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 <lbckmnn@mailbox.org>]

On Thu, May 21, 2026 at 12:49:52PM -0400, Sasha Levin wrote:
> 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 :)
Done in 
https://lore.kernel.org/stable/20260522213120.1205100-1-lbckmnn@mailbox.org/
> -- 
> Thanks,
> Sasha

Thanks,
Lukas