Re: sched/fair: DELAY_DEQUEUE causes ~25% pipe IPC regression on Raspberry Pi 5

[Tom Gebhardt]

Hi Mike,

thank you for testing — but I notice that your data only shows 6.12.75
with DELAY_DEQUEUE on/off. There is no 6.6 baseline in your results.
That is the comparison that matters: 6.6 vs 6.12 on the same hardware
with the same tool.

There is also a version difference worth noting: you are running
stress-ng 0.21.00, while our measurements used stress-ng 0.15.06 —
which was the version available on Raspberry Pi OS Bookworm at the
time of the original report. The pipe stressor calculation changed
significantly between those versions.

This creates an uncomfortable coincidence: a measurable scheduler
regression was introduced in 6.12 (DELAY_DEQUEUE), and around the same
time the standard measurement tool changed how it calculates the pipe
benchmark score. The result is that the regression becomes invisible
when comparing with the newer tool version — not because it was fixed,
but because the metric changed.

Our measurements were internally consistent: 6.6.78, 6.12.75, 6.12.81
and 6.18.21 were all compared using the same stress-ng 0.15.06 on the
same hardware. The −34% regression on 6.12 relative to 6.6 is real
within that methodology.

Additionally, our perf stat results are completely independent of
stress-ng version:

Kernel pipe bogo ops/s sched_switch
6.6.78 2 339 827 21 151 454
6.12.81 1 163 540 39 911 525

89% more context switches on 6.12 with DELAY_DEQUEUE active — that is
a kernel difference, not a measurement artifact.

Would you be willing to run the 6.6 comparison with stress-ng 0.15.06
on your Pi 5?

Best regards,
Thomas Gebhardt