[PATCH v6 0/2] cpuidle: teo: Introduce util-awareness

Kajetan Puchalski
2023-01-05T14:52:23Z

Hi,

At the moment, none of the available idle governors take any scheduling
information into account. They also tend to overestimate the idle
duration quite often, which causes them to select excessively deep idle
states, thus leading to increased wakeup latency and lower performance with no
power saving. For 'menu' while web browsing on Android for instance, those
types of wakeups ('too deep') account for over 24% of all wakeups.

At the same time, on some platforms idle state 0 can be power efficient
enough to warrant wanting to prefer it over idle state 1. This is because
the power usage of the two states can be so close that sufficient amounts
of too deep state 1 sleeps can completely offset the state 1 power saving to the
point where it would've been more power efficient to just use state 0 instead.
This is of course for systems where state 0 is not a polling state, such as
arm-based devices.

Sleeps that happened in state 0 while they could have used state 1 ('too shallow') only
save less power than they otherwise could have. Too deep sleeps, on the other
hand, harm performance and nullify the potential power saving from using state 1 in
the first place. While taking this into account, it is clear that on balance it
is preferable for an idle governor to have more too shallow sleeps instead of
more too deep sleeps on those kinds of platforms.

Currently the best available governor under this metric is TEO which on average results in less than
half the percentage of too deep sleeps compared to 'menu', getting much better wakeup latencies and
increased performance in the process.

This patchset specifically tunes TEO to prefer shallower idle states in order to reduce wakeup latency
and achieve better performance. To this end, before selecting the next idle state it uses the avg_util
signal of a CPU's runqueue in order to determine to what extent the CPU is being utilized.
This util value is then compared to a threshold defined as a percentage of the cpu's capacity
(capacity >> 6 ie. ~1.5% in the current implementation). If the util is above the threshold, the idle
state selected by TEO metrics will be reduced by 1, thus selecting a shallower state. If the util is
below the threshold, the governor defaults to the TEO metrics mechanism to try to select the deepest
available idle state based on the closest timer event and its own correctness.

The main goal of this is to reduce latency and increase performance for some workloads. Under some
workloads it will result in an increase in power usage (Geekbench 5) while for other workloads it
will also result in a decrease in power usage compared to TEO (PCMark Web, Jankbench, Speedometer).

As of v2 the patch includes a 'fast exit' path for arm-based and similar systems where only 2 idle
states are present. If there's just 2 idle states and the CPU is utilized, we can directly select
the shallowest state and save cycles by skipping the entire metrics mechanism.

Under the current implementation, the state will not be reduced by 1 if the change would lead to
selecting a polling state instead of a non-polling state.

This approach can outperform all the other currently available governors, at least on mobile device
workloads, which is why I think it is worth keeping as an option.

There is no particular attachment or reliance on TEO for this mechanism, I simply chose to base
it on TEO because it performs the best out of all the available options and I didn't think there was
any point in reinventing the wheel on the side of computing governor metrics. If a
better approach comes along at some point, there's no reason why the same idle aware mechanism
couldn't be used with any other metrics algorithm. That would, however, require implemeting it as
a separate governor rather than a TEO add-on.

As for how the extension performs in practice, below I'll add some benchmark results I got while
testing this patchset. All the benchmarks were run after holding the phone in the fridge for exactly
an hour each time to minimise the impact of thermal issues.

Pixel 6 (Android 12, mainline kernel 5.18, with newer mainline CFS patches):

1. Geekbench 5 (latency-sensitive, heavy load test)

The values below are gmean values across 3 back to back iteration of Geekbench 5.
As GB5 is a heavy benchmark, after more than 3 iterations intense throttling kicks in on mobile devices
resulting in skewed benchmark scores, which makes it difficult to collect reliable results. The actual
values for all of the governors can change between runs as the benchmark might be affected by factors
other than just latency. Nevertheless, on the runs I've seen, util-aware TEO frequently achieved better
scores than all the other governors.

