[PATCH net-next v2 0/3] tcp: remove obsolete RFC3517/RFC6675 code

From: Neal Cardwell <ncardwell@google.com>

RACK-TLP loss detection has been enabled as the default loss detection
algorithm for Linux TCP since 2018, in:

 commit b38a51fec1c1 ("tcp: disable RFC6675 loss detection")

In case users ran into unexpected bugs or performance regressions,
that commit allowed Linux system administrators to revert to using
RFC3517/RFC6675 loss recovery by setting net.ipv4.tcp_recovery to 0.

In the seven years since 2018, our team has not heard reports of
anyone reverting Linux TCP to use RFC3517/RFC6675 loss recovery, and
we can't find any record in web searches of such a revert.

RACK-TLP was published as a standards-track RFC, RFC8985, in February
2021.

Several other major TCP implementations have default-enabled RACK-TLP
at this point as well.

RACK-TLP offers several significant performance advantages over
RFC3517/RFC6675 loss recovery, including much better performance in
the common cases of tail drops, lost retransmissions, and reordering.

It is now time to remove the obsolete and unused RFC3517/RFC6675 loss
recovery code. This will allow a substantial simplification of the
Linux TCP code base, and removes 12 bytes of state in every tcp_sock
for 64-bit machines (8 bytes on 32-bit machines).

To arrange the commits in reasonable sizes, this patch series is split
into 3 commits:

(1) Removes the core RFC3517/RFC6675 logic.

(2) Removes the RFC3517/RFC6675 hint state and the first layer of logic that
    updates that state.

(3) Removes the emptied-out tcp_clear_retrans_hints_partial() helper function
    and all of its call sites.

v2: fix compiler warnings from unused variables

Neal Cardwell (3):
  tcp: remove obsolete and unused RFC3517/RFC6675 loss recovery code
  tcp: remove RFC3517/RFC6675 hint state: lost_skb_hint, lost_cnt_hint
  tcp: remove RFC3517/RFC6675 tcp_clear_retrans_hints_partial()

 Documentation/networking/ip-sysctl.rst        |   8 +-
 .../networking/net_cachelines/tcp_sock.rst    |   2 -
 include/linux/tcp.h                           |   3 -
 include/net/tcp.h                             |   6 -
 net/ipv4/tcp.c                                |   3 +-
 net/ipv4/tcp_input.c                          | 158 ++----------------
 net/ipv4/tcp_output.c                         |   6 -
 7 files changed, 16 insertions(+), 170 deletions(-)

-- 
2.50.0.rc1.591.g9c95f17f64-goog