Re: [REGRESSION] 6.12.y: d66792919d4f (sched/deadline: Use revised wakeup rule for dl_server) causes latencies up to 50ms with PREEMPT_RT

On Sun, 2026-05-10 at 22:57 +0200, Lukas Beckmann wrote:
> Hi,

Greetings!

> I am reporting a regression which was introduced by d66792919d4f on 6.12.y.
> Since this commit, cyclictest reports latencies up to 50 milliseconds, 
> on kernels with CONFIG_PREEMPT_RT=y.
> 
> Steps to reproduce:
> 1. run a load (e.g. stress-ng --cpu 4 --io 2 --vm 2 --vm-bytes 128M)
> 2. run cyclictest (e.g. cyclictest -a -t -m -p 80 -i 250 -d 0)

...


> I found this, because Debian updated its rt kernel from 6.12.74 to 6.12.85.
> The issue was also present with upstream 6.12.85 and HEAD, but not with 
> 6.12.74, so I started bisecting and eventually found d66792919d4f.
> 
> Is it possible to revert the commit?
> 
> I can provide traces or help wth testing if needed.

FWIW, my box says this *may* be due to a fix (+follow-ups) that didn't
wander back to stable.

cccb45d7c429 ("sched/deadline: Less agressive dl_server handling")
   Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")

Follow-ups:
4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")
a3a70caf7906 ("sched/deadline: Fix dl_server behaviour")

Local 6.12-rt tree containing the above (et al) failed to reproduce in
the hour I let it try to, whereas a build excluding only all locally
added fix backports reproduced in fairly short order.

Suspects NOT confirmed in total isolation...

	-Mike