net: collect LLE cache statistics

[kevans]

Summary

Track cache lookup/hits. A future commit will export these for display
in arp(8).

Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #66

[melifaro]

Would it be possible to describe a use case for the feature?

sys/net/if_ethersubr.c

326

If we really want the data, can we avoid atomics here and use counter(9) instead?

sys/net/if_llatbl.h

91

struct llentry fields are specifically ordered in the way that all fields used by the fast path are in the first cache line.

sys/netinet/in.c

1581

If we really want the data, can we avoid atomics here and use counter(9) instead?

[kevans]

In D32352#730092, @melifaro wrote:

Would it be possible to describe a use case for the feature?

NetApp uses it downstream for validation and testing -- we could stash it off behind a config option if you'd feel better about that?

[melifaro]

In D32352#732151, @kevans wrote:

In D32352#730092, @melifaro wrote:

Would it be possible to describe a use case for the feature?

NetApp uses it downstream for validation and testing -- we could stash it off behind a config option if you'd feel better about that?

I have some changes related to linking nhops with llentries, so the datapath avoids looking up / referencing llentries for routes with gateway. Per-lle counters may be a bit hard to support in that scenario.
Are other methods of getting the counters (dtrace, ipfw counters) out of the table?