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

Andrea Righi
2025-10-29T19:11:34Z

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