@@ -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 +245,6 @@ 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);
@@ -283,12 +258,12 @@ 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)
- * cpu_max_freq
+ * ps->freq * scale_cpu
+ * ps->performance = -------------------- (1)
+ * cpu_max_freq
*
* So, ignoring the costs of idle states (which are not available in
* the EM), the energy consumed by this CPU at that performance state
@@ -296,9 +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
@@ -308,9 +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'.
@@ -320,11 +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;
}
/**
@@ -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;
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). Signed-off-by: Lukasz Luba <lukasz.luba@arm.com> --- include/linux/energy_model.h | 54 ++++++++++-------------------------- kernel/power/energy_model.c | 7 ++--- 2 files changed, 17 insertions(+), 44 deletions(-)