High CPU usage from a vacuum process but does nothing

After migrating the server to a bigger machine, I'm noticing an odd
behaviour. If I begin vacuuming manually, it starts OK:

<snip>
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO:  analyzing "public.table024484"
INFO:  "table024484": scanned 111123 of 111123 pages, containing 4909226
live rows and 0 dead rows; 150000 rows in sample, 4909226 estimated
total rows

But then it gets stuck indefinitely with high CPU usage and command
never exits. kill -TERM isn't helping, process stays on with ~99% CPU
usage. Stracing this process gets me just:

strace: Process 102011 attached
strace: [ Process PID=102011 runs in x32 mode. ]

Which is an odd message, all other processes give normal strace output.

latest pgsql 9.5 (same on old and new machine)
Ubuntu 16.04

Values in "cat /proc/102011/io" stay the same.

Autovacuum and other processes, queries aren't affected and can be
straced normally. Disk IO is low (checked with iostat)

Any ideas on what else to try?