For cgroup v1, if turned on, and there's any cgroup in the "cpu" hierarchy it
needs an RT budget assigned, otherwise the processes in it will not be able to
get RT at all. The problem with RT group scheduling is that it requires the
budget assigned but there's no way we could assign a default budget, since the
values to assign are both upper and lower time limits, are absolute, and need to
be sum up to < 1 for each individal cgroup. That means we cannot really come up
with values that would work by default in the general case.[1]

For cgroup v2, it's almost unusable as well. If it turned on, the cpu controller
can only be enabled when all RT processes are in the root cgroup. But it will
lose the benefits of cgroup v2 if all RT process were placed in the same cgroup.

Red Hat, Gentoo, Arch Linux and Debian all disable it. systemd also doesn't
support it.[2]

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1229700
[2]: https://github.com/systemd/systemd/issues/13781#issuecomment-549164383

---
Changes in v3:
- Remove Fixes: tag because of it's not suitable.
- Link to v2: https://lore.kernel.org/r/20240823-fix-riscv-rt_group_sched-v2-0-e4dbae24f7e1@gmail.com

Changes in v2:
* Remove MIPS and PowerPC patch because of merged.
* Link to v1: https://lore.kernel.org/lkml/20240530111947.549474-8-CoelacanthusHex@gmail.com/T

Celeste Liu (6):
  riscv: defconfig: drop RT_GROUP_SCHED=y
  loongarch: defconfig: drop RT_GROUP_SCHED=y
  sh: defconfig: drop RT_GROUP_SCHED=y from sdk7786/urquell
  arm: defconfig: drop RT_GROUP_SCHED=y from bcm2855/tegra/omap2plus

 arch/arm/configs/bcm2835_defconfig         | 1 -
 arch/arm/configs/omap2plus_defconfig       | 1 -
 arch/arm/configs/tegra_defconfig           | 1 -
 arch/loongarch/configs/loongson3_defconfig | 1 -
 arch/riscv/configs/defconfig               | 1 -
 arch/sh/configs/sdk7786_defconfig          | 1 -
 arch/sh/configs/urquell_defconfig          | 1 -
 7 files changed, 7 deletions(-)

--
2.45.1

---
Celeste Liu (4):
      riscv: defconfig: drop RT_GROUP_SCHED=y
      loongarch: defconfig: drop RT_GROUP_SCHED=y
      sh: defconfig: drop RT_GROUP_SCHED=y from sdk7786/urquell
      arm: defconfig: drop RT_GROUP_SCHED=y from bcm2835/tegra/omap2plus

 arch/arm/configs/bcm2835_defconfig         | 1 -
 arch/arm/configs/omap2plus_defconfig       | 1 -
 arch/arm/configs/tegra_defconfig           | 1 -
 arch/loongarch/configs/loongson3_defconfig | 1 -
 arch/riscv/configs/defconfig               | 1 -
 arch/sh/configs/sdk7786_defconfig          | 1 -
 arch/sh/configs/urquell_defconfig          | 1 -
 7 files changed, 7 deletions(-)
---
base-commit: afcd48134c58d6af45fb3fdb648f1260b20f2326
change-id: 20240627-fix-riscv-rt_group_sched-d2e014be48da

Best regards,
-- 
Celeste Liu <CoelacanthusHex@gmail.com>