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

[Sasha Levin <sashal@kernel.org>]

On Sun, May 10, 2026 at 10:57:46PM +0200, Lukas Beckmann wrote:
> 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.
[...]
> Is it possible to revert the commit?
>
> I can provide traces or help with testing if needed.

Thanks for the detailed report. Before I revert d66792919d4f from 6.12.y,
I'd like to confirm whether the underlying issue is the missing dl_server
rework chain on 6.12.y rather than the revised wakeup rule itself.

Mike's reply notes that his local 6.12-rt tree carrying the following
three commits in cannot reproduce, while the same tree without them
reproduces quickly:

  cccb45d7c429 ("sched/deadline: Less agressive dl_server handling")
  4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")
  a3a70caf7906 ("sched/deadline: Fix dl_server behaviour")

d66792919d4f's upstream commit message explicitly says it relies on the
state established by a3a70caf7906, and none of the three are in 6.12.y.

Could you give those three commits a spin on top of 6.12.y (keeping
d66792919d4f in place) and see whether the latency goes away?

-- 
Thanks,
Sasha