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Message-ID: <CANn89iLwLxzSLKtobE9y+ZBU_eizfNo6FUfBa61KeUXsodA2FQ@mail.gmail.com>
Date: Thu, 27 Jul 2023 21:17:41 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: menglong8.dong@...il.com
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
dsahern@...nel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH net-next 1/3] net: tcp: send zero-window ACK when no memory
On Thu, Jul 27, 2023 at 2:51 PM <menglong8.dong@...il.com> wrote:
>
> From: Menglong Dong <imagedong@...cent.com>
>
> For now, skb will be dropped when no memory, which makes client keep
> retrans util timeout and it's not friendly to the users.
>
> In this patch, we reply an ACK with zero-window in this case to update
> the snd_wnd of the sender to 0. Therefore, the sender won't timeout the
> connection and will probe the zero-window with the retransmits.
>
> Signed-off-by: Menglong Dong <imagedong@...cent.com>
> ---
> include/net/inet_connection_sock.h | 3 ++-
> net/ipv4/tcp_input.c | 4 ++--
> net/ipv4/tcp_output.c | 14 +++++++++++---
> 3 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> index c2b15f7e5516..be3c858a2ebb 100644
> --- a/include/net/inet_connection_sock.h
> +++ b/include/net/inet_connection_sock.h
> @@ -164,7 +164,8 @@ enum inet_csk_ack_state_t {
> ICSK_ACK_TIMER = 2,
> ICSK_ACK_PUSHED = 4,
> ICSK_ACK_PUSHED2 = 8,
> - ICSK_ACK_NOW = 16 /* Send the next ACK immediately (once) */
> + ICSK_ACK_NOW = 16, /* Send the next ACK immediately (once) */
> + ICSK_ACK_NOMEM = 32,
> };
>
> void inet_csk_init_xmit_timers(struct sock *sk,
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 3cd92035e090..03111af6115d 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5061,7 +5061,8 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
> reason = SKB_DROP_REASON_PROTO_MEM;
> NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
> sk->sk_data_ready(sk);
> - goto drop;
> + inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOMEM;
Also set ICSK_ACK_NOW ?
We also need to make sure to send an immediate ACK WIN 0 in the case we queued
the skb in an empty receive queue and we were under pressure.
We do not want to have a delayed ACK sending a normal RWIN,
then wait for another packet that we will probably drop.
Look at the code :
if (skb_queue_len(&sk->sk_receive_queue) == 0)
sk_forced_mem_schedule(sk, skb->truesize);
else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
and refactor it to make sure to set ICSK_ACK_NOMEM even on the first packet
that bypassed the rmem_schedule().
> + goto out_of_window;
Why forcing quickack mode ? Please leave the "goto drop;"
> }
>
> eaten = tcp_queue_rcv(sk, skb, &fragstolen);
> @@ -5102,7 +5103,6 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
> out_of_window:
> tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
> inet_csk_schedule_ack(sk);
> -drop:
> tcp_drop_reason(sk, skb, reason);
> return;
> }
>
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