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

Andrea Righi
2025-11-05T13:47:53Z

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?

Thanks,
-Andrea