[PATCH v3 0/8] sched: RT throttling cleanup and a bit of documentation

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

People used to disable the RT throttling mechanism by writing -1 to
/proc/sys/kernel/sched_rt_runtime_us. Since v6.12 they can still do this
but they don't disable RT throttling anymore. Instead this throttling
mechanism has been taken over by the fair_server which can be configured
via debugfs.
This is not really documented by a hint appeared a few cycles later in
Documentation/scheduler/sched-rt-group.rst. This can be found once you
know what you are looking for because this document describes the RT
group scheduling (CONFIG_RT_GROUP_SCHED). The fair_server is used even
without it.
The then there is global DL resource control which also shares this
file proc file (which is stuck if CONFIG_RT_GROUP_SCHED is enabled).

This little series:
- Make sure the fair_serve configuration can only be changed while the
  CPU is active. Mathieu tries to fix this and allow it instead. That
  would be an alternative.

- Marks CONFIG_RT_GROUP_SCHED as broken because I was not allowed to
  remove it.

- Untangles DL usage of sched_rt_runtime_us and moves it to
    /sys/kernel/sched/dl_period_us
    /sys/kernel/sched/dl_runtime_us

  This keeps the _us sematic that we used to have /proc. It _could_ be
  changed to ns instead. Or replaced with a per-CPU/RQ setting if this
  is simpler/ preferred on the DL side.

- Moves rcutorture away from the procfs interface since it tries to
  shutdown the fair_server for its testing.

- Adds sysfs interface under /sys/kernel/sched/cpuX to represent the
  runqueues. The only knob for now is fair_server_{period|runtime}
  representing the fair_server parameters. If all goes well, debugfs
  could be removed. The ext server could be moved, too.

- I tried to correct/ update documentation while at it. I added the -doc
  folks on Cc so they are aware. I can drop them in the next posting, I
  expect that this gets routed via the sched/-tip.

v2…v3: https://patch.msgid.link/20261110-sched_dl-v2-0-cc9399f7aeb1@linutronix.de
  - Corrected in wording sched-rt-group.rst (Pointed out by Randy)
  - RCU-torture now disables the fair_server from its boost-thread. The
    code expects the CPU to be active which is after the CPU-hotplug
    notifier is run. An alternative would be to not start the
    fair_server (in the RCU-torture built-in case).
  - sched_dl_try_boundary() has now a private mutex to perform the
    update. sashiko complains that the values are read outside of
    sched_domains_mutex and written back while it is held, making it
    possible to write stale period/ runtime values back.
  - proper u64 casting for NSEC_PER_USEC multiplication.
  - sysfs_dl_runtime is not static and accessed in
    dl_bandwidth_enabled(). This was missed earlier.
    The remaining sysctl_sched_rt_{runtime|period} are now also hidden
    behind CONFIG_RT_GROUP_SCHED.

v1…v2: https://lore.kernel.org/all/20261105164724.yxONBd03@linutronix.de/
  - Tejun and PeterZ asked for marking CONFIG_RT_GROUP_SCHED broken
    first.
  - Everything after patch #1 that got queued up.

RFC…v1: https://lore.kernel.org/all/20261023161348.t3lpPLQZ@linutronix.de/
  - Try to remove RT-GROUP-sched after figuring out that it is useless.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

---
Sebastian Andrzej Siewior (8):
      sched/deadline: Reject debugfs dl_server writes for not-active CPUs.
      sched: Mark RT_GROUP_SCHED broken
      rcutorture: Don't try to disable to RT throttling.
      sched: rcutorture: Allow to disable DL fair_server
      sched: Move DL's admission control to sysfs
      sched: Limit sysctl sched_rt_{period|runtime}_us to RT_GROUP_SCHED
      sched: Add sysfs files for the per-CPU fair_server knobs
      Documentation/sched: Add fairserver to sched-deadline

 Documentation/arch/arm64/asymmetric-32bit.rst |   2 +-
 Documentation/scheduler/sched-deadline.rst    |  36 ++-
 Documentation/scheduler/sched-rt-group.rst    |   7 +-
 init/Kconfig                                  |   2 +-
 kernel/rcu/rcu.h                              |   2 -
 kernel/rcu/rcutorture.c                       |  37 +--
 kernel/sched/deadline.c                       | 346 +++++++++++++++++++++++++-
 kernel/sched/debug.c                          |  56 +----
 kernel/sched/features.h                       |   2 +
 kernel/sched/rt.c                             |  65 ++---
 kernel/sched/sched.h                          |  63 +++--
 11 files changed, 439 insertions(+), 179 deletions(-)
---
base-commit: fd9230005e85472b81c1c7c16afe93dd878996b1
change-id: 20261110-sched_dl-a9727787112f

Best regards,
--  
Sebastian Andrzej Siewior <bigeasy@linutronix.de>