WD_PASSIVE kernel based tickling of the watchdog - request for ideas

I've got two requests for a clue ... er... ideas today on how to 
implement the WD_PASSIVE flag in the watchdog(9) interface:

- How would I best implement the tickling of the watchdog at specified 
intervals (e.g. 1/2 the watchdog timeout value)?

By using a

 	- timeout
 	- kernel thread sleeping most of the time
 	- hardclock()

considering the trade-off between large variance in frequency vs. making 
sure we notice a (permanent) freeze in the kernel somewhere. For example 
burning a CD sometimes makes my laptop freeze for a short period. Will 
the watchdog fire in that case?

- What do we consider a 'bad' situation in which the watchdog should not 
be tickled? What kind of checks would we need to perform?

I'd be interested to hear your thoughts.

Nick

P.S.: I'm personally not interested in passive tickling of the watchdog, 
so if there is no response, I'll leave the implementation as is (return 
EOPNOTSUPP at the moment).