Confusing log message "Can't get entropy from getrandom()"

A relay operator on #tor shared these log lines:

Dec 01 16:33:00.000 [notice] Tor 0.3.1.8 (git-868c1b2b1eb7225a) opening log file.
Dec 01 16:33:00.515 [warn] Can't get entropy from getrandom().
Dec 01 16:33:00.534 [notice] Tor 0.3.1.8 (git-868c1b2b1eb7225a) running on Linux with Libevent 2.0.21-stable, OpenSSL 1.0.2g, Zlib 1.2.8, Liblzma 5.1.0alpha[...]

The middle line is confusing -- why can't we get the entropy from it? Does that mean Tor has failed at something important? What should the relay operator do?

If the relay operator shouldn't do anything, because it's fine, this should be a notice or less. If the relay operator ought to do something, because it's not fine, then we should suggest what to do and/or what the problem is with doing nothing.