[root@siva-vrs-2 ~]# stap --all-modules dropwatch.stp
semantic error: while resolving probe point: identifier 'kernel' at dropwatch.stp:18:7
        source: probe kernel.trace("kfree_skb") { locations[$location] <<< 1 }
                      ^

semantic error: no match (similar tracepoints: map, kfree, unmap, console, aer_event)

Pass 2: analysis failed.  [man error::pass2]