Energy Model of devices

1. Overview

The Energy Model (EM) framework serves as an interface between drivers knowing the power consumed by devices at various performance levels, and the kernel subsystems willing to use that information to make energy-aware decisions.

The source of the information about the power consumed by devices can vary greatly from one platform to another. These power costs can be estimated using devicetree data in some cases. In others, the firmware will know better. Alternatively, userspace might be best positioned. And so on. In order to avoid each and every client subsystem to re-implement support for each and every possible source of information on its own, the EM framework intervenes as an abstraction layer which standardizes the format of power cost tables in the kernel, hence enabling to avoid redundant work.

The power values might be expressed in milli-Watts or in an ‘abstract scale’. Multiple subsystems might use the EM and it is up to the system integrator to check that the requirements for the power value scale types are met. An example can be found in the Energy-Aware Scheduler documentation Energy Aware Scheduling. For some subsystems like thermal or powercap power values expressed in an ‘abstract scale’ might cause issues. These subsystems are more interested in estimation of power used in the past, thus the real milli-Watts might be needed. An example of these requirements can be found in the Intelligent Power Allocation in Power allocator governor tunables. Kernel subsystems might implement automatic detection to check whether EM registered devices have inconsistent scale (based on EM internal flag). Important thing to keep in mind is that when the power values are expressed in an ‘abstract scale’ deriving real energy in milli-Joules would not be possible.

The figure below depicts an example of drivers (Arm-specific here, but the approach is applicable to any architecture) providing power costs to the EM framework, and interested clients reading the data from it:

+---------------+  +-----------------+  +---------------+
| Thermal (IPA) |  | Scheduler (EAS) |  |     Other     |
+---------------+  +-----------------+  +---------------+
        |                   | em_cpu_energy()   |
        |                   | em_cpu_get()      |
        +---------+         |         +---------+
                  |         |         |
                  v         v         v
                 +---------------------+
                 |    Energy Model     |
                 |     Framework       |
                 +---------------------+
                    ^       ^       ^
                    |       |       | em_dev_register_perf_domain()
         +----------+       |       +---------+
         |                  |                 |
 +---------------+  +---------------+  +--------------+
 |  cpufreq-dt   |  |   arm_scmi    |  |    Other     |
 +---------------+  +---------------+  +--------------+
         ^                  ^                 ^
         |                  |                 |
 +--------------+   +---------------+  +--------------+
 | Device Tree  |   |   Firmware    |  |      ?       |
 +--------------+   +---------------+  +--------------+

In case of CPU devices the EM framework manages power cost tables per ‘performance domain’ in the system. A performance domain is a group of CPUs whose performance is scaled together. Performance domains generally have a 1-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are required to have the same micro-architecture. CPUs in different performance domains can have different micro-architectures.

2. Core APIs

2.1 Config options

CONFIG_ENERGY_MODEL must be enabled to use the EM framework.

2.2 Registration of performance domains

Registration of ‘advanced’ EM

The ‘advanced’ EM gets it’s name due to the fact that the driver is allowed to provide more precised power model. It’s not limited to some implemented math formula in the framework (like it’s in ‘simple’ EM case). It can better reflect the real power measurements performed for each performance state. Thus, this registration method should be preferred in case considering EM static power (leakage) is important.

Drivers are expected to register performance domains into the EM framework by calling the following API:

int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
              struct em_data_callback *cb, cpumask_t *cpus, bool milliwatts);

Drivers must provide a callback function returning <frequency, power> tuples for each performance state. The callback function provided by the driver is free to fetch data from any relevant location (DT, firmware, …), and by any mean deemed necessary. Only for CPU devices, drivers must specify the CPUs of the performance domains using cpumask. For other devices than CPUs the last argument must be set to NULL. The last argument ‘milliwatts’ is important to set with correct value. Kernel subsystems which use EM might rely on this flag to check if all EM devices use the same scale. If there are different scales, these subsystems might decide to: return warning/error, stop working or panic. See Section 3. for an example of driver implementing this callback, or Section 2.4 for further documentation on this API

Registration of EM using DT

The EM can also be registered using OPP framework and information in DT “operating-points-v2”. Each OPP entry in DT can be extended with a property “opp-microwatt” containing micro-Watts power value. This OPP DT property allows a platform to register EM power values which are reflecting total power (static + dynamic). These power values might be coming directly from experiments and measurements.

