# Re: [PATCH v2 1/1] PM: EM: Increase energy calculation precision

**From: **Greg KH

**Date: ** Tue Jul 20 2021 - 05:46:09 EST

On Tue, Jul 20, 2021 at 10:41:53AM +0100, Lukasz Luba wrote:

>* The Energy Model (EM) provides useful information about device power in*

>* each performance state to other subsystems like: Energy Aware Scheduler*

>* (EAS). The energy calculation in EAS does arithmetic operation based on*

>* the EM em_cpu_energy(). Current implementation of that function uses*

>* em_perf_state::cost as a pre-computed cost coefficient equal to:*

>* cost = power * max_frequency / frequency.*

>* The 'power' is expressed in milli-Watts (or in abstract scale).*

>* *

>* There are corner cases when the EAS energy calculation for two Performance*

>* Domains (PDs) return the same value. The EAS compares these values to*

>* choose smaller one. It might happen that this values are equal due to*

>* rounding error. In such scenario, we need better resolution, e.g. 1000*

>* times better. To provide this possibility increase the resolution in the*

>* em_perf_state::cost for 64-bit architectures. The costs for increasing*

>* resolution in 32-bit architectures are pretty high (64-bit division) and*

>* the returns do not justify the increased costs.*

>* *

>* This patch allows to avoid the rounding to milli-Watt errors, which might*

>* occur in EAS energy estimation for each Performance Domains (PD). The*

>* rounding error is common for small tasks which have small utilization*

>* value.*

>* *

>* There are two places in the code where it makes a difference:*

>* 1. In the find_energy_efficient_cpu() where we are searching for*

>* best_delta. We might suffer there when two PDs return the same result,*

>* like in the example below.*

>* *

>* Scenario:*

>* Low utilized system e.g. ~200 sum_util for PD0 and ~220 for PD1. There*

>* are quite a few small tasks ~10-15 util. These tasks would suffer for*

>* the rounding error. Such system utilization has been seen while playing*

>* some simple games. In such condition our partner reported 5..10mA less*

>* battery drain.*

>* *

>* Some details:*

>* We have two Perf Domains (PDs): PD0 (big) and PD1 (little)*

>* Let's compare w/o patch set ('old') and w/ patch set ('new')*

>* We are comparing energy w/ task and w/o task placed in the PDs*

>* *

>* a) 'old' w/o patch set, PD0*

>* task_util = 13*

>* cost = 480*

>* sum_util_w/o_task = 215*

>* sum_util_w_task = 228*

>* scale_cpu = 1024*

>* energy_w/o_task = 480 * 215 / 1024 = 100.78 => 100*

>* energy_w_task = 480 * 228 / 1024 = 106.87 => 106*

>* energy_diff = 106 - 100 = 6*

>* (this is equal to 'old' PD1's energy_diff in 'c)')*

>* *

>* b) 'new' w/ patch set, PD0*

>* task_util = 13*

>* cost = 480 * 1000 = 480000*

>* sum_util_w/o_task = 215*

>* sum_util_w_task = 228*

>* energy_w/o_task = 480000 * 215 / 1024 = 100781*

>* energy_w_task = 480000 * 228 / 1024 = 106875*

>* energy_diff = 106875 - 100781 = 6094*

>* (this is not equal to 'new' PD1's energy_diff in 'd)')*

>* *

>* c) 'old' w/o patch set, PD1*

>* task_util = 13*

>* cost = 160*

>* sum_util_w/o_task = 283*

>* sum_util_w_task = 293*

>* scale_cpu = 355*

>* energy_w/o_task = 160 * 283 / 355 = 127.55 => 127*

>* energy_w_task = 160 * 296 / 355 = 133.41 => 133*

>* energy_diff = 133 - 127 = 6*

>* (this is equal to 'old' PD0's energy_diff in 'a)')*

>* *

>* d) 'new' w/ patch set, PD1*

>* task_util = 13*

>* cost = 160 * 1000 = 160000*

>* sum_util_w/o_task = 283*

>* sum_util_w_task = 293*

>* scale_cpu = 355*

>* energy_w/o_task = 160000 * 283 / 355 = 127549*

>* energy_w_task = 160000 * 296 / 355 = 133408*

>* energy_diff = 133408 - 127549 = 5859*

>* (this is not equal to 'new' PD0's energy_diff in 'b)')*

>* *

>* 2. Difference in the the last find_energy_efficient_cpu(): margin filter.*

>* With this patch the margin comparison also has better resolution,*

>* so it's possible to have better task placement thanks to that.*

>* *

>* Fixes: 27871f7a8a341ef ("PM: Introduce an Energy Model management framework")*

>* Reported-by: CCJ Yeh <CCj.Yeh@xxxxxxxxxxxx>*

>* Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>*

>* ---*

>* include/linux/energy_model.h | 16 ++++++++++++++++*

>* kernel/power/energy_model.c | 3 ++-*

>* 2 files changed, 18 insertions(+), 1 deletion(-)*

>* *

<formletter>

This is not the correct way to submit patches for inclusion in the

stable kernel tree. Please read:

https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html

for how to do this properly.

</formletter>