[PATCH v3 net-next 0/9] tcp: avoid struct socket cache line miss in tcp_check_space()

[Eric Dumazet <edumazet@kernel.org>]

tcp_check_space() is called on every incoming ACK and every transmitted
packet, and tests SOCK_NOSPACE in sk->sk_socket->flags.  Because struct
socket lives in its own cache line and the TCP fast paths do not touch
it for anything else, that test pulls in an extra cache line that is
cold when the working set of active sockets is large.

This series mirrors SOCK_NOSPACE into a new u8 field, tp->tcp_nospace,
placed right after tp->chrono_type in the tcp_sock_write_tx cache line
group (fitting in an existing 3-byte hole before chrono_start, so no
other field moves and sizeof(struct tcp_sock) is unchanged).  Both the
transmit and ACK fast paths already touch that cache line, making the
fast-path test in tcp_check_space() free of extra cache misses while
keeping __tcp_check_space() authoritative on SOCK_NOSPACE.

To maintain the invariant (SOCK_NOSPACE set => tp->tcp_nospace set) from
a single choke point:

 - Patch 1 fixes a long-standing bug in dlm where SOCKWQ_ASYNC_NOSPACE
   was tested and cleared on con->sock->flags instead of SOCK_NOSPACE.
 - Patch 2 introduces sk_set_nospace() and sk_clear_nospace() and
   converts the core networking setters and clearers.
 - Patches 3-8 convert the remaining in-kernel callers (sunrpc, rds,
   dlm, drbd, nvme-tcp, libceph) so that no open-coded set_bit() or
   clear_bit() of SOCK_NOSPACE remains in the tree.
 - Patch 9 adds tp->tcp_nospace, wires it into sk_set_nospace() and
   sk_clear_nospace(), and switches tcp_check_space() to test it.

v3:
 - Rebase and CC subsystem maintainers (Jakub)
 - Link to v2: https://lore.kernel.org/netdev/20260924134729.2047213-1-edumazet@google.com/

v2:
 - Order set_bit(SOCK_NOSPACE) before tp->tcp_nospace = 1 in
   sk_set_nospace() and tp->tcp_nospace = 0 before
   clear_bit(SOCK_NOSPACE) in sk_clear_nospace() so a concurrent
   lockless tcp_poll() cannot leave SOCK_NOSPACE set with
   tp->tcp_nospace cleared (Sashiko)
 - Move tp->tcp_nospace to the 3-byte hole after tp->chrono_type so no
   field in struct tcp_sock shifts after the AccECN bitfield additions
   (Sashiko)
 - Clarify comment and Patch 2 changelog wording (Sashiko)
 - Link to v1: https://lore.kernel.org/netdev/20260922122721.3568295-1-edumazet@google.com/

Eric Dumazet (9):
  dlm: fix send buffer backpressure handling
  net: add sk_set_nospace() and sk_clear_nospace()
  sunrpc: use sk_set_nospace() and sk_clear_nospace()
  rds: use sk_set_nospace()
  dlm: use sk_set_nospace() and sk_clear_nospace()
  drbd: use sk_set_nospace()
  nvme-tcp: use sk_clear_nospace()
  libceph: use sk_clear_nospace()
  tcp: add tp->tcp_nospace

 .../networking/net_cachelines/tcp_sock.rst    |  1 +
 drivers/block/drbd/drbd_worker.c              |  3 +-
 drivers/nvme/host/tcp.c                       |  2 +-
 drivers/nvme/target/tcp.c                     |  2 +-
 fs/dlm/lowcomms.c                             | 10 ++--
 include/linux/tcp.h                           |  3 ++
 include/net/sock.h                            |  2 +
 include/net/tcp.h                             | 30 +++++++++++-
 net/ceph/messenger.c                          |  2 +-
 net/core/sock.c                               | 46 ++++++++++++++++++-
 net/core/stream.c                             |  6 +--
 net/ipv4/tcp.c                                |  5 +-
 net/ipv4/tcp_bpf.c                            |  2 +-
 net/ipv4/tcp_input.c                          |  8 +++-
 net/kcm/kcmsock.c                             |  4 +-
 net/mptcp/protocol.c                          |  4 +-
 net/mptcp/subflow.c                           |  5 ++
 net/rds/tcp_send.c                            |  5 +-
 net/smc/af_smc.c                              |  2 +-
 net/smc/smc_tx.c                              |  6 +--
 net/sunrpc/svcsock.c                          |  4 +-
 net/sunrpc/xprtsock.c                         |  4 +-
 net/tls/tls_sw.c                              |  2 +-
 23 files changed, 121 insertions(+), 37 deletions(-)

-- 
2.56.0.rc1.315.gc6ed9934b7-goog