[0/6] defconfig: drop RT_GROUP_SCHED=y

Message

Celeste Liu May 30, 2024, 11:19 a.m. UTC

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]

I leave tools/testing/selftests/bpf/config.{s390x,aarch64} untouched because
I don't whether bpf testing requires it.

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


Celeste Liu (6):
  riscv: defconfig: drop RT_GROUP_SCHED=y
  loongarch: defconfig: drop RT_GROUP_SCHED=y
  mips: defconfig: drop RT_GROUP_SCHED=y from generic/db1xxx/eyeq5
  powerpc: defconfig: drop RT_GROUP_SCHED=y from ppc6xx_defconfig
  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/mips/configs/db1xxx_defconfig         | 1 -
 arch/mips/configs/eyeq5_defconfig          | 1 -
 arch/mips/configs/generic_defconfig        | 1 -
 arch/powerpc/configs/ppc6xx_defconfig      | 1 -
 arch/riscv/configs/defconfig               | 1 -
 arch/sh/configs/sdk7786_defconfig          | 1 -
 arch/sh/configs/urquell_defconfig          | 1 -
 11 files changed, 11 deletions(-)

Comments

Michael Ellerman June 20, 2024, 12:49 p.m. UTC | #1

On Thu, 30 May 2024 19:19:48 +0800, Celeste Liu wrote:
> 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]
> 
> [...]

Patch 4 applied to powerpc/next.

[4/6] powerpc: defconfig: drop RT_GROUP_SCHED=y from ppc6xx_defconfig
      https://git.kernel.org/powerpc/c/2bac6caee94e25f59ee47e2d365d7e07465089ba

cheers