Re: [PATCH 6/6] arm: defconfig: drop RT_GROUP_SCHED=y from bcm2855/tegra/omap2plus

From: Kevin Hilman <khilman@kernel.org>
To: "Celeste Liu" <coelacanthushex@gmail.com>,
	"Heinrich Schuchardt" <heinrich.schuchardt@canonical.com>,
	"Anup Patel" <anup@brainfault.org>, "Guo Ren" <guoren@kernel.org>,
	"Palmer Dabbelt" <palmer@rivosinc.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Huacai Chen" <chenhuacai@kernel.org>,
	"WANG Xuerui" <kernel@xen0n.name>,
	"Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
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	"Gregory CLEMENT" <gregory.clement@bootlin.com>,
	"Théo Lebrun" <theo.lebrun@bootlin.com>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Christophe Leroy" <christophe.leroy@csgroup.eu>,
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	"Broadcom internal kernel review list"
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	linux-riscv@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	linux-rpi-kernel@lists.infradead.org, linux-omap@vger.kernel.org,
	linux-tegra@vger.kernel.org, loongarch@lists.linux.dev,
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Cc: linux-kselftest@vger.kernel.org, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Lennart Poettering <lennart@poettering.net>,
	Icenowy Zheng <uwu@icenowy.me>,
	Celeste Liu <CoelacanthusHex@gmail.com>
Subject: Re: [PATCH 6/6] arm: defconfig: drop RT_GROUP_SCHED=y from bcm2855/tegra/omap2plus
Date: Mon, 08 Jul 2024 16:49:01 -0700	[thread overview]
Message-ID: <7hv81f78cy.fsf@baylibre.com> (raw)
In-Reply-To: <20240530111947.549474-14-CoelacanthusHex@gmail.com>

Celeste Liu <coelacanthushex@gmail.com> writes:

> 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
>
> Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
> ---
>  arch/arm/configs/bcm2835_defconfig   | 1 -
>  arch/arm/configs/omap2plus_defconfig | 1 -
>  arch/arm/configs/tegra_defconfig     | 1 -

For omap2plus_defconfig:

Acked-by: Kevin Hilman <khilman@baylibre.com>