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 <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c2317919fc14..e85862352084 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3504,7 +3504,8 @@ static void schedule_3rdack_retransmission(struct sock *ssk)
 	timeout += jiffies;
 
 	WARN_ON_ONCE(icsk->icsk_ack.pending & ICSK_ACK_TIMER);
-	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(ssk, &icsk->icsk_delack_timer, timeout);
 }