Dominik Brodowski <linux@brodo.de>
some additions and corrections by Nico Golde <nico@ngolde.de>
+ Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+ Viresh Kumar <viresh.kumar@linaro.org>
2.3 Userspace
2.4 Ondemand
2.5 Conservative
+2.6 Schedutil
3. The Governor Interface in the CPUfreq Core
+4. References
1. What Is A CPUFreq Governor?
==============================
Most cpufreq drivers (except the intel_pstate and 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/target_index"
-call instead of the existing "->setpolicy" call. For "longrun", all
-stays the same, though.
+cpu frequency scaling algorithms only allow the CPU frequency to be set
+to predefined fixed values. 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/target_index/fast_switch()" call instead of the
+"->setpolicy()" call. For set_policy drivers, 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.
+That's done using "cpufreq governors".
Basically, it's the following flow graph:
/ the limits of policy->{min,max}
/ \
/ \
- Using the ->setpolicy call, Using the ->target/target_index call,
+ Using the ->setpolicy call, Using the ->target/target_index/fast_switch call,
the limits and the the frequency closest
"policy" is set. to target_freq is set.
It is assured that it
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_COMMON is set, the limit is 10ms fixed.
-If CONFIG_NO_HZ_COMMON 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.
-
-powersave_bias: this parameter takes a value between 0 to 1000. It
-defines the percentage (times 10) value of the target frequency that
-will be shaved off of the target. For example, when set to 100 -- 10%,
-when ondemand governor would have targeted 1000 MHz, it will target
-1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
-(disabled) by default.
-When AMD frequency sensitivity powersave bias driver --
-drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
-defines the workload frequency sensitivity threshold in which a lower
-frequency is chosen instead of ondemand governor's original target.
-The frequency sensitivity is a hardware reported (on AMD Family 16h
-Processors and above) value between 0 to 100% that tells software how
-the performance of the workload running on a CPU will change when
-frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
-will not perform any better on higher core frequency, whereas a
-workload with sensitivity of 100% (CPU-bound) will perform better
-higher the frequency. When the driver is loaded, this is set to 400
-by default -- for CPUs running workloads with sensitivity value below
-40%, a lower frequency is chosen. Unloading the driver or writing 0
-will disable this feature.
+The CPUfreq governor "ondemand" sets the CPU frequency depending on the
+current system load. Load estimation is triggered by the scheduler
+through the update_util_data->func hook; when triggered, cpufreq checks
+the CPU-usage statistics over the last period and the governor sets the
+CPU accordingly. The CPU must have the capability to switch the
+frequency very quickly.
+
+Sysfs files:
+
+* 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_COMMON is set, the limit is 10ms fixed.
+ - If CONFIG_NO_HZ_COMMON 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:
+
+ This 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.
+
+* powersave_bias:
+
+ This parameter takes a value between 0 to 1000. It defines the
+ percentage (times 10) value of the target frequency that will be
+ shaved off of the target. For example, when set to 100 -- 10%, when
+ ondemand governor would have targeted 1000 MHz, it will target
+ 1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
+ (disabled) by default.
+
+ When AMD frequency sensitivity powersave bias driver --
+ drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
+ defines the workload frequency sensitivity threshold in which a lower
+ frequency is chosen instead of ondemand governor's original target.
+ The frequency sensitivity is a hardware reported (on AMD Family 16h
+ Processors and above) value between 0 to 100% that tells software how
+ the performance of the workload running on a CPU will change when
+ frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
+ will not perform any better on higher core frequency, whereas a
+ workload with sensitivity of 100% (CPU-bound) will perform better
+ higher the frequency. When the driver is loaded, this is set to 400 by
+ default -- for CPUs running workloads with sensitivity value below
+ 40%, a lower frequency is chosen. Unloading the driver or writing 0
+ will disable this feature.
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.
-
-sampling_down_factor: similar functionality as in "ondemand" governor.
-But in "conservative", it controls the rate at which the kernel makes
-a decision on when to decrease the frequency while running in any
-speed. Load for frequency increase is still evaluated every
-sampling rate.
+governor, sets the CPU frequency 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 is 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.
+
+* sampling_down_factor:
+
+ Similar functionality as in "ondemand" governor. But in
+ "conservative", it controls the rate at which the kernel makes a
+ decision on when to decrease the frequency while running in any speed.
+ Load for frequency increase is still evaluated every sampling rate.
+
+
+2.6 Schedutil
+-------------
+
+The "schedutil" governor aims at better integration with the Linux
+kernel scheduler. Load estimation is achieved through the scheduler's
+Per-Entity Load Tracking (PELT) mechanism, which also provides
+information about the recent load [1]. This governor currently does
+load based DVFS only for tasks managed by CFS. RT and DL scheduler tasks
+are always run at the highest frequency. Unlike all the other
+governors, the code is located under the kernel/sched/ directory.
+
+Sysfs files:
+
+* rate_limit_us:
+
+ This contains a value in microseconds. The governor waits for
+ rate_limit_us time before reevaluating the load again, after it has
+ evaluated the load once.
+
+For an in-depth comparison with the other governors refer to [2].
+
3. The Governor Interface in the CPUfreq Core
=============================================
"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.
+governor->name - A unique name for this governor.
+governor->owner - .THIS_MODULE for the governor module (if appropriate).
+plus a set of hooks to the functions implementing the governor's logic.
The CPUfreq governor may call the CPU processor driver using one of
these two functions:
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.
+What's the difference between these two functions? When your governor is
+in a direct code path of a call to governor callbacks, like
+governor->start(), the policy->rwsem 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 take policy->rwsem
+before the command is passed to the cpufreq driver.
+
+4. References
+=============
+
+[1] Per-entity load tracking: https://lwn.net/Articles/531853/
+[2] Improvements in CPU frequency management: https://lwn.net/Articles/682391/
projects / linux.git / blobdiff