On 02/08/2021 08:21, Lukasz Luba wrote:

Hi Peter, Vincent,

Gentle ping.


On 7/20/21 10:41 AM, Lukasz Luba wrote:

[...]

quoted

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.

s/The costs ... increased costs./The cost of increasing resolution on
32-bit is pretty high (64-bit division) and is not justified since there
are no new 32bit big.LITTLE EAS systems expected which would benefit
from this higher resolution./ ?

quoted

This patch allows to avoid the rounding to milli-Watt errors, which might
occur in EAS energy estimation for each Performance Domains (PD). The

s/Performance Domains (PD)/PD.

[...]

quoted

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.

Hard to digest: Maybe s/Such system ... battery drain./These utilization
values are typical when running games on Android. One of our partners
has reported 5..10mA less battery drain when running with increased
resolution./ ?

quoted

Some details:
We have two Perf Domains (PDs): PD0 (big) and PD1 (little)

s/Perf Domains (PDs)/PDs

[...]

quoted

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

s/in the the last find_energy_efficient_cpu(): margin filter/in the 6%
energy margin filter at the end of find_energy_efficient_cpu()/ ?

quoted

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 <redacted>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
---
  include/linux/energy_model.h | 16 ++++++++++++++++
  kernel/power/energy_model.c  |  3 ++-
  2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
index 3f221dbf5f95..1834752c5617 100644
--- a/include/linux/energy_model.h
+++ b/include/linux/energy_model.h

@@ -53,6 +53,22 @@ struct em_perf_domain {

  #ifdef CONFIG_ENERGY_MODEL
  #define EM_MAX_POWER 0xFFFF
  +/*
+ * Increase resolution of energy estimation calculations for 64-bit
+ * architectures. The extra resolution improves decision made by EAS
for the
+ * task placement when two Performance Domains might provide similar
energy
+ * estimation values (w/o better resolution the values could be equal).
+ *
+ * We increase resolution only if we have enough bits to allow this
increased
+ * resolution (i.e. 64-bit). The costs for increasing resolution when
32-bit
+ * are pretty high and the returns do not justify the increased costs.
+ */
+#ifdef CONFIG_64BIT
+#define em_scale_power(p) ((p) * 1000)
+#else
+#define em_scale_power(p) (p)
+#endif
+
  struct em_data_callback {
      /**
       * active_power() - Provide power at the next performance state of

diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
index 0f4530b3a8cd..bf312c04c514 100644
--- a/kernel/power/energy_model.c
+++ b/kernel/power/energy_model.c

@@ -170,7 +170,8 @@ static int em_create_perf_table(struct device

*dev, struct em_perf_domain *pd,
      /* Compute the cost of each performance state. */
      fmax = (u64) table[nr_states - 1].frequency;
      for (i = 0; i < nr_states; i++) {
-        table[i].cost = div64_u64(fmax * table[i].power,
+        unsigned long power_res = em_scale_power(table[i].power);
+        table[i].cost = div64_u64(fmax * power_res,
                        table[i].frequency);
      }

Otherwise, LGTM.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

This is now an EM only patch (Task scheduler (i.e. CFS/EAS) is only
effected via compute_energy() -> em_cpu_energy().