[SRU][O:aws][PATCH 0/1] Disable ACPI_HOTPLUG_CPU for arm64

[Koichiro Den]

BugLink: https://bugs.launchpad.net/bugs/2088047

[Impact]

On AWS EC2 a1.metal and c6g.8xlarge instances, issues caused by unexpected
discrepancies within ACPI table have been observed. These are indicated by
kernel warn messages and taint but, more critically, result in fewer usable
CPUs than expected for end users:
* Oracular on a1.metal: paying for 16 CPUs but only 4 are usable
* Oracular on c6g.8xlarge: paying for 32 CPUs but only 16 are usable

Given the possibility for other problematic ACPI table patterns, disable
ACPI_HOTPLUG_CPU on arm64 instance types until the issues are fixed at the
firmware level.

Note: as of 2024/12/11, fresh EC2 instances of the above-mentioned types
still exhibit the issue.

[Test Plan]

With this patch applied, verify that the kernel warning messages no longer
appear. Additionally, easily confirm with pre-installed tools like htop
that all expected CPUs are actively usable.

[Where problems could occur]

The likelihood of regression is minimal. It's also unlikely that end users
expect to perform virtual hot-unplugging for some instance types, especially
since they are basically billed on an hourly basis.

[Other Info]

This forcefully disables the option by editting Kconfig file because
ACPI_PROCESSOR=n did not work on real instances and HOTPLUG_CPU cannot be
disabled without side effects. That's why I added "UBUNTU: SAUCE" to the
Subject line, rather than "UBUNTU: [Config]".


Koichiro Den (1):
  UBUNTU: SAUCE: arm64: Kconfig: Disable ACPI_HOTPLUG_CPU

 arch/arm64/Kconfig               | 1 -
 debian.master/config/annotations | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

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