DTrace gone quiet?

I just upgraded my kernel and userspace to head (r249552) and I notice
that DTrace doesn't output anything until I hit ctrl-c.  All previous
"hits" on the probe appear lost.  For example:

# dtrace -n 'fbt::ether_output:entry'
dtrace: description 'fbt::ether_output:entry' matched 1 probe

(No output here.  I waited a long time before the ctrl-c and I know the
system is actively transmitting and receiving Ethernet traffic).

^C
CPU     ID                    FUNCTION:NAME
  9  29354               ether_output:entry
  8  29354               ether_output:entry
  8  29354               ether_output:entry
  8  29354               ether_output:entry


Can anyone confirm or contradict this on a recent HEAD (around r249552
in my case)?

Regards,
Navdeep