PM: EM: Optimize em_cpu_energy() and remove division

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>

1 files changed, 3 insertions, 4 deletions

diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
index b192b0ac8c6e..a631d7d52c40 100644
--- a/kernel/power/energy_model.c
+++ b/kernel/power/energy_model.c
@@ -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;