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

[Mike Galbraith]

Greetings,

On Thu, 2026-04-16 at 14:22 +0200, Tom Gebhardt wrote:
> Hi Peter,
> 
> I would like to report a measurable pipe IPC throughput regression introduced by
> commit 152e11f ("sched/fair: Implement delayed dequeue"), first
> present in v6.12-rc1.
> 
> This has been independently confirmed on the official Raspberry Pi
> Linux issue tracker
> (raspberrypi/linux #7308), where the RPi kernel team directed the
> issue upstream.
> 
> 
> Hardware / Software
> -------------------
> - Raspberry Pi 5 Model B, BCM2712 (C1 stepping), 8 GB RAM
> - Raspberry Pi OS Bookworm (arm64)
> - Kernels tested: 6.6.78-v8-16k+ (rpi-6.6.y), 6.12.75+rpt-rpi-2712,
> 6.12.81-v8-16k+ (custom)
> - Benchmark: stress-ng 0.15.06, --pipe 4 --timeout 20s --metrics-brief
> 
> 
> Observed regression
> -------------------
> Comparing pipe IPC throughput across kernels (overclocked, arm_freq=2800):
> 
>   Kernel        pipe bogo ops/s   vs. 6.6
>   6.6.78            2 487 746      100%
>   6.12.75           1 651 427      -34%
>   6.18.21           2 049 701      -18%
> 
> This regression pattern is consistent across two separate Raspberry Pi
> 5 units and has
> been independently reproduced by the RPi kernel team with 20-run averages:
> 6.6=2065 Kops/s, 6.12=1662, 6.18=1805, 7.0=1570 (lowest).

Does reproduction require more than the posted stress-ng args?

Raw data from my pi5+bookworm+6.12.75+rpt-rpi-2712 (no overclock)

rpi5:..debug/sched # stress-ng --version
stress-ng, version 0.21.00 (gcc 12.2.0, aarch64 Linux 6.12.75+rpt-rpi-2712) 💻🔥
rpi5:..debug/sched # stress-ng --pipe 4 --timeout 20s --metrics-brief
stress-ng: info:  [2965] setting to a 20 secs run per stressor
stress-ng: info:  [2965] dispatching hogs: 4 pipe
stress-ng: metrc: [2965] stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
stress-ng: metrc: [2965]                           (secs)    (secs)    (secs)   (real time) (usr+sys time)
stress-ng: metrc: [2965] pipe           57731375     20.00      6.69     73.24   2886616.84      722246.62
stress-ng: info:  [2965] skipped: 0
stress-ng: info:  [2965] passed: 4: pipe (4)
stress-ng: info:  [2965] failed: 0
stress-ng: info:  [2965] metrics untrustworthy: 0
stress-ng: info:  [2965] successful run completed in 20.00 secs
rpi5:..debug/sched # echo NO_DELAY_DEQUEUE > features
rpi5:..debug/sched # stress-ng --pipe 4 --timeout 20s --metrics-brief
stress-ng: info:  [3048] setting to a 20 secs run per stressor
stress-ng: info:  [3048] dispatching hogs: 4 pipe
stress-ng: metrc: [3048] stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
stress-ng: metrc: [3048]                           (secs)    (secs)    (secs)   (real time) (usr+sys time)
stress-ng: metrc: [3048] pipe           58814508     20.00      6.78     73.17   2940713.90      735679.81
stress-ng: info:  [3048] skipped: 0
stress-ng: info:  [3048] passed: 4: pipe (4)
stress-ng: info:  [3048] failed: 0
stress-ng: info:  [3048] metrics untrustworthy: 0
stress-ng: info:  [3048] successful run completed in 20.00 secs
rpi5:..debug/sched # echo DELAY_DEQUEUE > features
rpi5:..debug/sched # stress-ng --pipe 4 --timeout 20s --metrics-brief
stress-ng: info:  [3349] setting to a 20 secs run per stressor
stress-ng: info:  [3349] dispatching hogs: 4 pipe
stress-ng: metrc: [3349] stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
stress-ng: metrc: [3349]                           (secs)    (secs)    (secs)   (real time) (usr+sys time)
stress-ng: metrc: [3349] pipe           58058262     20.00      6.62     73.32   2902974.63      726327.83
stress-ng: info:  [3349] skipped: 0
stress-ng: info:  [3349] passed: 4: pipe (4)
stress-ng: info:  [3349] failed: 0
stress-ng: info:  [3349] metrics untrustworthy: 0
stress-ng: info:  [3349] successful run completed in 20.00 secs