Re: [PATCHSET v10 sched_ext/for-6.19] Add a deadline server for sched_ext tasks

Peter Zijlstra
2025-11-05T13:59:19Z

On Wed, Nov 05, 2025 at 02:47:39PM +0100, Andrea Righi wrote:
> On Wed, Oct 29, 2025 at 08:08:37PM +0100, Andrea Righi wrote:
> > sched_ext tasks can be starved by long-running RT tasks, especially since
> > RT throttling was replaced by deadline servers to boost only SCHED_NORMAL
> > tasks.
> >
> > Several users in the community have reported issues with RT stalling
> > sched_ext tasks. This is fairly common on distributions or environments
> > where applications like video compositors, audio services, etc. run as RT
> > tasks by default.
> >
> > Example trace (showing a per-CPU kthread stalled due to the sway Wayland
> > compositor running as an RT task):
> >
> > runnable task stall (kworker/0:0[106377] failed to run for 5.043s)
> > ...
> > CPU 0 : nr_run=3 flags=0xd cpu_rel=0 ops_qseq=20646200 pnt_seq=45388738
> > curr=sway[994] class=rt_sched_class
> > R kworker/0:0[106377] -5043ms
> > scx_state/flags=3/0x1 dsq_flags=0x0 ops_state/qseq=0/0
> > sticky/holding_cpu=-1/-1 dsq_id=0x8000000000000002 dsq_vtime=0 slice=20000000
> > cpus=01
> >
> > This is often perceived as a bug in the BPF schedulers, but in reality
> > schedulers can't do much: RT tasks run outside their control and can
> > potentially consume 100% of the CPU bandwidth.
> >
> > Fix this by adding a sched_ext deadline server, so that sched_ext tasks are
> > also boosted and do not suffer starvation.
> >
> > Two kselftests are also provided to verify the starvation fixes and
> > bandwidth allocation is correct.
>
> Peter, Juri, this has now been tested quite extensively on our side and
> we're considering applying these patches to Tejun's sched_ext branch.
>
> Do you have any objections or concerns?

Yeah, I want to finish this first:

https://lkml.kernel.org/r/20251101000057.GA2184199@noisy.programming.kicks-ass.net

Because as is, the whole dl_server stuff isn't quite right.