I was working with the default x86_64 + kvm kernel config and a kernel module which uses the pr_debug() printk logging method.
On load, modprobe prints:
FATAL: Error inserting modulename: Unknown symbol in module, or unknown parameter (see dmesg)
And dmesg contains:
Unknown symbol __dynamic_pr_debug
I do have the CONFIG_DYNAMIC_DEBUG=y parameter set in my .config.
I found reference to this upstream at: https://lkml.org/lkml/2012/8/30/378 where Jim Crombie said "Ok, transient error, went away with a clean build".
I'd done a make; make modules_install; make install and reboot but that didn't work for me.
Like Jim found, a make clean then recompile worked for me.
You need to mount debugfs to use pr_debug(), see dynamic-debug-howto.txt in the kernel Documentation directory.