tcp: add a fast path in tcp_delack_timer()

delack timer is not stopped from inet_csk_clear_xmit_timer()
because we do not define INET_CSK_CLEAR_TIMERS.

This is a conscious choice : inet_csk_clear_xmit_timer()
is often called from another cpu. Calling del_timer()
would cause false sharing and lock contention.

This means that very often, tcp_delack_timer() is called
at the timer expiration, while there is no ACK to transmit.

This can be detected very early, avoiding the socket spinlock.

Notes:
- test about tp->compressed_ack is racy,
  but in the unlikely case there is a race, the dedicated
  compressed_ack_timer hrtimer would close it.

- Even if the fast path is not taken, reading
  icsk->icsk_ack.pending and tp->compressed_ack
  before acquiring the socket spinlock reduces
  acquisition time and chances of contention.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241002173042.917928-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 4d0407301603..08772395690d 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -4224,7 +4224,8 @@ void tcp_send_delayed_ack(struct sock *sk)
 		if (!time_before(timeout, icsk->icsk_ack.timeout))
 			timeout = icsk->icsk_ack.timeout;
 	}
-	icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
+	smp_store_release(&icsk->icsk_ack.pending,
+			  icsk->icsk_ack.pending | ICSK_ACK_SCHED | ICSK_ACK_TIMER);
 	icsk->icsk_ack.timeout = timeout;
 	sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
 }