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

[Andrea Righi]

On Wed, Nov 05, 2025 at 03:20:50PM +0100, Juri Lelli wrote:
> On 05/11/25 14:59, Peter Zijlstra wrote:
> > 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > Because as is, the whole dl_server stuff isn't quite right.
> > 
> 
> And I'm spending time on "[PATCH 04/11] sched/deadline: Add support to
> initialize and remove dl_server bandwidth" which I am still not 100%
> sure is correct (or that is correct how we handle setting runtime to 0
> for fair_server today). Apologies, had some travelling and pto, but
> should be able to write something more about it in the next few days.

No problem and no rush, I just wanted to follow up to make sure I didn't
miss anything. :)

Thank you both,
–Andrea