy *policy,
253 unsigned int target_freq,
254 unsigned int relation);
255
256int __cpufreq_driver_target(struct cpufreq_policy *policy,
257 unsigned int target_freq,
258 unsigned int relation);
259
260target_freq must be within policy->min and policy->max, of course.
261What's the difference between these two functions? When your governor
262still is in a direct code path of a call to governor->governor, the
263per-CPU cpufreq lock is still held in the cpufreq core, and there's
264no need to lock it again (in fact, this would cause a deadlock). So
265use __cpufreq_driver_target only in these cases. In all other cases
266(for example, when there's a "daemonized" function that wakes up
267every second), use cpufreq_driver_target to lock the cpufreq per-CPU
268lock before the command is passed to the cpufreq processor driver.
269