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

Thierry Reding <[email protected]> Thu, 6 Mar 2025 20:19:46 +0100

Newsgroups	gmane.linux.ports.tegra,gmane.linux.ports.riscv,gmane.linux.kernel,gmane.linux.ports.sh.devel,gmane.linux.ports.arm.kernel,gmane.linux.kernel.rpi,gmane.linux.ports.arm.omap
Message-ID	<t2dustbykx2qd24wazjeiw5hch5nwr6z2ewmaf4srg6r2grwrf@rdw47chzkef2>

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 <[email protected]>
> Acked-by: Kevin Hilman <khilman-rdvid1DuHRBWk0Htik3J/[email protected]>
> Acked-by: Thierry Reding <[email protected]>
> Signed-off-by: Celeste Liu <[email protected]>
> ---
>  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.

Thierry

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