These days. In the (pre-ACPI) past some machines would change CPU frequency on AC events in system management mode, without any real way for Linux to know that

These days. In the (pre-ACPI) past some machines would change CPU frequency on AC events in system management mode, without any real way for Linux to know that it had happened. If you booted on AC and then went to battery then you'd probably be ok because kernel delays would just get longer, but booting on battery and then going to AC could result in these timeouts being significantly shorter than the hardware required. Once Intel actually documented their Speedstep implementation, and once ACPI gained support for expressing CPU performance states and notifying the OS, this mostly went away. But it was a real issue in the past. (Nowadays we just have to deal with vendors who've decided that their firmware can do a better job of CPU power management than the OS and change frequency without telling us, but that's Enterprise so it's ok)