[RFC PATCH net-next 0/6] Always use tx interrupt for virtio-net

[Jason Wang <jasowang@redhat.com>]

According to David, proper accounting and queueing (at all levels, not
just TCP sockets) is more important than trying to skim a bunch of
cycles by avoiding TX interrupts. Having an event to free the SKB is
absolutely essential for the stack to operate correctly.

This series tries to enable tx interrupt for virtio-net. The idea is
simple: enable tx interrupt and schedule a tx napi to free old xmit
skbs.

Several notes:
- Tx interrupt storm avoidance when queue is about to be full is
  kept. Since we may enable callbacks in both ndo_start_xmit() and tx
  napi, patch 1 adds a check to make sure used event never go
  back. This will let the napi not enable the callbacks wrongly after
  delayed callbacks was used.
- For bulk dequeuing, there's no need to enable tx interrupt for each
  packet. The last patch only enable tx interrupt for the final skb in
  the chain through xmit_more and a new helper to publish current avail
  idx as used event.

This series fixes several issues of original rfc pointed out by Michael.

Smoking test is done, and will do complete netperf test. Send the
seires for early review.

Thanks

Jason Wang (6):
  virtio: make sure used event never go backwards
  virtio: introduce virtio_enable_cb_avail()
  virtio-net: small optimization on free_old_xmit_skbs()
  virtio-net: return the number of packets sent in free_old_xmit_skbs()
  virtio-net: enable tx interrupt
  virtio-net: enable tx interrupt only for the final skb in the chain

 drivers/net/virtio_net.c     |  125 ++++++++++++++++++++++++++++++------------
 drivers/virtio/virtio_ring.c |   25 ++++++++-
 include/linux/virtio.h       |    2 +
 3 files changed, 115 insertions(+), 37 deletions(-)