[gentoo-user] BFQ scheduler ... undefined symbol while shutdown ?

Hi,

I am using a good ole' harddisc with some rotating metal plates...
;)

To get more throughput I had set  CONFIG_IOSCHED_BFQ / BFQ scheduler
in the kernel sources of 4.15.1 /.2 /.3 and while the kernel was
running anything seems to be fine.

Then while the box was shutting down the kernel, the shutdown process
hangs and error messages were printed to the screen: Anythong about
"missing/unreferenced symbols".

I am using kernels downloaded from kernel.org.

Two question: 
1) What kills the BFQ that hard?
2) Is it recommended for spinning disks anyhow and what may be better?

Thanks a lot for any help!

Cheers
Meino