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Message-ID: <CAL+tcoDNTLoOc9yZsCGu-tt7SqgbJf=hdfkaW_isjR7Cntc5AA@mail.gmail.com>
Date: Thu, 3 Oct 2024 08:19:14 +0900
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>, netdev@...r.kernel.org,
eric.dumazet@...il.com
Subject: Re: [PATCH net-next 3/3] tcp: add a fast path in tcp_delack_timer()
Hello Eric,
On Thu, Oct 3, 2024 at 2:31 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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@...gle.com>
> ---
> include/net/inet_connection_sock.h | 5 +++--
> net/ipv4/inet_connection_sock.c | 4 ++--
> net/ipv4/tcp_output.c | 3 ++-
> net/ipv4/tcp_timer.c | 9 +++++++++
> net/mptcp/protocol.c | 3 ++-
> 5 files changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> index 914d1977270449241f6fc6da2055f3af02a75f99..3c82fad904d4c6c51069e2e703673d667bb36d06 100644
> --- 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
> --- 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
> --- 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
> --- 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))
I wonder what the use of single READ_ONCE() is here without a
WRITE_ONCE() pair? It cannot guarantee that the result of reading
compressed_ack is accurate. What if we use without this READ_ONCE()
here?
Thanks,
Jason
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