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

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Message-Id: <20240910-fix-riscv-rt_group_sched-v3-4-486e75e5ae6d@gmail.com>
Date: Tue, 10 Sep 2024 20:51:10 +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, Stefan Wahren <wahrenst@....net>, 
 Kevin Hilman <khilman@...libre.com>, 
 Celeste Liu <CoelacanthusHex@...il.com>
Subject: [PATCH v3 4/4] arm: defconfig: drop RT_GROUP_SCHED=y from
 bcm2835/tegra/omap2plus

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(-)

diff --git a/arch/arm/configs/bcm2835_defconfig b/arch/arm/configs/bcm2835_defconfig
index b5f0bd8dd536990a0726e73a6855d25c1286a768..27dc3bf6b124c2012b7a1a768f2f900f49903cd1 100644
--- a/arch/arm/configs/bcm2835_defconfig
+++ b/arch/arm/configs/bcm2835_defconfig
@@ -7,7 +7,6 @@ CONFIG_BSD_PROCESS_ACCT=y
 CONFIG_BSD_PROCESS_ACCT_V3=y
 CONFIG_LOG_BUF_SHIFT=18
 CONFIG_CFS_BANDWIDTH=y
-CONFIG_RT_GROUP_SCHED=y
 CONFIG_CGROUP_FREEZER=y
 CONFIG_CPUSETS=y
 CONFIG_CGROUP_DEVICE=y
diff --git a/arch/arm/configs/omap2plus_defconfig b/arch/arm/configs/omap2plus_defconfig
index 3a166c2f02bd80634fee3bd8c4579fdbad84ccfa..9cb265c8d4144ea9a41f02640dbc8a06b710323b 100644
--- a/arch/arm/configs/omap2plus_defconfig
+++ b/arch/arm/configs/omap2plus_defconfig
@@ -13,7 +13,6 @@ CONFIG_MEMCG=y
 CONFIG_BLK_CGROUP=y
 CONFIG_CGROUP_SCHED=y
 CONFIG_CFS_BANDWIDTH=y
-CONFIG_RT_GROUP_SCHED=y
 CONFIG_CGROUP_FREEZER=y
 CONFIG_CPUSETS=y
 CONFIG_CGROUP_DEVICE=y
diff --git a/arch/arm/configs/tegra_defconfig b/arch/arm/configs/tegra_defconfig
index d2a094ad360cefefbc3bd35226c5d23eadce42c1..3a9bda2bf422b14085c60b39a2c3df86fe0994bb 100644
--- a/arch/arm/configs/tegra_defconfig
+++ b/arch/arm/configs/tegra_defconfig
@@ -6,7 +6,6 @@ CONFIG_IKCONFIG=y
 CONFIG_IKCONFIG_PROC=y
 CONFIG_CGROUPS=y
 CONFIG_CGROUP_SCHED=y
-CONFIG_RT_GROUP_SCHED=y
 CONFIG_CGROUP_FREEZER=y
 CONFIG_CGROUP_CPUACCT=y
 CONFIG_CGROUP_DEBUG=y

-- 
2.46.0