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>

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 914d1977270449241f6fc6da2055f3af02a75f99..3c82fad904d4c6c51069e2e703673d667bb36d06 100644 (file)
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -202,7 +202,7 @@ static inline void inet_csk_clear_xmit_timer(struct sock *sk, const int what)
                sk_stop_timer(sk, &icsk->icsk_retransmit_timer);
 #endif
        } else if (what == ICSK_TIME_DACK) {
-               icsk->icsk_ack.pending = 0;
+               smp_store_release(&icsk->icsk_ack.pending, 0);
                icsk->icsk_ack.retry = 0;
 #ifdef INET_CSK_CLEAR_TIMERS
                sk_stop_timer(sk, &icsk->icsk_delack_timer);
@@ -233,7 +233,8 @@ static inline void inet_csk_reset_xmit_timer(struct sock *sk, const int what,
                icsk->icsk_timeout = jiffies + when;
                sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
        } else if (what == ICSK_TIME_DACK) {
-               icsk->icsk_ack.pending |= ICSK_ACK_TIMER;
+               smp_store_release(&icsk->icsk_ack.pending,
+                                 icsk->icsk_ack.pending | ICSK_ACK_TIMER);
                icsk->icsk_ack.timeout = jiffies + when;
                sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
        } else {

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 8c53385cc808c61097898514fd91a322e3a08d31..12e975ed4910d8c7cca79b1812f365589a5d469a 100644 (file)
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -776,7 +776,7 @@ void inet_csk_clear_xmit_timers(struct sock *sk)
        struct inet_connection_sock *icsk = inet_csk(sk);
 
        smp_store_release(&icsk->icsk_pending, 0);
-       icsk->icsk_ack.pending = 0;
+       smp_store_release(&icsk->icsk_ack.pending, 0);
 
        sk_stop_timer(sk, &icsk->icsk_retransmit_timer);
        sk_stop_timer(sk, &icsk->icsk_delack_timer);
@@ -792,7 +792,7 @@ void inet_csk_clear_xmit_timers_sync(struct sock *sk)
        sock_not_owned_by_me(sk);
 
        smp_store_release(&icsk->icsk_pending, 0);
-       icsk->icsk_ack.pending = 0;
+       smp_store_release(&icsk->icsk_ack.pending, 0);
 
        sk_stop_timer_sync(sk, &icsk->icsk_retransmit_timer);
        sk_stop_timer_sync(sk, &icsk->icsk_delack_timer);

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 4d04073016035dcf62ba5e0ad23aac86e54e65c7..08772395690d13a0c3309a273543a51aa0dd3fdc 100644 (file)
--- 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);
 }

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index b7266b9101ce5933776bd38d086287667e3a7f18..c3a7442332d4926a6089812f789e89ee23081306 100644 (file)
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -361,6 +361,14 @@ static void tcp_delack_timer(struct timer_list *t)
                        from_timer(icsk, t, icsk_delack_timer);
        struct sock *sk = &icsk->icsk_inet.sk;
 
+       /* Avoid taking socket spinlock if there is no ACK to send.
+        * The compressed_ack check is racy, but a separate hrtimer
+        * will take care of it eventually.
+        */
+       if (!(smp_load_acquire(&icsk->icsk_ack.pending) & ICSK_ACK_TIMER) &&
+           !READ_ONCE(tcp_sk(sk)->compressed_ack))
+               goto out;
+
        bh_lock_sock(sk);
        if (!sock_owned_by_user(sk)) {
                tcp_delack_timer_handler(sk);
@@ -371,6 +379,7 @@ static void tcp_delack_timer(struct timer_list *t)
                        sock_hold(sk);
        }
        bh_unlock_sock(sk);
+out:
        sock_put(sk);
 }
 

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c2317919fc148a67a81ded795359bd613c9b0dff..e85862352084907582ec884dcb96832356419fa5 100644 (file)
--- 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);
 }