148 lines
5.6 KiB
ReStructuredText
148 lines
5.6 KiB
ReStructuredText
====================
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Energy Model of CPUs
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====================
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1. Overview
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-----------
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The Energy Model (EM) framework serves as an interface between drivers knowing
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the power consumed by CPUs at various performance levels, and the kernel
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subsystems willing to use that information to make energy-aware decisions.
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The source of the information about the power consumed by CPUs can vary greatly
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from one platform to another. These power costs can be estimated using
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devicetree data in some cases. In others, the firmware will know better.
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Alternatively, userspace might be best positioned. And so on. In order to avoid
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each and every client subsystem to re-implement support for each and every
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possible source of information on its own, the EM framework intervenes as an
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abstraction layer which standardizes the format of power cost tables in the
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kernel, hence enabling to avoid redundant work.
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The figure below depicts an example of drivers (Arm-specific here, but the
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approach is applicable to any architecture) providing power costs to the EM
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framework, and interested clients reading the data from it::
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+---------------+ +-----------------+ +---------------+
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| Thermal (IPA) | | Scheduler (EAS) | | Other |
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+---------------+ +-----------------+ +---------------+
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| | em_pd_energy() |
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| | em_cpu_get() |
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+---------+ | +---------+
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| | |
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v v v
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+---------------------+
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| Energy Model |
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| Framework |
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+---------------------+
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^ ^ ^
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| | | em_register_perf_domain()
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+----------+ | +---------+
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| | |
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+---------------+ +---------------+ +--------------+
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| cpufreq-dt | | arm_scmi | | Other |
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+---------------+ +---------------+ +--------------+
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^ ^ ^
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| | |
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+--------------+ +---------------+ +--------------+
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| Device Tree | | Firmware | | ? |
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+--------------+ +---------------+ +--------------+
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The EM framework manages power cost tables per 'performance domain' in the
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system. A performance domain is a group of CPUs whose performance is scaled
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together. Performance domains generally have a 1-to-1 mapping with CPUFreq
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policies. All CPUs in a performance domain are required to have the same
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micro-architecture. CPUs in different performance domains can have different
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micro-architectures.
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2. Core APIs
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------------
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2.1 Config options
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^^^^^^^^^^^^^^^^^^
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CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
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2.2 Registration of performance domains
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Drivers are expected to register performance domains into the EM framework by
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calling the following API::
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int em_register_perf_domain(cpumask_t *span, unsigned int nr_states,
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struct em_data_callback *cb);
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Drivers must specify the CPUs of the performance domains using the cpumask
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argument, and provide a callback function returning <frequency, power> tuples
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for each capacity state. The callback function provided by the driver is free
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to fetch data from any relevant location (DT, firmware, ...), and by any mean
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deemed necessary. See Section 3. for an example of driver implementing this
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callback, and kernel/power/energy_model.c for further documentation on this
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API.
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2.3 Accessing performance domains
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Subsystems interested in the energy model of a CPU can retrieve it using the
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em_cpu_get() API. The energy model tables are allocated once upon creation of
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the performance domains, and kept in memory untouched.
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The energy consumed by a performance domain can be estimated using the
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em_pd_energy() API. The estimation is performed assuming that the schedutil
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CPUfreq governor is in use.
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More details about the above APIs can be found in include/linux/energy_model.h.
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3. Example driver
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-----------------
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This section provides a simple example of a CPUFreq driver registering a
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performance domain in the Energy Model framework using the (fake) 'foo'
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protocol. The driver implements an est_power() function to be provided to the
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EM framework::
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-> drivers/cpufreq/foo_cpufreq.c
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01 static int est_power(unsigned long *mW, unsigned long *KHz, int cpu)
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02 {
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03 long freq, power;
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04
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05 /* Use the 'foo' protocol to ceil the frequency */
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06 freq = foo_get_freq_ceil(cpu, *KHz);
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07 if (freq < 0);
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08 return freq;
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09
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10 /* Estimate the power cost for the CPU at the relevant freq. */
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11 power = foo_estimate_power(cpu, freq);
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12 if (power < 0);
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13 return power;
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14
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15 /* Return the values to the EM framework */
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16 *mW = power;
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17 *KHz = freq;
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18
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19 return 0;
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20 }
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21
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22 static int foo_cpufreq_init(struct cpufreq_policy *policy)
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23 {
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24 struct em_data_callback em_cb = EM_DATA_CB(est_power);
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25 int nr_opp, ret;
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26
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27 /* Do the actual CPUFreq init work ... */
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28 ret = do_foo_cpufreq_init(policy);
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29 if (ret)
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30 return ret;
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31
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32 /* Find the number of OPPs for this policy */
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33 nr_opp = foo_get_nr_opp(policy);
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34
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35 /* And register the new performance domain */
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36 em_register_perf_domain(policy->cpus, nr_opp, &em_cb);
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37
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38 return 0;
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39 }
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