Somewhere back around 3.17 the kmem cache "tcp_bind_bucket" dropped out of /proc/slabinfo. It turns out the ss command was dumpster diving in slabinfo to determine the number of bound sockets and now it always reports 0.
Not sure why, the cache is still created but it doesn't show in slabinfo. Could it be some part of making slab/slub common code (or network namespaces). The cache is created in tcp_init but not visible. Any ideas?