[PATCH net 0/2] tcp: Fix fastreuse cache initialization on first bind

[Zhewei Yuan <yzwlhm@gmail.com>]

This series fixes TCP bind reuse cache initialization and adds eight
KUnit regression cases.

Since commit 8002d44fe84d, tb2 is already linked to tb->bhash2 when
inet_csk_update_fastreuse() checks for an empty list. First binds miss
fastreuse=1 for SO_REUSEADDR and FASTREUSEPORT_ANY for SO_REUSEPORT,
causing unnecessary conflict scans. Patch 1 recognizes the sole,
ownerless address bucket; patch 2 tests initialization and conflicts.

Bind-only results on historical net f4377bf85887 (not the submission
base): TCP/IPv4, one-vCPU KVM, GCC 11.5.0, identical configs/programs.
Means of 3 guest boots x 3 rounds; only bind() is timed, with all sockets
kept open. Each case uses one reuse option, the same UID and port.
"same" uses 127.0.0.1; "alternate" interleaves 127.0.0.1/127.0.0.2.

  16000 sockets                 before (ms)    after (ms)
  SO_REUSEADDR, same                 4256.956        20.004
  SO_REUSEADDR, alternate            2476.914        19.983
  SO_REUSEPORT, same                   20.600        19.557
  SO_REUSEPORT, alternate            2491.294        19.830

Same-IP SO_REUSEPORT measurements overlap, so no stable gain is claimed
there. These are bind microbenchmarks, not application throughput.
A minimal reproducer is included at the end of this letter.

Validation on submission base 7076f684d513:
- KUnit: six expected first-bind failures and two passing conflict
  guards before the fix; all eight pass after it (same tests/config).
- bind_wildcard, reuseaddr_conflict, reuseport_addr_any,
  reuseaddr_ports_exhausted and tcp_port_share: pass without skips.
- Full x86_64 allmodconfig W=1: baseline and patched builds pass with
  identical configs, GCC 11.5.0 and CONFIG_WERROR=n. Existing warnings
  remain, but normalized diagnostics and counts match: no new warnings.
- Full allyesconfig is blocked by the MCTP KUnit static_assert at
  drivers/net/mctp/mctp-usblib-test.c:819. A targeted unpatched build
  reproduces the same error; this is not a full allyesconfig pass.

KUnit A/B and patched allyesconfig/allmodconfig W=1 builds also passed
on historical f4377bf85887 (GCC 11.5.0, CONFIG_WERROR=n); those builds
are auxiliary evidence, not substitutes for the submission-base checks.

Found during manual code inspection and GDB tracing. Codex assisted
with analysis, test/benchmark development, validation and patch text.
Requests covered real first binds, cache flags, automatic ports,
conflict preservation and separate reuseaddr/reuseport measurements.
No failure injection was used; KUnit inspects state after real binds.

Reproducer (not for the merge commit). It gives matching results on
the submission base 7076f684d513 (16000 sockets, same order as the
table, before/after in ms: 4178/19.4, 2416/19.4, 19.8/19.4,
2428/19.5). It needs loopback up and an fd limit above N, e.g.
"ulimit -n 70000; ./bind_bench reuseaddr same 16000".

  // Time bind() of N TCP sockets with one reuse option to one port.
  // Usage: bind_bench reuseaddr|reuseport same|alternate N
  // same: 127.0.0.1; alternate: 127.0.0.1/127.0.0.2. Sockets stay open.
  #include <netinet/in.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <sys/socket.h>
  #include <time.h>

  int main(int argc, char **argv)
  {
  	struct sockaddr_in sa = { .sin_family = AF_INET,
  				  .sin_port = htons(40000) };
  	int opt, alt, n, one = 1, i, fd;
  	struct timespec a, b;
  	long long ns = 0;

  	if (argc != 4)
  		return fprintf(stderr, "usage: %s reuseaddr|reuseport same|alternate N\n",
  			       argv[0]), 1;
  	opt = strcmp(argv[1], "reuseport") ? SO_REUSEADDR : SO_REUSEPORT;
  	alt = !strcmp(argv[2], "alternate");
  	n = atoi(argv[3]);

  	for (i = 0; i < n; i++) {
  		fd = socket(AF_INET, SOCK_STREAM, 0);
  		if (fd < 0 || setsockopt(fd, SOL_SOCKET, opt, &one, sizeof(one)))
  			return perror("socket/setsockopt"), 1;
  		sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK + (alt ? i % 2 : 0));
  		clock_gettime(CLOCK_MONOTONIC, &a);
  		if (bind(fd, (struct sockaddr *)&sa, sizeof(sa)))
  			return perror("bind"), 1;
  		clock_gettime(CLOCK_MONOTONIC, &b);
  		ns += (b.tv_sec - a.tv_sec) * 1000000000LL + b.tv_nsec - a.tv_nsec;
  	}
  	printf("%s %s %d: %.3f ms\n", argv[1], argv[2], n, ns / 1e6);
  	return 0;
  }

Zhewei Yuan (2):
  tcp: Fix fastreuse cache initialization on first bind
  tcp: Add KUnit tests for bind fastreuse caches

 net/ipv4/.kunitconfig                |   8 +
 net/ipv4/Kconfig                     |  14 ++
 net/ipv4/Makefile                    |   1 +
 net/ipv4/inet_connection_sock.c      |   3 +-
 net/ipv4/inet_connection_sock_test.c | 303 +++++++++++++++++++++++++++
 5 files changed, 328 insertions(+), 1 deletion(-)
 create mode 100644 net/ipv4/.kunitconfig
 create mode 100644 net/ipv4/inet_connection_sock_test.c


base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5
-- 
2.52.0