Re: [PATCH net] tcp: introduce a compack timer handler in sack compression

[Eric Dumazet <edumazet@google.com>]

On Mon, May 29, 2023 at 1:38 PM fuyuanli <fuyuanli@didiglobal.com> wrote:
>
> We've got some issues when sending a compressed ack is deferred to
> release phrase due to the socket owned by another user:
> 1. a compressed ack would not be sent because of lack of ICSK_ACK_TIMER
> flag.

Are you sure ? Just add it then, your patch will be a one-liner
instead of a complex one adding
more code in a fast path.

> 2. the tp->compressed_ack counter should be decremented by 1.
> 3. we cannot pass timeout check and reset the delack timer in
> tcp_delack_timer_handler().
> 4. we are not supposed to increment the LINUX_MIB_DELAYEDACKS counter.
> ...
>
> The reason why it could happen is that we previously reuse the delayed
> ack logic when handling the sack compression. With this patch applied,
> the sack compression logic would go into the same function
> (tcp_compack_timer_handler()) whether we defer sending ack or not.
> Therefore, those two issued could be easily solved.
>
> Here are more details in the old logic:
> When sack compression is triggered in the tcp_compressed_ack_kick(),
> if the sock is owned by user, it will set TCP_DELACK_TIMER_DEFERRED and
> then defer to the release cb phrase. Later once user releases the sock,
> tcp_delack_timer_handler() should send a ack as expected, which, however,
> cannot happen due to lack of ICSK_ACK_TIMER flag. Therefore, the receiver
> would not sent an ack until the sender's retransmission timeout. It
> definitely increases unnecessary latency.
>
> This issue happens rarely in the production environment. I used kprobe
> to hook some key functions like tcp_compressed_ack_kick, tcp_release_cb,
> tcp_delack_timer_handler and then found that when tcp_delack_timer_handler
> was called, value of icsk_ack.pending was 1, which means we only had
> flag ICSK_ACK_SCHED set, not including ICSK_ACK_TIMER. It was against
> our expectations.
>
> In conclusion, we chose to separate the sack compression from delayed
> ack logic to solve issues only happening when the process is deferred.
>
> Fixes: 5d9f4262b7ea ("tcp: add SACK compression")
> Signed-off-by: fuyuanli <fuyuanli@didiglobal.com>
> Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
> ---
>  include/linux/tcp.h   |  2 ++
>  include/net/tcp.h     |  1 +
>  net/ipv4/tcp_output.c |  4 ++++
>  net/ipv4/tcp_timer.c  | 28 +++++++++++++++++++---------
>  4 files changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index b4c08ac86983..cd15a9972c48 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -461,6 +461,7 @@ enum tsq_enum {
>         TCP_MTU_REDUCED_DEFERRED,  /* tcp_v{4|6}_err() could not call
>                                     * tcp_v{4|6}_mtu_reduced()
>                                     */
> +       TCP_COMPACK_TIMER_DEFERRED, /* tcp_compressed_ack_kick() found socket was owned */
>  };
>
>  enum tsq_flags {
> @@ -470,6 +471,7 @@ enum tsq_flags {
>         TCPF_WRITE_TIMER_DEFERRED       = (1UL << TCP_WRITE_TIMER_DEFERRED),
>         TCPF_DELACK_TIMER_DEFERRED      = (1UL << TCP_DELACK_TIMER_DEFERRED),
>         TCPF_MTU_REDUCED_DEFERRED       = (1UL << TCP_MTU_REDUCED_DEFERRED),
> +       TCPF_COMPACK_TIMER_DEFERRED     = (1UL << TCP_DELACK_TIMER_DEFERRED),
>  };
>
>  #define tcp_sk(ptr) container_of_const(ptr, struct tcp_sock, inet_conn.icsk_inet.sk)
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 18a038d16434..e310d7bf400c 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -342,6 +342,7 @@ void tcp_release_cb(struct sock *sk);
>  void tcp_wfree(struct sk_buff *skb);
>  void tcp_write_timer_handler(struct sock *sk);
>  void tcp_delack_timer_handler(struct sock *sk);
> +void tcp_compack_timer_handler(struct sock *sk);
>  int tcp_ioctl(struct sock *sk, int cmd, unsigned long arg);
>  int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb);
>  void tcp_rcv_established(struct sock *sk, struct sk_buff *skb);
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index cfe128b81a01..1703caab6632 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1110,6 +1110,10 @@ void tcp_release_cb(struct sock *sk)
>                 tcp_delack_timer_handler(sk);
>                 __sock_put(sk);
>         }
> +       if (flags & TCPF_COMPACK_TIMER_DEFERRED) {
> +               tcp_compack_timer_handler(sk);
> +               __sock_put(sk);
> +       }

Please do not add another test in the fast path.

Just make sure tcp_delack_timer_handler() handles the case (this
certainly was my intent)


>         if (flags & TCPF_MTU_REDUCED_DEFERRED) {
>                 inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
>                 __sock_put(sk);
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index b839c2f91292..069f6442069b 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -318,6 +318,23 @@ void tcp_delack_timer_handler(struct sock *sk)
>         }
>  }
>
> +/* Called with BH disabled */
> +void tcp_compack_timer_handler(struct sock *sk)
> +{
> +       struct tcp_sock *tp = tcp_sk(sk);
> +
> +       if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
> +               return;
> +
> +       if (tp->compressed_ack) {
> +               /* Since we have to send one ack finally,
> +                * subtract one from tp->compressed_ack to keep
> +                * LINUX_MIB_TCPACKCOMPRESSED accurate.
> +                */
> +               tp->compressed_ack--;
> +               tcp_send_ack(sk);
> +       }
> +}
>
>  /**
>   *  tcp_delack_timer() - The TCP delayed ACK timeout handler
> @@ -757,16 +774,9 @@ static enum hrtimer_restart tcp_compressed_ack_kick(struct hrtimer *timer)
>
>         bh_lock_sock(sk);
>         if (!sock_owned_by_user(sk)) {
> -               if (tp->compressed_ack) {
> -                       /* Since we have to send one ack finally,
> -                        * subtract one from tp->compressed_ack to keep
> -                        * LINUX_MIB_TCPACKCOMPRESSED accurate.
> -                        */
> -                       tp->compressed_ack--;
> -                       tcp_send_ack(sk);
> -               }
> +               tcp_compack_timer_handler(sk);
>         } else {
> -               if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED,

See, I was clearly intending to let tcp_delack_timer_handler() deal with this.

> +               if (!test_and_set_bit(TCP_COMPACK_TIMER_DEFERRED,
>                                       &sk->sk_tsq_flags))
>                         sock_hold(sk);
>         }
> --
> 2.17.1
>