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

[Eric Dumazet <edumazet@google.com>]

On Tue, May 30, 2023 at 9:45 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> On Tue, May 30, 2023 at 3:12 PM Eric Dumazet <edumazet@google.com> wrote:
> >
> > 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.
>
> Honestly, at the very beginning, we just added one line[1] to fix
> this. After I digged more into this part, I started to doubt if we
> should reuse the delayed ack logic.
>
> Because in the sack compression logic there is no need to do more
> things as delayed ack does in the tcp_delack_timer_handler() function.
>
> Besides, here are some things extra to be done if we defer to send an
> ack in sack compression:
> 1) decrease tp->compressed_ack. The same as "tp->compressed_ack--;" in
> tcp_compressed_ack_kick().
> 2) initialize icsk->icsk_ack.timeout. Actually we don't need to do
> this because we don't modify the expiration time in the sack
> compression hrtimer.

Yes, we do not need this, see my following comment.

> 3) don't need to count the LINUX_MIB_DELAYEDACKS counter.
> 4) I wonder even if those checks about the ack schedule or ping pong
> mode in tcp_delack_timer_handler() for sack compression? I'm not sure
> about it.
>
> So one line cannot solve it perfectly. That's the reason why we
> introduce a new logic which can be clearer.
>
> I'm wondering if adding one check in the fast path is really that
> unacceptable (it may hurt performance?) because a new logic would be
> clearer for the whole sack compression.

We definitely can solve the minor issue by not polluting the fast path.

We also want simple/localised fixes, not something invasive (and risky).

>
> [1]
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index cc072d2cfcd8..d9e76d761cc6 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5568,6 +5568,7 @@ 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_TIMER;

Why not simply use existing storage/variables (tp->compressed_ack),
instead of trying
to reuse something else or add another bit, then complain that this
does not work well ?

Again, just fix tcp_delack_timer_handler(), it already can fetch existing state.

As a bonus, no need to send one patch for net, and another in net-next,
trying to 'fix' issues that should have been fixed cleanly in a single patch.

Something like:

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index b839c2f91292f7346f33d6dcbf597594473a5aca..16bc4cedceb8a5e88f61f9abc2c0a8cc9322676a
100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -290,10 +290,20 @@ static int tcp_write_timeout(struct sock *sk)
 void tcp_delack_timer_handler(struct sock *sk)
 {
        struct inet_connection_sock *icsk = inet_csk(sk);
+       struct tcp_sock *tp = tcp_sk(sk);

-       if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
-           !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
+       if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
+               return;
+
+       if (!(icsk->icsk_ack.pending & ICSK_ACK_TIMER) &&
+           !tp->compressed_ack)
+               return;
+
+       if (tp->compressed_ack) {
+               tcp_mstamp_refresh(tp);
+               tcp_sack_compress_send_ack(sk);
                return;
+       }

        if (time_after(icsk->icsk_ack.timeout, jiffies)) {
                sk_reset_timer(sk, &icsk->icsk_delack_timer,
icsk->icsk_ack.timeout);
@@ -312,7 +322,7 @@ void tcp_delack_timer_handler(struct sock *sk)
                        inet_csk_exit_pingpong_mode(sk);
                        icsk->icsk_ack.ato      = TCP_ATO_MIN;
                }
-               tcp_mstamp_refresh(tcp_sk(sk));
+               tcp_mstamp_refresh(tp);
                tcp_send_ack(sk);
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKS);
        }