arm: defconfig: drop RT_GROUP_SCHED=y from bcm2835/tegra/omap2plus

Commit 673ce00 ("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]: systemd/systemd@f4e74be
[2]: https://bugzilla.redhat.com/show_bug.cgi?id=1229700

Tested-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Celeste Liu <uwu@coelacanthus.name>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Author: CoelacanthusHex

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

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

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