linux-kernel - Re: [PATCH v4 4/4] arm: defconfig: drop RT_GROUP

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Message-ID: <967278a1-a2e9-44c3-90ee-ae9a57a54421@broadcom.com>
Date: Thu, 16 Jan 2025 10:04:14 -0800
From: Florian Fainelli <florian.fainelli@...adcom.com>
To: Celeste Liu <uwu@...lacanthus.name>,
 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>,
 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>,
 Thierry Reding <thierry.reding@...il.com>,
 Jonathan Hunter <jonathanh@...dia.com>, Aaro Koskinen
 <aaro.koskinen@....fi>, Andreas Kemnade <andreas@...nade.info>,
 Kevin Hilman <khilman@...libre.com>, Roger Quadros <rogerq@...nel.org>
Cc: 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>,
 Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH v4 4/4] arm: defconfig: drop RT_GROUP_SCHED=y from
 bcm2835/tegra/omap2plus

On 1/14/25 12:41, 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>
> Acked-by: Thierry Reding <treding@...dia.com>
> Signed-off-by: Celeste Liu <uwu@...lacanthus.name>
> ---
>   arch/arm/configs/bcm2835_defconfig   | 1 -

Acked-by: Florian Fainelli <florian.fainelli@...adcom.com> # 
bcm2835_defconfig
-- 
Florian