netdev - Re: [PATCH net-next] tcp: handle the deferred sack compression when it should be canceled

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Message-ID: <CANn89i+xRVxqx1-tSQaf_Kh7AG07Y2niGROPc0--mGf7_wz7Qw@mail.gmail.com>
Date: Tue, 30 May 2023 09:09:32 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	dsahern@...nel.org, ncardwell@...gle.com, ycheng@...gle.com, toke@...e.dk, 
	fuyuanli@...iglobal.com, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] tcp: handle the deferred sack compression when
 it should be canceled

On Tue, May 30, 2023 at 4:37 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> In the previous commits, we have not implemented to deal with the case
> where we're going to abort sending a ack triggered in the timer. So
> introducing a new flag ICSK_ACK_COMP_TIMER for sack compression only
> could satisify this requirement.
>


Sorry, I can not parse this changelog.

What is the problem you want to fix ?


> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> Comment on this:
> This patch is based on top of a commit[1]. I don't think the current
> patch is fixing a bug, but more like an improvement. So I would like
> to target at net-next tree.
>
> [1]: https://patchwork.kernel.org/project/netdevbpf/patch/20230529113804.GA20300@didi-ThinkCentre-M920t-N000/
> ---
>  include/net/inet_connection_sock.h | 3 ++-
>  net/ipv4/tcp_input.c               | 6 +++++-
>  net/ipv4/tcp_output.c              | 3 +++
>  net/ipv4/tcp_timer.c               | 5 ++++-
>  4 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> index c2b15f7e5516..34ff6d27471d 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_COMP_TIMER  = 32       /* Used for sack compression */
>  };
>
>  void inet_csk_init_xmit_timers(struct sock *sk,
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index cc072d2cfcd8..3980f77dcdff 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4540,6 +4540,9 @@ static void tcp_sack_compress_send_ack(struct sock *sk)
>         if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
>                 __sock_put(sk);
>
> +       /* It also deal with the case where the sack compression is deferred */
> +       inet_csk(sk)->icsk_ack.pending &= ~ICSK_ACK_COMP_TIMER;
> +
>         /* Since we have to send one ack finally,
>          * substract one from tp->compressed_ack to keep
>          * LINUX_MIB_TCPACKCOMPRESSED accurate.
> @@ -5555,7 +5558,7 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
>                 goto send_now;
>         }
>         tp->compressed_ack++;
> -       if (hrtimer_is_queued(&tp->compressed_ack_timer))
> +       if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_COMP_TIMER)
>                 return;

I do not see why adding a new bit, while hrtimer() already has one ?

>
>         /* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */
> @@ -5568,6 +5571,7 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
>                       READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns),
>                       rtt * (NSEC_PER_USEC >> 3)/20);
>         sock_hold(sk);
> +       inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_COMP_TIMER;
>         hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
>                                READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns),
>                                HRTIMER_MODE_REL_PINNED_SOFT);
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 02b58721ab6b..83840daad142 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -188,6 +188,9 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
>                 tp->compressed_ack = 0;
>                 if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
>                         __sock_put(sk);
> +
> +               /* It also deal with the case where the sack compression is deferred */
> +               inet_csk(sk)->icsk_ack.pending &= ~ICSK_ACK_COMP_TIMER;

This is adding a lot of code in fast path.

>         }
>
>         if (unlikely(rcv_nxt != tp->rcv_nxt))
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index b3f8933b347c..a970336d1383 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -323,9 +323,12 @@ void tcp_compack_timer_handler(struct sock *sk)
>  {
>         struct tcp_sock *tp = tcp_sk(sk);
>
> -       if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
> +       if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
> +           !(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_COMP_TIMER))
>                 return;
>
> +       inet_csk(sk)->icsk_ack.pending &= ~ICSK_ACK_COMP_TIMER;
> +
>         if (tp->compressed_ack) {
>                 /* Since we have to send one ack finally,
>                  * subtract one from tp->compressed_ack to keep
> --
> 2.37.3
>

An ACK is an ACK, I do not see why you need to differentiate them.