Benchmark scores

+-----------------+-------------+---------+-------------+
| metric | kernel | value | perc_diff |
|-----------------+-------------+---------+-------------|
| multicore_score | menu | 2826.5 | 0.0% |
| multicore_score | teo | 2764.8 | -2.18% |
| multicore_score | teo_util_v3 | 2849 | 0.8% |
| multicore_score | teo_util_v4 | 2865 | 1.36% |
| score | menu | 1053 | 0.0% |
| score | teo | 1050.7 | -0.22% |
| score | teo_util_v3 | 1059.6 | 0.63% |
| score | teo_util_v4 | 1057.6 | 0.44% |
+-----------------+-------------+---------+-------------+

Idle misses

The numbers are percentages of too deep and too shallow sleeps computed using the new trace
event - cpu_idle_miss. The percentage is obtained by counting the two types of misses over
the course of a run and then dividing them by the total number of wakeups in that run.

+-------------+-------------+--------------+
| wa_path | type | count_perc |
|-------------+-------------+--------------|
| menu | too deep | 14.994% |
| teo | too deep | 9.649% |
| teo_util_v3 | too deep | 4.298% |
| teo_util_v4 | too deep | 4.02 % |
| menu | too shallow | 2.497% |
| teo | too shallow | 5.963% |
| teo_util_v3 | too shallow | 13.773% |
| teo_util_v4 | too shallow | 14.598% |
+-------------+-------------+--------------+

Power usage [mW]

+--------------+----------+-------------+---------+-------------+
| chan_name | metric | kernel | value | perc_diff |
|--------------+----------+-------------+---------+-------------|
| total_power | gmean | menu | 2551.4 | 0.0% |
| total_power | gmean | teo | 2606.8 | 2.17% |
| total_power | gmean | teo_util_v3 | 2670.1 | 4.65% |
| total_power | gmean | teo_util_v4 | 2722.3 | 6.7% |
+--------------+----------+-------------+---------+-------------+

Task wakeup latency

+-----------------+----------+-------------+-------------+-------------+
| comm | metric | kernel | value | perc_diff |
|-----------------+----------+-------------+-------------+-------------|
| AsyncTask #1 | gmean | menu | 78.16μs | 0.0% |
| AsyncTask #1 | gmean | teo | 61.60μs | -21.19% |
| AsyncTask #1 | gmean | teo_util_v3 | 74.34μs | -4.89% |
| AsyncTask #1 | gmean | teo_util_v4 | 54.45μs | -30.34% |
| labs.geekbench5 | gmean | menu | 88.55μs | 0.0% |
| labs.geekbench5 | gmean | teo | 100.97μs | 14.02% |
| labs.geekbench5 | gmean | teo_util_v3 | 53.57μs | -39.5% |
| labs.geekbench5 | gmean | teo_util_v4 | 59.60μs | -32.7% |
+-----------------+----------+-------------+-------------+-------------+

In case of this benchmark, the difference in latency does seem to translate into better scores.

2. PCMark Web Browsing (non latency-sensitive, normal usage web browsing test)

The table below contains gmean values across 20 back to back iterations of PCMark 2 Web Browsing.

Benchmark scores

+----------------+-------------+---------+-------------+
| metric | kernel | value | perc_diff |
|----------------+-------------+---------+-------------|
| PcmaWebV2Score | menu | 5232 | 0.0% |
| PcmaWebV2Score | teo | 5219.8 | -0.23% |
| PcmaWebV2Score | teo_util_v3 | 5273.5 | 0.79% |
| PcmaWebV2Score | teo_util_v4 | 5239.9 | 0.15% |
+----------------+-------------+---------+-------------+

Idle misses

+-------------+-------------+--------------+
| wa_path | type | count_perc |
|-------------+-------------+--------------|
| menu | too deep | 24.814% |
| teo | too deep | 11.65% |
| teo_util_v3 | too deep | 3.481% |
| teo_util_v4 | too deep | 3.662% |
| menu | too shallow | 3.101% |
| teo | too shallow | 8.578% |
| teo_util_v3 | too shallow | 18.326% |
| teo_util_v4 | too shallow | 18.692% |
+-------------+-------------+--------------+

Power usage [mW]

+--------------+----------+-------------+---------+-------------+
| chan_name | metric | kernel | value | perc_diff |
|--------------+----------+-------------+---------+-------------|
| total_power | gmean | menu | 179.2 | 0.0% |
| total_power | gmean | teo | 184.8 | 3.1% |
| total_power | gmean | teo_util_v3 | 177.4 | -1.02% |
| total_power | gmean | teo_util_v4 | 184.1 | 2.71% |
+--------------+----------+-------------+---------+-------------+

