linux-kernel - Re: [PATCH v3 4/4] arm: defconfig: drop RT_GROUP

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Message-ID: <6m77wg6b76abyk2ebfniayxvrabvzur7onwar4isajb6tvfxbv@7a24p2xkmevj>
Date: Thu, 24 Oct 2024 19:10:23 +0200
From: Thierry Reding <thierry.reding@...il.com>
To: Celeste Liu <coelacanthushex@...il.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>, 
	Anup Patel <anup@...infault.org>, Heinrich Schuchardt <heinrich.schuchardt@...onical.com>, 
	Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
	Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, 
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>, Russell King <linux@...linux.org.uk>, 
	Florian Fainelli <florian.fainelli@...adcom.com>, 
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, Ray Jui <rjui@...adcom.com>, 
	Scott Branden <sbranden@...adcom.com>, Tony Lindgren <tony@...mide.com>, 
	Jonathan Hunter <jonathanh@...dia.com>, Palmer Dabbelt <palmer@...osinc.com>, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-sh@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-rpi-kernel@...ts.infradead.org, linux-omap@...r.kernel.org, linux-tegra@...r.kernel.org, 
	Stefan Wahren <wahrenst@....net>, Kevin Hilman <khilman@...libre.com>
Subject: Re: [PATCH v3 4/4] arm: defconfig: drop RT_GROUP_SCHED=y from
 bcm2835/tegra/omap2plus

On Tue, Sep 10, 2024 at 08:51:10PM +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@....net>
> Acked-by: Kevin Hilman <khilman@...libre.com>
> Signed-off-by: Celeste Liu <CoelacanthusHex@...il.com>
> ---
>  arch/arm/configs/bcm2835_defconfig   | 1 -
>  arch/arm/configs/omap2plus_defconfig | 1 -
>  arch/arm/configs/tegra_defconfig     | 1 -
>  3 files changed, 3 deletions(-)

Any idea who will want to pick this up? Probably something the ARM SoC
maintainers can do directly, in which case:

Acked-by: Thierry Reding <treding@...dia.com>

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