Registration of ‘artificial’ EM

There is an option to provide a custom callback for drivers missing detailed knowledge about power value for each performance state. The callback .get_cost() is optional and provides the ‘cost’ values used by the EAS. This is useful for platforms that only provide information on relative efficiency between CPU types, where one could use the information to create an abstract power model. But even an abstract power model can sometimes be hard to fit in, given the input power value size restrictions. The .get_cost() allows to provide the ‘cost’ values which reflect the efficiency of the CPUs. This would allow to provide EAS information which has different relation than what would be forced by the EM internal formulas calculating ‘cost’ values. To register an EM for such platform, the driver must set the flag ‘milliwatts’ to 0, provide .get_power() callback and provide .get_cost() callback. The EM framework would handle such platform properly during registration. A flag EM_PERF_DOMAIN_ARTIFICIAL is set for such platform. Special care should be taken by other frameworks which are using EM to test and treat this flag properly.

Registration of ‘simple’ EM

The ‘simple’ EM is registered using the framework helper function cpufreq_register_em_with_opp(). It implements a power model which is tight to math formula:

Power = C * V^2 * f

The EM which is registered using this method might not reflect correctly the physics of a real device, e.g. when static power (leakage) is important.

2.3 Accessing performance domains

There are two API functions which provide the access to the energy model: em_cpu_get() which takes CPU id as an argument and em_pd_get() with device pointer as an argument. It depends on the subsystem which interface it is going to use, but in case of CPU devices both functions return the same performance domain.

Subsystems interested in the energy model of a CPU can retrieve it using the em_cpu_get() API. The energy model tables are allocated once upon creation of the performance domains, and kept in memory untouched.

The energy consumed by a performance domain can be estimated using the em_cpu_energy() API. The estimation is performed assuming that the schedutil CPUfreq governor is in use in case of CPU device. Currently this calculation is not provided for other type of devices.

More details about the above APIs can be found in <linux/energy_model.h> or in Section 2.4

2.4 Description details of this API

struct em_perf_state

Performance state of a performance domain

Definition

struct em_perf_state {
  unsigned long frequency;
  unsigned long power;
  unsigned long cost;
  unsigned long flags;
};

Members

frequency
The frequency in KHz, for consistency with CPUFreq
power
The power consumed at this level (by 1 CPU or by a registered device). It can be a total power: static and dynamic.
cost
The cost coefficient associated with this level, used during energy calculation. Equal to: power * max_frequency / frequency
flags
see “em_perf_state flags” description below.
struct em_perf_domain

Performance domain

Definition

struct em_perf_domain {
  struct em_perf_state *table;
  int nr_perf_states;
  unsigned long flags;
  unsigned long cpus[];
};

Members

table
List of performance states, in ascending order
nr_perf_states
Number of performance states
flags
See “em_perf_domain flags”
cpus
Cpumask covering the CPUs of the domain. It’s here for performance reasons to avoid potential cache misses during energy calculations in the scheduler and simplifies allocating/freeing that memory region.

Description

In case of CPU device, a “performance domain” represents a group of CPUs whose performance is scaled together. All CPUs of a performance domain must have the same micro-architecture. Performance domains often have a 1-to-1 mapping with CPUFreq policies. In case of other devices the cpus field is unused.

struct em_perf_state * em_pd_get_efficient_state(struct em_perf_domain *pd, unsigned long freq)

Get an efficient performance state from the EM

Parameters

struct em_perf_domain *pd
Performance domain for which we want an efficient frequency
unsigned long freq
Frequency to map with the EM

Description

It is called from the scheduler code quite frequently and as a consequence doesn’t implement any check.

