Quoted from the  https://www.kernel.org/doc/local/pr_debug.txt

pr_debug()    Some files call pr_debug(), which is ordinarily an empty macro that discards    its arguments at compile time.  To enable debugging output, build the    appropriate file with -DDEBUG by adding      CFLAGS_[filename].o := -DDEBUG    to the makefile.    For example, to see all attempts to spawn a usermode helper (such as    /sbin/hotplug), add to lib/Makefile the line:        CFLAGS_kobject_uevent.o := -DDEBUG    Then boot the new kernel, do something that spawns a usermode helper, and    use the "dmesg" command to view the pr_debug() output.