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

[Lukas Beckmann <lukas.beckmann@mailbox.org>]

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