Merge branch 'packet_rollover'

Willem de Bruijn says:

====================
refine packet socket rollover:

1. mitigate a case of lock contention
2. avoid exporting resource exhaustion to other sockets,
   by migrating only to a victim socket that has ample room
3. avoid reordering of most flows on the socket,
   by migrating first the flow responsible for load imbalance
4. help processes detect load imbalance,
   by exporting rollover counters

Context: rollover implements flow migration in packet socket fanout
groups in case of extreme load imbalance. It is a specific
implementation of migration that minimizes reordering by selecting
the same victim socket when possible (and by selecting subsequent
victims in a round robin fashion, from which its name derives).

Changes:
  v2 -> v3:
    - statistics: replace unsigned long with __aligned_u64
  v1 -> v2:
    - huge flow detection: run lockless
    - huge flow detection: replace stored index with random
    - contention avoidance: test in packet_poll while lock held
    - contention avoidance: clear pressure sooner

          packet_poll and packet_recvmsg would clear only if the sock
          is empty to avoid taking the necessary lock. But,
          * packet_poll already holds this lock, so a lockless variant
            __packet_rcv_has_room is cheap.
          * packet_recvmsg is usually called only for non-ring sockets,
            which also runs lockless.

    - preparation: drop "single return" patch

          packet_rcv_has_room is now a locked wrapper around
          __packet_rcv_has_room, achieving the same (single footer).

The benchmark mentioned in the patches is at
https://github.com/wdebruij/kerneltools/blob/master/tests/bench_rollover.c
====================

Signed-off-by: David S. Miller <davem@davemloft.net>