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tcp: add a fast path in tcp_delack_timer()

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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>
This commit is contained in:
Eric Dumazet 2024-10-02 17:30:42 +00:00 committed by Jakub Kicinski
parent 3b78429301
commit 81df4fa94e
5 changed files with 18 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
include/net/inet_connection_sock.h
View File

@ -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 {

4
net/ipv4/inet_connection_sock.c
View File

@ -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);

3
net/ipv4/tcp_output.c
View File

@ -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);
}

9
net/ipv4/tcp_timer.c
View File

@ -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);
}

3
net/mptcp/protocol.c
View File

@ -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);
}