I pulled down the most recent Fedora rawhide kernel today:
4.0.0-0.rc2.git0.1.fc23.x86_64
...and with that, I can't reproduce this. The ftrace_event_field slab
(which is shared by the fasync_struct cache) seems to stay under
control. I see it hover around 3-4M in size while the test is running
but the box isn't falling over or anything.
Perhaps this was an MM or RCU bug that is now fixed? Can you confirm
whether you're still able to reproduce it with the most recent mainline
kernels?