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- CPU frequency and voltage scaling code in the Linux(TM) kernel
-
-
- L i n u x C P U F r e q
-
- C P U F r e q G o v e r n o r s
-
- - information for users and developers -
-
-
- Dominik Brodowski <linux@brodo.de>
- some additions and corrections by Nico Golde <nico@ngolde.de>
-
-
-
- Clock scaling allows you to change the clock speed of the CPUs on the
- fly. This is a nice method to save battery power, because the lower
- the clock speed, the less power the CPU consumes.
-
-
-Contents:
----------
-1. What is a CPUFreq Governor?
-
-2. Governors In the Linux Kernel
-2.1 Performance
-2.2 Powersave
-2.3 Userspace
-2.4 Ondemand
-2.5 Conservative
-
-3. The Governor Interface in the CPUfreq Core
-
-
-
-1. What Is A CPUFreq Governor?
-==============================
-
-Most cpufreq drivers (in fact, all except one, longrun) or even most
-cpu frequency scaling algorithms only offer the CPU to be set to one
-frequency. In order to offer dynamic frequency scaling, the cpufreq
-core must be able to tell these drivers of a "target frequency". So
-these specific drivers will be transformed to offer a "->target"
-call instead of the existing "->setpolicy" call. For "longrun", all
-stays the same, though.
-
-How to decide what frequency within the CPUfreq policy should be used?
-That's done using "cpufreq governors". Two are already in this patch
--- they're the already existing "powersave" and "performance" which
-set the frequency statically to the lowest or highest frequency,
-respectively. At least two more such governors will be ready for
-addition in the near future, but likely many more as there are various
-different theories and models about dynamic frequency scaling
-around. Using such a generic interface as cpufreq offers to scaling
-governors, these can be tested extensively, and the best one can be
-selected for each specific use.
-
-Basically, it's the following flow graph:
-
-CPU can be set to switch independently | CPU can only be set
- within specific "limits" | to specific frequencies
-
- "CPUfreq policy"
- consists of frequency limits (policy->{min,max})
- and CPUfreq governor to be used
- / \
- / \
- / the cpufreq governor decides
- / (dynamically or statically)
- / what target_freq to set within
- / the limits of policy->{min,max}
- / \
- / \
- Using the ->setpolicy call, Using the ->target call,
- the limits and the the frequency closest
- "policy" is set. to target_freq is set.
- It is assured that it
- is within policy->{min,max}
-
-
-2. Governors In the Linux Kernel
-================================
-
-2.1 Performance
----------------
-
-The CPUfreq governor "performance" sets the CPU statically to the
-highest frequency within the borders of scaling_min_freq and
-scaling_max_freq.
-
-
-2.2 Powersave
--------------
-
-The CPUfreq governor "powersave" sets the CPU statically to the
-lowest frequency within the borders of scaling_min_freq and
-scaling_max_freq.
-
-
-2.3 Userspace
--------------
-
-The CPUfreq governor "userspace" allows the user, or any userspace
-program running with UID "root", to set the CPU to a specific frequency
-by making a sysfs file "scaling_setspeed" available in the CPU-device
-directory.
-
-
-2.4 Ondemand
-------------
-
-The CPUfreq governor "ondemand" sets the CPU depending on the
-current usage. To do this the CPU must have the capability to
-switch the frequency very quickly. There are a number of sysfs file
-accessible parameters:
-
-sampling_rate: measured in uS (10^-6 seconds), this is how often you
-want the kernel to look at the CPU usage and to make decisions on
-what to do about the frequency. Typically this is set to values of
-around '10000' or more. It's default value is (cmp. with users-guide.txt):
-transition_latency * 1000
-Be aware that transition latency is in ns and sampling_rate is in us, so you
-get the same sysfs value by default.
-Sampling rate should always get adjusted considering the transition latency
-To set the sampling rate 750 times as high as the transition latency
-in the bash (as said, 1000 is default), do:
-echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
- >ondemand/sampling_rate
-
-sampling_rate_min:
-The sampling rate is limited by the HW transition latency:
-transition_latency * 100
-Or by kernel restrictions:
-If CONFIG_NO_HZ is set, the limit is 10ms fixed.
-If CONFIG_NO_HZ is not set or nohz=off boot parameter is used, the
-limits depend on the CONFIG_HZ option:
-HZ=1000: min=20000us (20ms)
-HZ=250: min=80000us (80ms)
-HZ=100: min=200000us (200ms)
-The highest value of kernel and HW latency restrictions is shown and
-used as the minimum sampling rate.
