6.0-CURRENT very unstable with PREEMPTION

[Stefan Ehmann]

Since it's default now, I turned PREEMPTION (with SCHED_4BSD) on again
in my KERNCONF on my 6.0-CURRENT i386 pc.

After about 10-15 minutes some windows stop beeing redrawn... new terms
can be started very slowly for a short period of time. Then everything
completely freezes.

Unfortunately I can't provide much more information. So far I've only
been able to reproduce this in X. Getting coredumps has not been working
for months now and I don't have a serial console setup.

Without PREEMPTION everything works very stable.