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

[Peter Zijlstra]

On Thu, Apr 23, 2026 at 06:51:20AM +0200, Mike Galbraith wrote:
> 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.

Humm, I'd almost forgotten about that. I suppose I'll have to go dust
that off again.