-
-up_threshold: defines what the average CPU usage between the samplings
-of 'sampling_rate' needs to be for the kernel to make a decision on
-whether it should increase the frequency. For example when it is set
-to its default value of '95' it means that between the checking
-intervals the CPU needs to be on average more than 95% in use to then
-decide that the CPU frequency needs to be increased.
-
-ignore_nice_load: this parameter takes a value of '0' or '1'. When
-set to '0' (its default), all processes are counted towards the
-'cpu utilisation' value. When set to '1', the processes that are
-run with a 'nice' value will not count (and thus be ignored) in the
-overall usage calculation. This is useful if you are running a CPU
-intensive calculation on your laptop that you do not care how long it
-takes to complete as you can 'nice' it and prevent it from taking part
-in the deciding process of whether to increase your CPU frequency.
-
-sampling_down_factor: this parameter controls the rate at which the
-kernel makes a decision on when to decrease the frequency while running
-at top speed. When set to 1 (the default) decisions to reevaluate load
-are made at the same interval regardless of current clock speed. But
-when set to greater than 1 (e.g. 100) it acts as a multiplier for the
-scheduling interval for reevaluating load when the CPU is at its top
-speed due to high load. This improves performance by reducing the overhead
-of load evaluation and helping the CPU stay at its top speed when truly
-busy, rather than shifting back and forth in speed. This tunable has no
-effect on behavior at lower speeds/lower CPU loads.
-
-
-2.5 Conservative
-----------------
-
-The CPUfreq governor "conservative", much like the "ondemand"
-governor, sets the CPU depending on the current usage. It differs in
-behaviour in that it gracefully increases and decreases the CPU speed
-rather than jumping to max speed the moment there is any load on the
-CPU. This behaviour more suitable in a battery powered environment.
-The governor is tweaked in the same manner as the "ondemand" governor
-through sysfs with the addition of:
-
-freq_step: this describes what percentage steps the cpu freq should be
-increased and decreased smoothly by. By default the cpu frequency will
-increase in 5% chunks of your maximum cpu frequency. You can change this
-value to anywhere between 0 and 100 where '0' will effectively lock your
-CPU at a speed regardless of its load whilst '100' will, in theory, make
-it behave identically to the "ondemand" governor.
-
-down_threshold: same as the 'up_threshold' found for the "ondemand"
-governor but for the opposite direction. For example when set to its
-default value of '20' it means that if the CPU usage needs to be below
-20% between samples to have the frequency decreased.
-
-3. The Governor Interface in the CPUfreq Core
-=============================================
-
-A new governor must register itself with the CPUfreq core using
-"cpufreq_register_governor". The struct cpufreq_governor, which has to
-be passed to that function, must contain the following values:
-
-governor->name - A unique name for this governor
-governor->governor - The governor callback function
-governor->owner - .THIS_MODULE for the governor module (if
- appropriate)
-
-The governor->governor callback is called with the current (or to-be-set)
-cpufreq_policy struct for that CPU, and an unsigned int event. The
-following events are currently defined:
-
-CPUFREQ_GOV_START: This governor shall start its duty for the CPU
- policy->cpu
-CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
- policy->cpu
-CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
- policy->min and policy->max.
-
-If you need other "events" externally of your driver, _only_ use the
-cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
-CPUfreq core to ensure proper locking.
-
-
-The CPUfreq governor may call the CPU processor driver using one of
-these two functions:
-
-int cpufreq_driver_target(struct cpufreq_policy *policy,
- unsigned int target_freq,
- unsigned int relation);
-
-int __cpufreq_driver_target(struct cpufreq_policy *policy,
- unsigned int target_freq,
- unsigned int relation);
-
-target_freq must be within policy->min and policy->max, of course.
-What's the difference between these two functions? When your governor
-still is in a direct code path of a call to governor->governor, the
-per-CPU cpufreq lock is still held in the cpufreq core, and there's
-no need to lock it again (in fact, this would cause a deadlock). So
-use __cpufreq_driver_target only in these cases. In all other cases
-(for example, when there's a "daemonized" function that wakes up
-every second), use cpufreq_driver_target to lock the cpufreq per-CPU
-lock before the command is passed to the cpufreq processor driver.
-