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

[Andrea Righi]

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