linux-kernel - [PATCH v3 1/4] riscv: defconfig: drop RT_GROUP

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Message-Id: <20240910-fix-riscv-rt_group_sched-v3-1-486e75e5ae6d@gmail.com>
Date: Tue, 10 Sep 2024 20:51:07 +0800
From: Celeste Liu <coelacanthushex@...il.com>
To: 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>, Thierry Reding <thierry.reding@...il.com>, 
 Jonathan Hunter <jonathanh@...dia.com>
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, Celeste Liu <CoelacanthusHex@...il.com>
Subject: [PATCH v3 1/4] riscv: defconfig: drop RT_GROUP_SCHED=y

Commit ba6cfef057e1 ("riscv: enable Docker requirements in defconfig")
introduced it because of Docker, but Docker has removed this requirement
since [1] (2023-04-19).

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.[3]

[1]: https://github.com/moby/moby/commit/005150ed69c540fb0b5323e0f2208608c1204536
[2]: https://bugzilla.redhat.com/show_bug.cgi?id=1229700
[3]: https://github.com/systemd/systemd/issues/13781#issuecomment-549164383

Acked-by: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
Signed-off-by: Celeste Liu <CoelacanthusHex@...il.com>
---
 arch/riscv/configs/defconfig | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/riscv/configs/defconfig b/arch/riscv/configs/defconfig
index 12dc8c73a8acfaa5c8f442968a807de303428d9e..de85c3ab261e6d62b2089a3c89bdc9d1b34fa792 100644
--- a/arch/riscv/configs/defconfig
+++ b/arch/riscv/configs/defconfig
@@ -9,7 +9,6 @@ CONFIG_CGROUPS=y
 CONFIG_MEMCG=y
 CONFIG_CGROUP_SCHED=y
 CONFIG_CFS_BANDWIDTH=y
-CONFIG_RT_GROUP_SCHED=y
 CONFIG_CGROUP_PIDS=y
 CONFIG_CGROUP_FREEZER=y
 CONFIG_CGROUP_HUGETLB=y

-- 
2.46.0