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 Thu, May 21, 2026 at 09:32:32AM +0200, Thorsten Leemhuis wrote:
>On 5/16/26 21:50, Lukas Beckmann wrote:
>> 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.
>>
>> Cyclictest is still running and looking good (latency-wise).
>> How should we proceed?
>
>Sasha, just wondering: is this still in your queue? It sounds like a
>clear case to pick those three up for 6.12.y (everybody: please correct
>me if I'm wrong). Or are you busy and should we ask Greg to pick them up?

Nope, sorry, I got sidetracked by a large series of commits from the merge window.

I'll plan to queue this up for the next release. It'll be even better if
someone can send us a more "official" backport request, ideally with a
tested-by too :)

-- 
Thanks,
Sasha