Locating a possible interrupt storm

[Peter Jeremy]

I suspect I have a machine that appears to be suffering from a severe
interrupt problem: It becomes non-responsive for all practical
purposes.  Entering 'Ctrl-Alt-Esc' eventually results in a very slow
dribble of characters with 'db> ' being reached after about 1/2 hr.
Typing characters at the prompt resulted in nothing being echoed after
about 2 hours (and which point it got reset).  This is a 2.4GB Xeon
CPU so I would expect a slightly zippier response :-).

My working hypothesis is that an interrupt storm is preventing the
system from doing anything other than acknowledge interrupts.  Does
this sound reasonable?  If not, can anyone suggest any alternate
hypotheses?

In either case, does anyone have any ideas on how to get a crashdump
when there's (effectively) no response to DDB?  Is enabling the
logical CPUs (machdep.cpu_idle_hlt=0) likely to have any effect?

The machine is running 4.9p1.  It was previously running 4.8p7 and
this problem was not noticed - though it's not clear that it wasn't
there.

-- 
Peter Jeremy