This RFC patchset introduces 4-tuple hash for connected udp sockets, to
make connected udp lookup faster. It is a tentative proposal and any
comment is welcome.

Patch1: Add a new counter for hslot2 named hash4_cnt, to avoid cache line
        miss when lookup.
Patch2 and 3: Implement 4-tuple hash for ipv4.

The detailed motivation is described in Patch 3.

AFAICT the patchset can be further improved by:
(a) Better interact with hash2/reuseport. Now hash4 hardly affects other
mechanisms, but maintaining sockets in both hash4 and hash2 lists seems
unnecessary.
(b) Support early demux and ipv6.

changelogs:
RFCv1 -> RFCv2:
- add a new struct for hslot2
- remove the sockopt UDP_HASH4 because it has little side effect for
  unconnected sockets
- add rehash in connect()
- re-organize the patch into 3 smaller ones
- other minor fix

RFCv1:
https://lore.kernel.org/all/20240913100941.8565-1-lulie@linux.alibaba.com/ (local)

Philo Lu (3):
  net/udp: Add a new struct for hash2 slot
  net/udp: Add 4-tuple hash list basis
  ipv4/udp: Add 4-tuple hash for connected socket

 include/linux/udp.h |   7 ++
 include/net/udp.h   |  44 ++++++++--
 net/ipv4/udp.c      | 197 ++++++++++++++++++++++++++++++++++++++------
 net/ipv6/udp.c      |  17 ++--
 4 files changed, 223 insertions(+), 42 deletions(-)

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2.32.0.3.g01195cf9f