I have no idea what HZ is used on my kernel, but i think
i understand the problem now.

I still can reproduce it with this:

	double t, ct, ts;

	ct = nsec()*1e-6;
	t = ct + msec;

	for(;;){
		ts = t - ct;
		sleep(ts);
		ct = nsec()*1e-6;
		if(t <= ct){
			t += msec;
		}
	}

The problem is that when sleep is a little bit too fast on every round,
t will never catch up with ct. so it sleeps shorter and shorter until
t - ct < 1.

It all works when changing t += msec to t = ct + msec. It may still
do a zero sleep, but the error will not add up.

making new kernel...

--
cinap