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.
== Highlights in this version ==
- wait for inactive_task_timer() to fire before removing the bandwidth reservation (Juri/Peter: please check if this new dl_server_remove_params() implementation makes sense to you) - removed the explicit dl_server_stop() from dequeue_task_scx() and rely on the delayed stop behavior (Juri/Peter: ditto)
This patchset is also available in the following git branch:
git://git.kernel.org/pub/scm/linux/kernel/git/arighi/linux.git scx-dl-server
Changes in v10: - reordered patches to better isolate sched_ext changes vs sched/deadline changes (Andrea Righi) - define ext_server only with CONFIG_SCHED_CLASS_EXT=y (Andrea Righi) - add WARN_ON_ONCE(!cpus) check in dl_server_apply_params() (Andrea Righi) - wait for inactive_task_timer to fire before removing the bandwidth reservation (Juri Lelli) - remove explicit dl_server_stop() in dequeue_task_scx() to reduce timer reprogramming overhead (Juri Lelli) - do not restart pick_task() when invoked by the dl_server (Tejun Heo) - rename rq_dl_server to dl_server (Peter Zijlstra) - fixed a missing dl_server start in dl_server_on() (Christian Loehle) - add a comment to the rt_stall selftest to better explain the 4% threshold (Emil Tsalapatis)
Changes in v9: - Drop the ->balance() logic as its functionality is now integrated into ->pick_task(), allowing dl_server to call pick_task_scx() directly - Link to v8: https://lore.kernel.org/all/20250903095008.162049-1-arighi@nvidia.com/
Changes in v8: - Add tj's patch to de-couple balance and pick_task and avoid changing sched/core callbacks to propagate @rf - Simplify dl_se->dl_server check (suggested by PeterZ) - Small coding style fixes in the kselftests - Link to v7: https://lore.kernel.org/all/20250809184800.129831-1-joelagnelf@nvidia.com/
Changes in v7: - Rebased to Linus master - Link to v6: https://lore.kernel.org/all/20250702232944.3221001-1-joelagnelf@nvidia.com/
Changes in v6: - Added Acks to few patches - Fixes to few nits suggested by Tejun - Link to v5: https://lore.kernel.org/all/20250620203234.3349930-1-joelagnelf@nvidia.com/
Changes in v5: - Added a kselftest (total_bw) to sched_ext to verify bandwidth values from debugfs - Address comment from Andrea about redundant rq clock invalidation - Link to v4: https://lore.kernel.org/all/20250617200523.1261231-1-joelagnelf@nvidia.com/
Changes in v4: - Fixed issues with hotplugged CPUs having their DL server bandwidth altered due to loading SCX - Fixed other issues - Rebased on Linus master - All sched_ext kselftests reliably pass now, also verified that the total_bw in debugfs (CONFIG_SCHED_DEBUG) is conserved with these patches - Link to v3: https://lore.kernel.org/all/20250613051734.4023260-1-joelagnelf@nvidia.com/
Changes in v3: - Removed code duplication in debugfs. Made ext interface separate - Fixed issue where rq_lock_irqsave was not used in the relinquish patch - Fixed running bw accounting issue in dl_server_remove_params - Link to v2: https://lore.kernel.org/all/20250602180110.816225-1-joelagnelf@nvidia.com/
Changes in v2: - Fixed a hang related to using rq_lock instead of rq_lock_irqsave - Added support to remove BW of DL servers when they are switched to/from EXT - Link to v1: https://lore.kernel.org/all/20250315022158.2354454-1-joelagnelf@nvidia.com/
Andrea Righi (5): sched/deadline: Add support to initialize and remove dl_server bandwidth sched_ext: Add a DL server for sched_ext tasks sched/deadline: Account ext server bandwidth sched_ext: Selectively enable ext and fair DL servers selftests/sched_ext: Add test for sched_ext dl_server
Joel Fernandes (6): sched/debug: Fix updating of ppos on server write ops sched/debug: Stop and start server based on if it was active sched/deadline: Clear the defer params sched/deadline: Add a server arg to dl_server_update_idle_time() sched/debug: Add support to change sched_ext server params selftests/sched_ext: Add test for DL server total_bw consistency
kernel/sched/core.c | 3 + kernel/sched/deadline.c | 169 +++++++++++--- kernel/sched/debug.c | 171 +++++++++++--- kernel/sched/ext.c | 144 +++++++++++- kernel/sched/fair.c | 2 +- kernel/sched/idle.c | 2 +- kernel/sched/sched.h | 8 +- kernel/sched/topology.c | 5 + tools/testing/selftests/sched_ext/Makefile | 2 + tools/testing/selftests/sched_ext/rt_stall.bpf.c | 23 ++ tools/testing/selftests/sched_ext/rt_stall.c | 222 ++++++++++++++++++ tools/testing/selftests/sched_ext/total_bw.c | 281 +++++++++++++++++++++++ 12 files changed, 955 insertions(+), 77 deletions(-) create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c create mode 100644 tools/testing/selftests/sched_ext/total_bw.c