Add kernel cmdline option for rt_group_sched

Despite RT_GROUP_SCHED is only available on cgroup v1, there are still
some (v1-bound) users of this feature. General purpose distros (e.g.
[1][2][3][4]) cannot enable CONFIG_RT_GROUP_SCHED easily:
- since it prevents creation of RT tasks unless RT runtime is determined
  and distributed into cgroup tree,
- grouping of RT threads is not what is desired by default on such
  systems,
- it prevents use of cgroup v2 with RT tasks.

This changeset aims at deferring the decision whether to have
CONFIG_RT_GROUP_SCHED or not up until the boot time.
By default RT groups are available as originally but the user can
pass rt_group_sched=0 kernel cmdline parameter that disables the
grouping and behavior is like with !CONFIG_RT_GROUP_SCHED (with certain
runtime overhead).

The series is organized as follows:

1) generic ifdefs cleanup, no functional changes,
2) preparing root_task_group to be used in places that take shortcuts in
   the case of !CONFIG_RT_GROUP_SCHED,
3) boot cmdline option that controls cgroup (v1) attributes,
4) conditional bypass of non-root task groups,
5) checks and comments refresh.

The crux are patches:
  sched: Skip non-root task_groups with disabled RT_GROUP
  sched: Bypass bandwitdh checks with runtime disabled RT_GROUP_SCHED

Further notes:
- it is not sched_feat() flag because that can be flipped any time
- runtime disablement is not implemented as infinite per-cgroup RT limit
  since that'd still employ group scheduling which is unlike
  !CONFIG_RT_GROUP_SCHED
- there remain two variants of various functions for
  CONFIG_RT_GROUP_SCHED and !CONFIG_RT_GROUP_SCHED, those could be
  folded into one and runtime evaluated guards in the folded functions
  could be used (I haven't posted it yet due to unclear performance
  benefit)
- I noticed some lockdep issues over rt_runtime_lock but those are also
  in an unpatched kernel (and they seem to have been present since a
  long time with CONFIG_RT_GROUP_SCHED)

Changes from RFC (https://lore.kernel.org/r/20241216201305.19761-1-mkoutny@...):
- fix macro CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED invocation
- rebase on torvalds/master

Changes from v1 (https://lore.kernel.org/all/20250210151239.50055-1-mkoutn...)
- add runtime deprecation warning

[1] Debian (https://salsa.debian.org/kernel-team/linux/-/blob/debian/...),
[2] ArchLinux (https://gitlab.archlinux.org/archlinux/packaging/packages...),
[3] Fedora (https://src.fedoraproject.org/rpms/kernel/blob/rawhide/f/...)
[4] openSUSE TW (https://github.com/SUSE/kernel-source/blob/stable/config/...)

Michal Koutný (10):
  sched: Convert CONFIG_RT_GROUP_SCHED macros to code conditions
  sched: Remove unneeed macro wrap
  sched: Always initialize rt_rq's task_group
  sched: Add commadline option for RT_GROUP_SCHED toggling
  sched: Skip non-root task_groups with disabled RT_GROUP_SCHED
  sched: Bypass bandwitdh checks with runtime disabled RT_GROUP_SCHED
  sched: Do not construct nor expose RT_GROUP_SCHED structures if
    disabled
  sched: Add RT_GROUP WARN checks for non-root task_groups
  sched: Add annotations to RT_GROUP_SCHED fields
  sched: Add deprecation warning for users of RT_GROUP_SCHED

 .../admin-guide/kernel-parameters.txt         |  5 ++
 init/Kconfig                                  | 11 +++
 kernel/sched/core.c                           | 70 +++++++++++++++----
 kernel/sched/rt.c                             | 51 +++++++++-----
 kernel/sched/sched.h                          | 34 +++++++--
 kernel/sched/syscalls.c                       |  5 +-
 6 files changed, 138 insertions(+), 38 deletions(-)


base-commit: 69e858e0b8b2ea07759e995aa383e8780d9d140c
-- 
2.48.1