A watchdog is intended to check that all vital services are being performed, and (for example) the processor is not stuck in an endless loop somewhere.
If you place the WWDG feeder in the systick interrupt, the system will be reset if systick does not get serviced.
But the processor could be stuck in an endless loop - not doing what it is supposed to do. And on each systick interrupt it obediently goes off, services systick, and returns to its endless loop. Under these circumstances, you probably do want the watchdog to time-out and the processor to be reset. So having the WWDG feeder inside systick might be unsatisfactory unless you make it conditional on genuine progress being observed on your main execution loop.
Where you have a real-time OS and several threads, you probably want to observe progress (i.e. having been around the code loop) on each and every one of these threads. So you might need to have a flag for each thread, and you feed the watchdog only when all flags have been set (at which point you clear all flags).
Hope this helps,
Danish
