This patchset introduces 4-tuple hash for connected udp sockets, to make
connected udp lookup faster.

Stress test results (with 1 cpu fully used) are shown below, in pps:
(1) _un-connected_ socket as server
    [a] w/o hash4: 1,825176
    [b] w/  hash4: 1,831750 (+0.36%)

(2) 500 _connected_ sockets as server
    [c] w/o hash4:   290860 (only 16% of [a])
    [d] w/  hash4: 1,889658 (+3.1% compared with [b])
With hash4, compute_score is skipped when lookup, so [d] is slightly
better than [b].

Patch1: Add a new counter for hslot2 named hash4_cnt, to avoid cache line
        miss when lookup.
Patch2: Add hslot/hlist_nulls for 4-tuple hash.
Patch3 and 4: Implement 4-tuple hash for ipv4 and ipv6.

The detailed motivation is described in Patch 3.

The 4-tuple hash increases the size of udp_sock and udp_hslot. Thus add it
with CONFIG_BASE_SMALL, i.e., it's a no op with CONFIG_BASE_SMALL.

changelogs:
v5 -> v6 (Paolo Abeni):
- move udp_table_hash4_init from patch2 to patch1
- use hlist_nulls for lookup-rehash race
- add test results in commit log
- add more comment, e.g., for rehash4 used in hash4
- add ipv6 support (Patch4), and refactor some functions for better
  sharing, without functionality change

v4 -> v5 (Paolo Abeni):
- add CONFIG_BASE_SMALL with which udp hash4 does nothing

v3 -> v4 (Willem de Bruijn):
- fix mistakes in udp_pernet_table_alloc()

RFCv2 -> v3 (Gur Stavi):
- minor fix in udp_hashslot2() and udp_table_init()
- add rcu sync in rehash4()

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

v5:
https://lore.kernel.org/all/20241018114535.35712-1-lulie@linux.alibaba.com/
v4:
https://lore.kernel.org/all/20241012012918.70888-1-lulie@linux.alibaba.com/
v3:
https://lore.kernel.org/all/20241010090351.79698-1-lulie@linux.alibaba.com/
RFCv2:
https://lore.kernel.org/all/20240924110414.52618-1-lulie@linux.alibaba.com/
RFCv1:
https://lore.kernel.org/all/20240913100941.8565-1-lulie@linux.alibaba.com/

Philo Lu (4):
  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
  ipv6/udp: Add 4-tuple hash for connected socket

 include/linux/udp.h |  11 +++
 include/net/udp.h   | 133 +++++++++++++++++++++++--
 net/ipv4/udp.c      | 231 +++++++++++++++++++++++++++++++++++++++-----
 net/ipv6/udp.c      | 111 ++++++++++++++++++---
 4 files changed, 445 insertions(+), 41 deletions(-)

--
2.32.0.3.g01195cf9f