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

Christian Loehle
2025-02-13T14:07:20Z

On 2/4/25 20:58, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> 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@linux.intel.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> 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'm curious, are Doug's numbers reproducible?
Or could you share the idle state usage numbers? Is that explainable?
Seems like a lot and it does worry me that I can't reproduce anything
as drastic.

I did a bit of x86 as well and got for Raptor Lake (I won't post the
non-x86 numbers now, but teo-tweak performs comparable to teo mainline.)

Idle 5 min:
device gov iter Joules idles idle_misses idle_miss_ratio belows aboves
teo 0 170.02 12690 646 0.051 371 275
teo 1 123.17 8361 517 0.062 281 236
teo 2 122.76 7741 347 0.045 262 85
teo 3 118.5 8699 668 0.077 307 361
teo 4 80.46 8113 443 0.055 264 179
teo-tweak 0 115.05 10223 803 0.079 323 480
teo-tweak 1 164.41 8523 631 0.074 263 368
teo-tweak 2 163.91 8409 711 0.085 256 455
teo-tweak 3 137.22 8581 721 0.084 261 460
teo-tweak 4 174.95 8703 675 0.078 261 414
teo 0 164.34 8443 516 0.061 303 213
teo 1 167.85 8767 492 0.056 256 236
teo 2 166.25 7835 406 0.052 263 143
teo 3 189.77 8865 493 0.056 276 217
teo 4 136.97 9185 467 0.051 286 181

At least in the idle case you can see an increase in 'above' idle_misses.

Firefox Youtube 4K video playback 2 min:
device gov iter Joules idles idle_misses idle_miss_ratio belows aboves
teo 0 260.09 67404 11189 0.166 1899 9290
teo 1 273.71 76649 12155 0.159 2233 9922
teo 2 231.45 59559 10344 0.174 1747 8597
teo 3 202.61 58223 10641 0.183 1748 8893
teo 4 217.56 61411 10744 0.175 1809 8935
teo-tweak 0 227.99 61209 11251 0.184 2110 9141
teo-tweak 1 222.44 61959 10323 0.167 1474 8849
teo-tweak 2 218.1 64380 11080 0.172 1845 9235
teo-tweak 3 207.4 60183 11267 0.187 1929 9338
teo-tweak 4 217.46 61253 10381 0.169 1620 8761
menu 0 225.72 87871 26032 0.296 25412 620
menu 1 200.36 86577 24712 0.285 24486 226
menu 2 214.79 84885 24750 0.292 24556 194
menu 3 206.07 88007 25938 0.295 25683 255
menu 4 216.48 88700 26504 0.299 26302 202

(Idle numbers aren't really reflective in energy used -> dominated by
active power.)