Xenomai is a real-time framework for Linux. The link given by the OP directly gives the reason for disabling APM / ACPI, which are very valid if RT is your priority:
Xenomai wrote:CONFIG_APM
The APM model assigns power management control to the BIOS, and BIOS code is never written with RT-latency in mind. If configured, APM routines are invoked with SMI priority, which breaks the rule that adeos-ipipe must be in charge of such things. CONFIG_XENO_HW_SMI_WORKAROUND_* doesn't help here.
CONFIG_ACPI_PROCESSOR
For systems with ACPI support in the BIOS, this ACPI sub-option installs an 'idle' handler that uses ACPI C2 and C3 processor states to save power. The CPU must 'warm-up' from these sleep states, increasing latency in ways dependent upon both the BIOS's ACPI tables and code. You may be able to suppress the sleeping with 'idle=poll' boot-arg, test to find out. With recents versions of Linux (probably starting around Linux 2.6.21), the acpi processor module disables the local APIC when loaded. This will cause Xenomai timer initialization to fail. This makes a second reason for disabling this option.
As an aside, this shows why you simply cannot do "hard" RTOS on your average desktop OS. RTOS and desktop OS have vastly different requirements, and I always cringe when people write about how they want to implement RTOS features on their general-purpose OS. (Mind you, I've been one of them, once upon a time, but I talked to actual people in the actual RTOS trade, and they
very quickly talked me out of it.
)