Return

An efficient performance state, high enough to meet freq requirement.

unsigned long em_cpu_energy(struct em_perf_domain *pd, unsigned long max_util, unsigned long sum_util, unsigned long allowed_cpu_cap)

Estimates the energy consumed by the CPUs of a performance domain

Parameters

struct em_perf_domain *pd
performance domain for which energy has to be estimated
unsigned long max_util
highest utilization among CPUs of the domain
unsigned long sum_util
sum of the utilization of all CPUs in the domain
unsigned long allowed_cpu_cap
maximum allowed CPU capacity for the pd, which might reflect reduced frequency (due to thermal)

Description

This function must be used only for CPU devices. There is no validation, i.e. if the EM is a CPU type and has cpumask allocated. It is called from the scheduler code quite frequently and that is why there is not checks.

Return

the sum of the energy consumed by the CPUs of the domain assuming a capacity state satisfying the max utilization of the domain.

int em_pd_nr_perf_states(struct em_perf_domain *pd)

Get the number of performance states of a perf. domain

Parameters

struct em_perf_domain *pd
performance domain for which this must be done

Return

the number of performance states in the performance domain table

struct em_perf_domain * em_pd_get(struct device *dev)

Return the performance domain for a device

Parameters

struct device *dev
Device to find the performance domain for

Description

Returns the performance domain to which dev belongs, or NULL if it doesn’t exist.

struct em_perf_domain * em_cpu_get(int cpu)

Return the performance domain for a CPU

Parameters

int cpu
CPU to find the performance domain for

Description

Returns the performance domain to which cpu belongs, or NULL if it doesn’t exist.

int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states, struct em_data_callback *cb, cpumask_t *cpus, bool milliwatts)

Register the Energy Model (EM) for a device

Parameters

struct device *dev
Device for which the EM is to register
unsigned int nr_states
Number of performance states to register
struct em_data_callback *cb
Callback functions providing the data of the Energy Model
cpumask_t *cpus
Pointer to cpumask_t, which in case of a CPU device is obligatory. It can be taken from i.e. ‘policy->cpus’. For other type of devices this should be set to NULL.
bool milliwatts
Flag indicating that the power values are in milliWatts or in some other scale. It must be set properly.

Description

Create Energy Model tables for a performance domain using the callbacks defined in cb.

The milliwatts is important to set with correct value. Some kernel sub-systems might rely on this flag and check if all devices in the EM are using the same scale.

If multiple clients register the same performance domain, all but the first registration will be ignored.

Return 0 on success

void em_dev_unregister_perf_domain(struct device *dev)

Unregister Energy Model (EM) for a device

Parameters

struct device *dev
Device for which the EM is registered

Description

Unregister the EM for the specified dev (but not a CPU device).

3. Example driver

The CPUFreq framework supports dedicated callback for registering the EM for a given CPU(s) ‘policy’ object: cpufreq_driver::register_em(). That callback has to be implemented properly for a given driver, because the framework would call it at the right time during setup. This section provides a simple example of a CPUFreq driver registering a performance domain in the Energy Model framework using the (fake) ‘foo’ protocol. The driver implements an est_power() function to be provided to the EM framework:

-> drivers/cpufreq/foo_cpufreq.c

01    static int est_power(struct device *dev, unsigned long *mW,
02                    unsigned long *KHz)
03    {
04            long freq, power;
05
06            /* Use the 'foo' protocol to ceil the frequency */
07            freq = foo_get_freq_ceil(dev, *KHz);
08            if (freq < 0);
09                    return freq;
10
11            /* Estimate the power cost for the dev at the relevant freq. */
12            power = foo_estimate_power(dev, freq);
13            if (power < 0);
14                    return power;
15
16            /* Return the values to the EM framework */
17            *mW = power;
18            *KHz = freq;
19
20            return 0;
21    }
22
23    static void foo_cpufreq_register_em(struct cpufreq_policy *policy)
24    {
25            struct em_data_callback em_cb = EM_DATA_CB(est_power);
26            struct device *cpu_dev;
27            int nr_opp;
28
29            cpu_dev = get_cpu_device(cpumask_first(policy->cpus));
30
31            /* Find the number of OPPs for this policy */
32            nr_opp = foo_get_nr_opp(policy);
33
34            /* And register the new performance domain */
35            em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus,
36                                        true);
37    }
38
39    static struct cpufreq_driver foo_cpufreq_driver = {
40            .register_em = foo_cpufreq_register_em,
41    };