Someone reported a bug in dtrace whereby execve() wasnt tracing.
I created a VM and started testing, and can confirm this. Looks
like dtrace is getting confused by the new rewritten syscall assembler.
I have a working version for this in my testbed, but I found that
changes to the IPI code in the kernel are making any dtrace probes
extremely unreliable - looks like a 1:N chance of seeing output (where
N is the number of cpus you have).

I have some similar issues in Ubuntu 14.04 - hopefully similar issues.