cx_lowest and CPU usage

[Andriy Gapon]

Report for 7.0-RC1 on quite old hardware: 440BX-based motherboard,
450Mhz Pentium III (Katmai).

cx_supported claims to support C1, C2, C3. If I set cx_lowest to C3 it
immediately gets backed out to C2 with a kernel message about too many
short sleeps. But that's not a problem.
There is a weird thing: if I change cx_lowest to C2 when the machine is
completely idle, top shows that CPU usage for interrupts immediately
jumps to almost 20%. Change cx_lowest to C1, CPU usage drops back to
almost 0%.
Is this normal ?
If not, does this indicate some problem in idle routine or is this just
incorrect statistics calculation ? Or maybe something with HW ?

-- 
Andriy Gapon