RE: [RFT][PATCH v2 0/3] cpuidle: teo: Do not check timers unconditionally every time

[Doug Smythies]

On 2023.08.03 14:33 Rafael wrote:
> On Thu, Aug 3, 2023 at 11:12 PM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
>>
>> Hi Folks,
>>
>> This is the second iteration of:
>>
>> https://lore.kernel.org/linux-pm/4511619.LvFx2qVVIh@kreacher/
>>
>> with an additional patch.
>>
>> There are some small modifications of patch [1/3] and the new
>> patch causes governor statistics to play a role in deciding whether
>> or not to stop the scheduler tick.
>>
>> Testing would be much appreciated!
>
> For convenience, this series is now available in the following git branch:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
> pm-cpuidle-teo

Hi Rafael,

Thank you for the git branch link.

I did some testing:

Disclaimer: I used areas of focus derived
from the original teo-util work last fall,
and did not check if they were still the best
places to look for issues.

CPU: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz
HWP: enabled
CPU frequency scaling driver: intel_pstate
CPU frequency scaling governor: performance
Kernel 1: 6.5-rc4 (1000 Hz tick rate)
Kernel 2: kernel 1 + this patch series (called "rjw")
System is extremely idle, other than the test work.

All tests were done with all idle governors:
menu, teo, ladder, rjw.

Test 1: 2 core ping pong sweep:

Pass a token between 2 CPUs on 2 different cores.
Do a variable amount of work at each stop.

Purpose: To utilize the shallowest idle states
and observe the transition from using more of 1
idle state to another.

Results: 
 
teo and rjw track fairly well, with
rjw reducing its use of idle state 0 before
teo as the work packet increases. The menu governor
does best overall, but performs worse over a greater
range of token loop times.

Details (power and idle stats; times):
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/2-1/perf/
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/2-1/2-core-ping-pong-sweep.png

Test 2: 6 core ping pong sweep:

Pass a token between 6 CPUs on 6 different cores.
Do a variable amount of work at each stop.

Purpose: To utilize the midrange idle states
and observe the transitions from between use of
idle states.

Results: There is some instability in the results
in the early stages.
For unknown reasons, the rjw governor sometimes works
slower and at lower power. The condition is not 100%
repeatable.

Overall teo completed the test fastest (54.9 minutes)
Followed by menu (56.2 minutes), then rjw (56.7 minutes),
then ladder (58.4 minutes). teo is faster throughout the
latter stages of the test, but at the cost of more power.
The differences seem to be in the transition from idle
state 1 to idle state 2 usage.

Details (power and idle stats; times):
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/6-2/perf/
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/6-2/6-core-ping-pong-sweep.png
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/6-2/6-core-ping-pong-sweep-detail-a.png
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/6-2/6-core-ping-pong-sweep-detail-b.png
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/6-2/6-core-ping-pong-sweep-diffs.png

a re-run power and idle stats, showing inconsistent behaviour.
teo and rjw only, and no timing data:
http://smythies.com/~doug/linux/idle/teo-util2/ping-sweep/6-1/perf/

Test 3: sleeping ebizzy - 128 threads.

Purpose: This test has given interesting results in the past.
The test varies the sleep interval between record lookups.
The result is varying usage of idle states.

Results: It can be difficult to see any differences in
the overall timing graph, but a graph of differences
is revealing. teo outperforms rjw in the longer intervals
region of the test, at the cost of more power.

Details: (power and idle stats; times):
http://smythies.com/~doug/linux/idle/teo-util2/ebizzy/perf/
http://smythies.com/~doug/linux/idle/teo-util2/ebizzy/ebizzy-128-perf.png
http://smythies.com/~doug/linux/idle/teo-util2/ebizzy/ebizzy-128-perf-diffs.png

Test 4: 2 X 2 pair token passing. Dwell test. Fast:

Purpose: Dwell under one set of conditions. Observe
noise and/or any bi-stability.

Results (reference time is menu):
rjw: 3.0723 usecs/loop average. +3.15%
teo: 2.9917 usecs/loop average. +0.44%
menu: 2.97845 usecs/loop average. Reference
ladder: 4.077375 usecs/loop average. +36.9%

Powers are all similar, with ladder a bit lower.

Details: (power and idle stats; times):
http://smythies.com/~doug/linux/idle/teo-util2/many-0-400000000-2/perf/
http://smythies.com/~doug/linux/idle/teo-util2/many-0-400000000-2/times.txt

Test 5: 2 X 2 pair token passing. Dwell test. Medium:

Purpose: Dwell under one set of conditions. Observe
noise and/or any bi-stability.

Results (reference time is menu):
rjw: 11.3406 usecs/loop average. -0.69%
teo: 11.36765 usecs/loop average. -0.45%
menu: 11.41905 usecs/loop average. reference
ladder: 11.9535 usecs/loop average. +4.68%

Powers are all similar.

Details:
http://smythies.com/~doug/linux/idle/teo-util2/many-3000-100000000-2/perf/
http://smythies.com/~doug/linux/idle/teo-util2/many-3000-100000000-2/times.txt

Test 6: 2 X 2 pair token passing. Dwell test. Slow:

Purpose: Dwell under one set of conditions. Observe
noise and/or any bi-stability.

Results (reference time is menu):
rjw: 2591.70 usecs/loop average. +0.26%
teo: 2566.34 usecs/loop average. -0.72%
menu: 2585.00 usecs/loop average. reference
ladder: 2635.36 usecs/loop average. +1.95%

Powers are all similar, with ladder a bit lower. 
Due to the strong temperature to power use curve,
a much longer dwell test would need to be run to
be sure to get to steady state power usage.

Details:
http://smythies.com/~doug/linux/idle/teo-util2/many-1000000-342000-2/perf/
http://smythies.com/~doug/linux/idle/teo-util2/many-1000000-342000-2/times.txt

Test 7: 500 low load threads.

Purpose: This test has given interesting results
in the past.

500 threads at approximately 10 hertz work/sleep frequency
and about 0.0163 load per thread, 8.15 total.
CPUs about 32% idle.

Results:
rjw executed 0.01% faster than teo.
rjw used 5% less energy than teo.

Details:
http://smythies.com/~doug/linux/idle/teo-util2/waiter/perf/
http://smythies.com/~doug/linux/idle/teo-util2/waiter/times.txt

Conclusions: Overall, I am not seeing a compelling reason to
proceed with this patch set.

... Doug