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

[Mike Galbraith <efault@gmx.de>]

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On Tue, 2026-04-21 at 05:55 +0200, Mike Galbraith wrote:
> On Mon, 2026-04-20 at 12:59 +0200, Tom Gebhardt wrote:
> 
> 
> > Would you be willing to run the 6.6 comparison with stress-ng 0.15.06
> > on your Pi 5?
> 
> No need, your reported DELAY_DEQUEUE impact for 6.12.75+rpt-rpi-2712
> appeared with stress-ng 0.15.06.
> 
> However, my local 6.12.y branch kernel, which has all eevdf fixes as
> well as Peter's still pending sched/ttwu series, still does NOT repro.
> 
> git@homer:..git/raspberrypi-kernel> quilt applied|grep delay
> patches/eevdf/WIP/0043-sched-fair-Removed-unsued-cfs_rq.h_nr_delayed.patch
> patches/eevdf/WIP/0047-sched-fair-Do-not-try-to-migrate-delayed-dequeue-task.patch           <== ?
> patches/eevdf/WIP/0065-sched-Change-ttwu_runnable-vs-sched_delayed.patch
> patches/eevdf/WIP/0066-sched-Add-ttwu_queue-support-for-delayed-tasks.patch
> patches/eevdf/WIP/0089-sched-fair-Prevent-negative-lag-increase-during-delayed-dequeue.patch <== ?

I've finally finished slogging through my quilt backport battery, and
my dinky/cute rpi5 asserts that what prevents the local heavily patched
rpi-6.12-y@6.12.81 kernel from reproducing has nothing to do with
pointed at suspects, rather Peter's long pending ttwu series is
entirely responsible.

Validation enabling tarball attached.

	-Mike

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