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PM: EM: Optimize em_cpu_energy() and remove division

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The Energy Model (EM) can be modified at runtime which brings new
possibilities. The em_cpu_energy() is called by the Energy Aware Scheduler
(EAS) in its hot path. The energy calculation uses power value for
a given performance state (ps) and the CPU busy time as percentage for that
given frequency.

It is possible to avoid the division by 'scale_cpu' at runtime, because
EM is updated whenever new max capacity CPU is set in the system.

Use that feature and do the needed division during the calculation of the
coefficient 'ps->cost'. That enhanced 'ps->cost' value can be then just
multiplied simply by utilization:

pd_nrg = ps->cost * \Sum cpu_util

to get the needed energy for whole Performance Domain (PD).

With this optimization and earlier removal of map_util_freq(), the
em_cpu_energy() should run faster on the Big CPU by 1.43x and on the Little
CPU by 1.69x (RockPi 4B board).

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Lukasz Luba 2024-02-08 11:55:49 +00:00 committed by Rafael J. Wysocki
parent e3f1164fc9
commit 1b600da510
2 changed files with 18 additions and 44 deletions
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51
include/linux/energy_model.h
View File

@ -115,27 +115,6 @@ struct em_perf_domain {
#define EM_MAX_NUM_CPUS 16
#endif
/*
* To avoid an overflow on 32bit machines while calculating the energy
* use a different order in the operation. First divide by the 'cpu_scale'
* which would reduce big value stored in the 'cost' field, then multiply by
* the 'sum_util'. This would allow to handle existing platforms, which have
* e.g. power ~1.3 Watt at max freq, so the 'cost' value > 1mln micro-Watts.
* In such scenario, where there are 4 CPUs in the Perf. Domain the 'sum_util'
* could be 4096, then multiplication: 'cost' * 'sum_util' would overflow.
* This reordering of operations has some limitations, we lose small
* precision in the estimation (comparing to 64bit platform w/o reordering).
*
* We are safe on 64bit machine.
*/
#ifdef CONFIG_64BIT
#define em_estimate_energy(cost, sum_util, scale_cpu) \
(((cost) * (sum_util)) / (scale_cpu))
#else
#define em_estimate_energy(cost, sum_util, scale_cpu) \
(((cost) / (scale_cpu)) * (sum_util))
#endif
struct em_data_callback {
/**
* active_power() - Provide power at the next performance state of
@ -249,8 +228,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
{
struct em_perf_table *em_table;
struct em_perf_state *ps;
unsigned long scale_cpu;
int cpu, i;
int i;
#ifdef CONFIG_SCHED_DEBUG
WARN_ONCE(!rcu_read_lock_held(), "EM: rcu read lock needed\n");
@ -267,9 +245,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
* max utilization to the allowed CPU capacity before calculating
* effective performance.
*/
cpu = cpumask_first(to_cpumask(pd->cpus));
scale_cpu = arch_scale_cpu_capacity(cpu);
max_util = map_util_perf(max_util);
max_util = min(max_util, allowed_cpu_cap);
/*
@ -282,11 +258,11 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
ps = &em_table->state[i];
/*
* The capacity of a CPU in the domain at the performance state (ps)
* can be computed as:
* The performance (capacity) of a CPU in the domain at the performance
* state (ps) can be computed as:
*
* ps->freq * scale_cpu
* ps->cap = -------------------- (1)
* ps->performance = -------------------- (1)
* cpu_max_freq
*
* So, ignoring the costs of idle states (which are not available in
@ -295,9 +271,10 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
*
* ps->power * cpu_util
* cpu_nrg = -------------------- (2)
* ps->cap
* ps->performance
*
* since 'cpu_util / ps->cap' represents its percentage of busy time.
* since 'cpu_util / ps->performance' represents its percentage of busy
* time.
*
* NOTE: Although the result of this computation actually is in
* units of power, it can be manipulated as an energy value
@ -307,9 +284,9 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
* By injecting (1) in (2), 'cpu_nrg' can be re-expressed as a product
* of two terms:
*
* ps->power * cpu_max_freq cpu_util
* cpu_nrg = ------------------------ * --------- (3)
* ps->freq scale_cpu
* ps->power * cpu_max_freq
* cpu_nrg = ------------------------ * cpu_util (3)
* ps->freq * scale_cpu
*
* The first term is static, and is stored in the em_perf_state struct
* as 'ps->cost'.
@ -319,11 +296,9 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
* total energy of the domain (which is the simple sum of the energy of
* all of its CPUs) can be factorized as:
*
* ps->cost * \Sum cpu_util
* pd_nrg = ------------------------ (4)
* scale_cpu
* pd_nrg = ps->cost * \Sum cpu_util (4)
*/
return em_estimate_energy(ps->cost, sum_util, scale_cpu);
return ps->cost * sum_util;
}
/**

7
kernel/power/energy_model.c
View File

@ -192,11 +192,9 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
unsigned long flags)
{
unsigned long prev_cost = ULONG_MAX;
u64 fmax;
int i, ret;
/* Compute the cost of each performance state. */
fmax = (u64) table[nr_states - 1].frequency;
for (i = nr_states - 1; i >= 0; i--) {
unsigned long power_res, cost;
@ -208,8 +206,9 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
return -EINVAL;
}
} else {
power_res = table[i].power;
cost = div64_u64(fmax * power_res, table[i].frequency);
/* increase resolution of 'cost' precision */
power_res = table[i].power * 10;
cost = power_res / table[i].performance;
}
table[i].cost = cost;