Task wakeup latency

+-----------------+----------+-------------+-------------+-------------+
| comm | metric | kernel | value | perc_diff |
|-----------------+----------+-------------+-------------+-------------|
| CrRendererMain | gmean | menu | 236.63μs | 0.0% |
| CrRendererMain | gmean | teo | 201.85μs | -14.7% |
| CrRendererMain | gmean | teo_util_v3 | 106.46μs | -55.01% |
| CrRendererMain | gmean | teo_util_v4 | 106.72μs | -54.9% |
| chmark:workload | gmean | menu | 100.30μs | 0.0% |
| chmark:workload | gmean | teo | 80.20μs | -20.04% |
| chmark:workload | gmean | teo_util_v3 | 65.88μs | -34.32% |
| chmark:workload | gmean | teo_util_v4 | 57.90μs | -42.28% |
| surfaceflinger | gmean | menu | 97.57μs | 0.0% |
| surfaceflinger | gmean | teo | 98.86μs | 1.31% |
| surfaceflinger | gmean | teo_util_v3 | 56.49μs | -42.1% |
| surfaceflinger | gmean | teo_util_v4 | 72.68μs | -25.52% |
+-----------------+----------+-------------+-------------+-------------+

In this case the large latency improvement does not translate into a notable increase in benchmark score as
this particular benchmark mainly responds to changes in operating frequency.

3. Jankbench (locked 60hz screen) (normal usage UI test)

Frame durations

+---------------+------------------+---------+-------------+
| variable | kernel | value | perc_diff |
|---------------+------------------+---------+-------------|
| mean_duration | menu_60hz | 13.9 | 0.0% |
| mean_duration | teo_60hz | 14.7 | 6.0% |
| mean_duration | teo_util_v3_60hz | 13.8 | -0.87% |
| mean_duration | teo_util_v4_60hz | 12.6 | -9.0% |
+---------------+------------------+---------+-------------+

Jank percentage

+------------+------------------+---------+-------------+
| variable | kernel | value | perc_diff |
|------------+------------------+---------+-------------|
| jank_perc | menu_60hz | 1.5 | 0.0% |
| jank_perc | teo_60hz | 2.1 | 36.99% |
| jank_perc | teo_util_v3_60hz | 1.3 | -13.95% |
| jank_perc | teo_util_v4_60hz | 1.3 | -17.37% |
+------------+------------------+---------+-------------+

Idle misses

+------------------+-------------+--------------+
| wa_path | type | count_perc |
|------------------+-------------+--------------|
| menu_60hz | too deep | 26.00% |
| teo_60hz | too deep | 11.00% |
| teo_util_v3_60hz | too deep | 2.33% |
| teo_util_v4_60hz | too deep | 2.54% |
| menu_60hz | too shallow | 4.74% |
| teo_60hz | too shallow | 11.89% |
| teo_util_v3_60hz | too shallow | 21.78% |
| teo_util_v4_60hz | too shallow | 21.93% |
+------------------+-------------+--------------+

Power usage [mW]

+--------------+------------------+---------+-------------+
| chan_name | kernel | value | perc_diff |
|--------------+------------------+---------+-------------|
| total_power | menu_60hz | 144.6 | 0.0% |
| total_power | teo_60hz | 136.9 | -5.27% |
| total_power | teo_util_v3_60hz | 134.2 | -7.19% |
| total_power | teo_util_v4_60hz | 121.3 | -16.08% |
+--------------+------------------+---------+-------------+

Task wakeup latency

+-----------------+------------------+-------------+-------------+
| comm | kernel | value | perc_diff |
|-----------------+------------------+-------------+-------------|
| RenderThread | menu_60hz | 139.52μs | 0.0% |
| RenderThread | teo_60hz | 116.51μs | -16.49% |
| RenderThread | teo_util_v3_60hz | 86.76μs | -37.82% |
| RenderThread | teo_util_v4_60hz | 91.11μs | -34.7% |
| droid.benchmark | menu_60hz | 135.88μs | 0.0% |
| droid.benchmark | teo_60hz | 105.21μs | -22.57% |
| droid.benchmark | teo_util_v3_60hz | 83.92μs | -38.24% |
| droid.benchmark | teo_util_v4_60hz | 83.18μs | -38.79% |
| surfaceflinger | menu_60hz | 124.03μs | 0.0% |
| surfaceflinger | teo_60hz | 151.90μs | 22.47% |
| surfaceflinger | teo_util_v3_60hz | 100.19μs | -19.22% |
| surfaceflinger | teo_util_v4_60hz | 87.65μs | -29.33% |
+-----------------+------------------+-------------+-------------+

