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
