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

On Tue, May 12, 2026 at 12:08:49AM +0200, Lukas Beckmann wrote:
> 
> On 5/11/26 16:21, Sasha Levin wrote:
> > 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?
> 
> If I apply the three commits on 6.12.y, the latencies indeed go away.
> This is running for a few hours now, and the latencies showed up after 30
> minutes tops, with plain 6.12.y before.
> I will leave this running.
> 
> Note:
> I also tried applying only cccb45d7c429 ("sched/deadline: Less agressive
> dl_server handling") before, and that also seems to fix the issue.
> 
> Thanks
> Lukas

Hey Sasha,

Cyclictest is still running and looking good (latency-wise).
How should we proceed?

Thanks
Lukas