softintr running on boot CPU only?

I am running 3.0.1 kernel on Dell SC430(two pentium cores) and run
vmstats saw softinterrupt only on boot CPU. Is it the way that the 3.0.1
kernel code supported to do? How about 4.0 and upcoming 5.0?

There is really not much documents on SMP/APCI implementation on x86
besides a few email threads. Does someone know more can share with me?

Thanks
Ming