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

[Andrea Righi <arighi@nvidia.com>]

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