RE: [RFT][PATCH v1] cpuidle: teo: Avoid selecting deepest idle state over-eagerly

[Doug Smythies]

Hi Rafael,

On 2025.02.04 12:58 Rafael J. Wysocki wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> It has been observed that the recent teo governor update which concluded
> with commit 16c8d7586c19 ("cpuidle: teo: Skip sleep length computation
> for low latency constraints") caused the max-jOPS score of the SPECjbb
> 2015 benchmark [1] on Intel Granite Rapids to decrease by around 1.4%.
> While it may be argued that this is not a significant increase, the
> previous score can be restored by tweaking the inequality used by teo
> to decide whether or not to preselect the deepest enabled idle state.
> That change also causes the critical-jOPS score of SPECjbb to increase
> by around 2%.
>
> Namely, the likelihood of selecting the deepest enabled idle state in
> teo on the platform in question has increased after commit 13ed5c4a6d9c
> ("cpuidle: teo: Skip getting the sleep length if wakeups are very
> frequent") because some timer wakeups were previously counted as non-
> timer ones and they were effectively added to the left-hand side of the
> inequality deciding whether or not to preselect the deepest idle state.
>
> Many of them are now (accurately) counted as timer wakeups, so the left-
> hand side of that inequality is now effectively smaller in some cases,
> especially when timer wakeups often occur in the range below the target
> residency of the deepest enabled idle state and idle states with target
> residencies below CPUIDLE_FLAG_POLLING are often selected, but the
> majority of recent idle intervals are still above that value most of
> the time.  As a result, the deepest enabled idle state may be selected
> more often than it used to be selected in some cases.
>
> To counter that effect, add the sum of the hits metric for all of the
> idle states below the candidate one (which is the deepest enabled idle
> state at that point) to the left-hand side of the inequality mentioned
> above.  This will cause it to be more balanced because, in principle,
> putting both timer and non-timer wakeups on both sides of it is more
> consistent than only taking into account the timer wakeups in the range
> above the target residency of the deepest enabled idle state.
>
> Link: https://www.spec.org/jbb2015/
> Tested-by: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  drivers/cpuidle/governors/teo.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> --- a/drivers/cpuidle/governors/teo.c
> +++ b/drivers/cpuidle/governors/teo.c
> @@ -349,13 +349,13 @@
>         }
>
>         /*
> -        * If the sum of the intercepts metric for all of the idle states
> -        * shallower than the current candidate one (idx) is greater than the
> +        * If the sum of the intercepts and hits metric for all of the idle
> +        * states below the current candidate one (idx) is greater than the
>          * sum of the intercepts and hits metrics for the candidate state and
>          * all of the deeper states, a shallower idle state is likely to be a
>          * better choice.
>          */
> -       if (2 * idx_intercept_sum > cpu_data->total - idx_hit_sum) {
> +       if (2 * (idx_intercept_sum + idx_hit_sum) > cpu_data->total) {
>                 int first_suitable_idx = idx;
>
>                 /*

I have only just started testing the recent idle governor changes,
and have not gotten very far yet.

There is a significant increase in processor package power during idle
with this patch, about 5 times increase (400%).

My processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz
Distro: Ubuntu 24.04.1, server, no desktop GUI.
CPU frequency scaling driver: intel_pstate
HWP: disabled.
CPU frequency scaling governor: performance

Idle states:
$ grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/name
/sys/devices/system/cpu/cpu0/cpuidle/state0/name:POLL
/sys/devices/system/cpu/cpu0/cpuidle/state1/name:C1_ACPI
/sys/devices/system/cpu/cpu0/cpuidle/state2/name:C2_ACPI
/sys/devices/system/cpu/cpu0/cpuidle/state3/name:C3_ACPI

Test durations were >= 45 minutes each.

Kernel 6.14-rc1: Includes cpuidle: teo: Cleanups and very frequent wakeups handling update
Average Idle Power: teo governor: 2.199 watts (+25.51%)
Average Idle power: menu governor: 1.873 watts (+6.91%)

Kernel 6.14-rc1-p: Added this patch for teo and "cpuidle: menu: Avoid discarding useful information when processing recent idle intervals" for menu
Average Idle Power: teo governor: 9.401 watts (+436.6%)
Only 69.61% idle is in the deepest idle state. More typically it would be 98% to 99%.
28.6531% idle time is in state 1. More typically it would be 0.03%
Average Idle Power: menu governor: 1.820 watts (+3.9%)

Kernel 6.13: before "cpuidle: teo: Cleanups and very frequent wakeups handling update"
Average Idle Power: teo governor: 1.752 watts (reference: 0.0%)
Average Idle power: menu governor: 1.909 watts (+9.0%)

... Doug