[RFC] GRO scalability

[Eric Dumazet <eric.dumazet@gmail.com>]

Current GRO cell is somewhat limited :

- It uses a single list (napi->gro_list) of pending skbs

- This list has a limit of 8 skbs (MAX_GRO_SKBS)

- Workloads with lot of concurrent flows have small GRO hit rate but
  pay high overhead (in inet_gro_receive())

- Increasing MAX_GRO_SKBS is not an option, because GRO
  overhead becomes too high.

- Packets can stay a long time held in GRO cell (there is
  no flush if napi never completes on a stressed cpu)

  Some elephant flows can stall interactive ones (if we receive
  flood of non TCP frames, we dont flush tcp packets waiting in
gro_list)

What we could do :

1) Use a hash to avoid expensive gro_list management and allow
   much more concurrent flows.

Use skb_get_rxhash(skb) to compute rxhash

If l4_rxhash not set -> not a GRO candidate.

If l4_rxhash set, use a hash lookup to immediately finds a 'same flow'
candidates.

(tcp stack could eventually use rxhash instead of its custom hash
computation ...)

2) Use a LRU list to eventually be able to 'flush' too old packets,
   even if the napi never completes. Each time we process a new packet,
   being a GRO candidate or not, we increment a napi->sequence, and we
   flush the oldest packet in gro_lru_list if its own sequence is too
   old.

  That would give a latency guarantee.