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![Gustavo A. R. Silva](/assets/img/avatar_default.png)
The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:
struct foo {
int stuff;
struct boo array[];
};
By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.
Also, notice that, dynamic memory allocations won't be affected by
this change:
"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]
This issue was found with the help of Coccinelle.
[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773293
("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
186 lines
5.8 KiB
C
186 lines
5.8 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_ENERGY_MODEL_H
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#define _LINUX_ENERGY_MODEL_H
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#include <linux/cpumask.h>
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#include <linux/jump_label.h>
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#include <linux/kobject.h>
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#include <linux/rcupdate.h>
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#include <linux/sched/cpufreq.h>
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#include <linux/sched/topology.h>
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#include <linux/types.h>
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/**
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* em_cap_state - Capacity state of a performance domain
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* @frequency: The CPU frequency in KHz, for consistency with CPUFreq
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* @power: The power consumed by 1 CPU at this level, in milli-watts
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* @cost: The cost coefficient associated with this level, used during
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* energy calculation. Equal to: power * max_frequency / frequency
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*/
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struct em_cap_state {
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unsigned long frequency;
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unsigned long power;
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unsigned long cost;
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};
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/**
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* em_perf_domain - Performance domain
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* @table: List of capacity states, in ascending order
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* @nr_cap_states: Number of capacity states
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* @cpus: Cpumask covering the CPUs of the domain
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*
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* A "performance domain" represents a group of CPUs whose performance is
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* scaled together. All CPUs of a performance domain must have the same
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* micro-architecture. Performance domains often have a 1-to-1 mapping with
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* CPUFreq policies.
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*/
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struct em_perf_domain {
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struct em_cap_state *table;
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int nr_cap_states;
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unsigned long cpus[];
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};
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#ifdef CONFIG_ENERGY_MODEL
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#define EM_CPU_MAX_POWER 0xFFFF
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struct em_data_callback {
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/**
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* active_power() - Provide power at the next capacity state of a CPU
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* @power : Active power at the capacity state in mW (modified)
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* @freq : Frequency at the capacity state in kHz (modified)
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* @cpu : CPU for which we do this operation
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*
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* active_power() must find the lowest capacity state of 'cpu' above
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* 'freq' and update 'power' and 'freq' to the matching active power
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* and frequency.
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*
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* The power is the one of a single CPU in the domain, expressed in
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* milli-watts. It is expected to fit in the [0, EM_CPU_MAX_POWER]
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* range.
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*
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* Return 0 on success.
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*/
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int (*active_power)(unsigned long *power, unsigned long *freq, int cpu);
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};
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#define EM_DATA_CB(_active_power_cb) { .active_power = &_active_power_cb }
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struct em_perf_domain *em_cpu_get(int cpu);
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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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/**
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* em_pd_energy() - Estimates the energy consumed by the CPUs of a perf. domain
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* @pd : performance domain for which energy has to be estimated
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* @max_util : highest utilization among CPUs of the domain
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* @sum_util : sum of the utilization of all CPUs in the domain
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*
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* Return: the sum of the energy consumed by the CPUs of the domain assuming
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* a capacity state satisfying the max utilization of the domain.
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*/
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static inline unsigned long em_pd_energy(struct em_perf_domain *pd,
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unsigned long max_util, unsigned long sum_util)
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{
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unsigned long freq, scale_cpu;
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struct em_cap_state *cs;
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int i, cpu;
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/*
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* In order to predict the capacity state, map the utilization of the
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* most utilized CPU of the performance domain to a requested frequency,
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* like schedutil.
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*/
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cpu = cpumask_first(to_cpumask(pd->cpus));
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scale_cpu = arch_scale_cpu_capacity(cpu);
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cs = &pd->table[pd->nr_cap_states - 1];
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freq = map_util_freq(max_util, cs->frequency, scale_cpu);
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/*
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* Find the lowest capacity state of the Energy Model above the
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* requested frequency.
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*/
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for (i = 0; i < pd->nr_cap_states; i++) {
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cs = &pd->table[i];
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if (cs->frequency >= freq)
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break;
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}
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/*
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* The capacity of a CPU in the domain at that capacity state (cs)
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* can be computed as:
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*
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* cs->freq * scale_cpu
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* cs->cap = -------------------- (1)
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* cpu_max_freq
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*
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* So, ignoring the costs of idle states (which are not available in
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* the EM), the energy consumed by this CPU at that capacity state is
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* estimated as:
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*
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* cs->power * cpu_util
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* cpu_nrg = -------------------- (2)
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* cs->cap
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*
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* since 'cpu_util / cs->cap' represents its percentage of busy time.
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*
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* NOTE: Although the result of this computation actually is in
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* units of power, it can be manipulated as an energy value
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* over a scheduling period, since it is assumed to be
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* constant during that interval.
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*
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* By injecting (1) in (2), 'cpu_nrg' can be re-expressed as a product
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* of two terms:
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*
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* cs->power * cpu_max_freq cpu_util
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* cpu_nrg = ------------------------ * --------- (3)
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* cs->freq scale_cpu
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*
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* The first term is static, and is stored in the em_cap_state struct
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* as 'cs->cost'.
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*
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* Since all CPUs of the domain have the same micro-architecture, they
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* share the same 'cs->cost', and the same CPU capacity. Hence, the
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* total energy of the domain (which is the simple sum of the energy of
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* all of its CPUs) can be factorized as:
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*
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* cs->cost * \Sum cpu_util
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* pd_nrg = ------------------------ (4)
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* scale_cpu
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*/
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return cs->cost * sum_util / scale_cpu;
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}
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/**
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* em_pd_nr_cap_states() - Get the number of capacity states of a perf. domain
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* @pd : performance domain for which this must be done
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*
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* Return: the number of capacity states in the performance domain table
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*/
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static inline int em_pd_nr_cap_states(struct em_perf_domain *pd)
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{
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return pd->nr_cap_states;
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}
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#else
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struct em_data_callback {};
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#define EM_DATA_CB(_active_power_cb) { }
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static inline int em_register_perf_domain(cpumask_t *span,
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unsigned int nr_states, struct em_data_callback *cb)
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{
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return -EINVAL;
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}
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static inline struct em_perf_domain *em_cpu_get(int cpu)
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{
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return NULL;
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}
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static inline unsigned long em_pd_energy(struct em_perf_domain *pd,
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unsigned long max_util, unsigned long sum_util)
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{
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return 0;
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}
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static inline int em_pd_nr_cap_states(struct em_perf_domain *pd)
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{
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return 0;
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}
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#endif
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#endif
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