Merge branch 'tcp-avoid-struct-socket-cache-line-miss-in-tcp_check_space'

Eric Dumazet says:

====================
tcp: avoid struct socket cache line miss in tcp_check_space()

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.
====================

Link: https://patch.msgid.link/20260929071743.23624-1-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 6a8c77719322..e4d1720f49f0 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -269,6 +269,9 @@ struct tcp_sock {
 				 */
 	u32	snd_sml;	/* Last byte of the most recently transmitted small packet */
 	u8	chrono_type;	/* current chronograph type */
+	u8	tcp_nospace;	/* mirrors SOCK_NOSPACE, must be set whenever
+				 * SOCK_NOSPACE is set.
+				 */
 	u32	chrono_start;	/* Start time in jiffies of a TCP chrono */
 	u32	chrono_stat[3];	/* Time in jiffies for chrono_stat stats */
 	u32	write_seq;	/* Tail(+1) of data held in tcp send buffer */