tcp: improving RACK cpu performance

This patch set improves the CPU consumption of the RACK TCP loss recovery algorithm, in particular for high-speed networks. Currently, for every ACK in recovery RACK can potentially iterate over all sent packets in the write queue. On large BDP networks with non-trivial losses the RACK write queue walk CPU usage becomes unreasonably high.

This patch introduces a new queue in TCP that keeps only skbs sent and not yet (s)acked or marked lost, in time order instead of sequence order. With that, RACK can examine this time-sorted list and only check packets that were sent recently, within the reordering window, per ACK. This is the fastest way without any write queue walks. The number of skbs examined per ACK is reduced by orders of magnitude.

Eric Dumazet (1):
tcp: new list for sent but unacked skbs for RACK recovery

Yuchung Cheng (2):
tcp: more efficient RACK loss detection
tcp: a small refactor of RACK loss detection

 include/linux/skbuff.h   | 11 ++++++++--
 include/linux/tcp.h      |  1 +
 include/net/tcp.h        | 24 +++++++++++++++++++++-
 net/ipv4/tcp.c           |  2 ++
 net/ipv4/tcp_input.c     |  9 +++++++--
 net/ipv4/tcp_minisocks.c |  1 +
 net/ipv4/tcp_output.c    | 42 ++++++++++++++++++++++++++++----------
 net/ipv4/tcp_recovery.c  | 52 ++++++++++++++++++------------------------------
 8 files changed, 93 insertions(+), 49 deletions(-)

2.14.2.920.gcf0c67979c-goog