Re: [PATCH v4 4/4] arm: defconfig: drop RT_GROUP_SCHED=y from bcm2835/tegra/omap2plus

[Huacai Chen]

On Fri, Mar 7, 2025 at 3:19 AM Thierry Reding <thierry.reding@xxxxxxxxx> wrote:
>
> On Wed, Jan 15, 2025 at 04:41:23AM +0800, Celeste Liu wrote:
> > Commit 673ce00c5d6c ("ARM: omap2plus_defconfig: Add support for distros
> > with systemd") said it's because of recommendation from systemd. But
> > systemd changed their recommendation later.[1]
> >
> > 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.[2]
> >
> > 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.
> >
> > [1]: https://github.com/systemd/systemd/commit/f4e74be1856b3ac058acbf1be321c31d5299f69f
> > [2]: https://bugzilla.redhat.com/show_bug.cgi?id=1229700
> >
> > Tested-by: Stefan Wahren <wahrenst@xxxxxxx>
> > Acked-by: Kevin Hilman <khilman@xxxxxxxxxxxx>
> > Acked-by: Thierry Reding <treding@xxxxxxxxxx>
> > Signed-off-by: Celeste Liu <uwu@xxxxxxxxxxxxxxxxx>
> > ---
> >  arch/arm/configs/bcm2835_defconfig   | 1 -
> >  arch/arm/configs/omap2plus_defconfig | 1 -
> >  arch/arm/configs/tegra_defconfig     | 1 -
> >  3 files changed, 3 deletions(-)
>
> Hi Arnd,
>
> is this something that you could pick up? I think so far only the RISC-V
> patch was picked up, but nobody seems to feel responsible for the ARM
> patch here.
I will take the LoongArch patch, but may be squashed to another one
about config file update.

Huacai

>
> Thierry