4. Speedometer 2 (heavy load web browsing test)

Benchmark scores

+-------------------+-------------+---------+-------------+
| metric | kernel | value | perc_diff |
|-------------------+-------------+---------+-------------|
| Speedometer Score | menu | 102 | 0.0% |
| Speedometer Score | teo | 104.9 | 2.88% |
| Speedometer Score | teo_util_v3 | 102.1 | 0.16% |
| Speedometer Score | teo_util_v4 | 103.8 | 1.83% |
+-------------------+-------------+---------+-------------+

Idle misses

+-------------+-------------+--------------+
| wa_path | type | count_perc |
|-------------+-------------+--------------|
| menu | too deep | 17.95% |
| teo | too deep | 6.46% |
| teo_util_v3 | too deep | 0.63% |
| teo_util_v4 | too deep | 0.64% |
| menu | too shallow | 3.86% |
| teo | too shallow | 8.21% |
| teo_util_v3 | too shallow | 14.72% |
| teo_util_v4 | too shallow | 14.43% |
+-------------+-------------+--------------+

Power usage [mW]

+--------------+----------+-------------+---------+-------------+
| chan_name | metric | kernel | value | perc_diff |
|--------------+----------+-------------+---------+-------------|
| total_power | gmean | menu | 2059 | 0.0% |
| total_power | gmean | teo | 2187.8 | 6.26% |
| total_power | gmean | teo_util_v3 | 2212.9 | 7.47% |
| total_power | gmean | teo_util_v4 | 2121.8 | 3.05% |
+--------------+----------+-------------+---------+-------------+

Task wakeup latency

+-----------------+----------+-------------+-------------+-------------+
| comm | metric | kernel | value | perc_diff |
|-----------------+----------+-------------+-------------+-------------|
| CrRendererMain | gmean | menu | 17.18μs | 0.0% |
| CrRendererMain | gmean | teo | 16.18μs | -5.82% |
| CrRendererMain | gmean | teo_util_v3 | 18.04μs | 5.05% |
| CrRendererMain | gmean | teo_util_v4 | 18.25μs | 6.27% |
| RenderThread | gmean | menu | 68.60μs | 0.0% |
| RenderThread | gmean | teo | 48.44μs | -29.39% |
| RenderThread | gmean | teo_util_v3 | 48.01μs | -30.02% |
| RenderThread | gmean | teo_util_v4 | 51.24μs | -25.3% |
| surfaceflinger | gmean | menu | 42.23μs | 0.0% |
| surfaceflinger | gmean | teo | 29.84μs | -29.33% |
| surfaceflinger | gmean | teo_util_v3 | 24.51μs | -41.95% |
| surfaceflinger | gmean | teo_util_v4 | 29.64μs | -29.8% |
+-----------------+----------+-------------+-------------+-------------+

Thank you for taking your time to read this!

--
Kajetan

v5 -> v6:
- amended some wording in the commit description & cover letter
- included test results in the commit description
- refactored checking the CPU utilized status to account for !SMP systems
- dropped the RFC from the patchset header

v4 -> v5:
- remove the restriction to only apply the mechanism for C1 candidate state
- clarify some code comments, fix comment style
- refactor the fast-exit path loop implementation
- move some cover letter information into the commit description

v3 -> v4:
- remove the chunk of code skipping metrics updates when the CPU was utilized
- include new test results and more benchmarks in the cover letter

v2 -> v3:
- add a patch adding an option to skip polling states in teo_find_shallower_state()
- only reduce the state if the candidate state is C1 and C0 is not a polling state
- add a check for polling states in the 2-states fast-exit path
- remove the ifdefs and Kconfig option

v1 -> v2:
- rework the mechanism to reduce selected state by 1 instead of directly selecting C0 (suggested by Doug Smythies)
- add a fast-exit path for systems with 2 idle states to not waste cycles on metrics when utilized
- fix typos in comments
- include a missing header

Kajetan Puchalski (2):
cpuidle: teo: Optionally skip polling states in teo_find_shallower_state()
cpuidle: teo: Introduce util-awareness

drivers/cpuidle/governors/teo.c | 100 ++++++++++++++++++++++++++++++--
1 file changed, 96 insertions(+), 4 deletions(-)

--
2.37.1