Re: [PATCH v2 00/13] sched/fair/schedutil: Better manage system response time

[Tom Gebhardt <tomge68@gmail.com>]

Hi Christian,

Good point -- I ran additional tests with `performance` and `ondemand` governors
side by side on the same kernel (7.0.0 + ttwu + patch 12 only):

  Clock     Governor      pipe bogo ops/s
  --------  ------------  ----------------
  2400 MHz  performance       2 095 187
  2400 MHz  ondemand          2 093 221
  2800 MHz  performance       2 415 817
  2800 MHz  ondemand          2 415 617

The difference between governors is <0.1% -- well within noise. So you are
right: the effect is not cpufreq-related. Whatever patch 12 changes, it
affects the scheduler path directly, not through frequency selection.

I also applied Vincent's fix [1] and benchmarked it:

  Kernel                  Clock     pipe bogo ops/s   Δ vs. 6.6.78
  ----------------------  --------  ----------------  ------------
  6.6.78                  2400 MHz       2 129 330        ±0%
  6.6.78                  2800 MHz       2 487 746        ±0%
  7.0 + ttwu + patch 12   2400 MHz       2 093 221        −1.7%
  7.0 + ttwu + patch 12   2800 MHz       2 415 617        −2.9%
  7.0 + ttwu + Vincent    2400 MHz       2 077 526        −2.4%
  7.0 + ttwu + Vincent    2800 MHz       2 458 151        −1.2%

Vincent's fix gets very close to 6.6 at 2800 MHz (−1.2%) and is similar to
patch 12 at 2400 MHz. Both are a large improvement over vanilla 7.0+ttwu
(−22% at 2800 MHz) and plain 7.0 stock (−26% at 2800 MHz).

Note: [1] applied with a manual context fixup for the DELAY_DEQUEUE hunk --
the rpi-7.0.y tree's dequeue_entity() differs slightly from mainline in that
block (no update_entity_lag() call inside the DELAY_DEQUEUE early-return).
The semantic intent of the hunk was preserved.

[1] https://lore.kernel.org/lkml/agRyoe1wHyZ-vMk9@vingu-cube/

Thanks for catching that and for offering to reproduce it.

Tom