Re: [PATCH net-next v2] tcp: fix condition for increasing pingpong count

[Neal Cardwell <ncardwell@google.com>]

On Wed, Jul 20, 2022 at 3:25 AM LemmyHuang <hlm3280@163.com> wrote:
>
> When CONFIG_HZ defaults to 1000Hz and the network transmission time is
> less than 1ms, lsndtime and lrcvtime are likely to be equal, which will
> lead to hundreds of interactions before entering pingpong mode.
>
> Fixes: 4a41f453bedf ("tcp: change pingpong threshold to 3")
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: LemmyHuang <hlm3280@163.com>
> ---
> v2:
>   * Use !after() wrapping the values. (Jakub Kicinski)
>
> v1: https://lore.kernel.org/netdev/20220719130136.11907-1-hlm3280@163.com/
> ---
>  net/ipv4/tcp_output.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 858a15cc2..c1c95dc40 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -172,7 +172,7 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
>          * and it is a reply for ato after last received packet,
>          * increase pingpong count.
>          */
> -       if (before(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
> +       if (!after(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
>             (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
>                 inet_csk_inc_pingpong_cnt(sk);
>
> --

Thanks for pointing out this problem!

AFAICT this patch would result in incorrect behavior.

With this patch, we could have cases where tp->lsndtime ==
icsk->icsk_ack.lrcvtime and (u32)(now - icsk->icsk_ack.lrcvtime) <
icsk->icsk_ack.ato and yet we do not really have a ping-pong exchange.

For example, with this patch we could have:

T1: jiffies=J1; host B receives RPC request from host A
T2: jiffies=J1; host B sends first RPC response data packet to host A;
      -> calls inet_csk_inc_pingpong_cnt()
T3: jiffies=J1; host B sends second RPC response data packet to host A;
      -> calls inet_csk_inc_pingpong_cnt()

In this scenario there is only one ping-pong exchange but the code
calls inet_csk_inc_pingpong_cnt() twice.

So I'm hoping we can come up with a better fix.

A simpler approach might be to simplify the model and go back to
having a single ping-pong interaction cause delayed ACKs to be enabled
on a connection endpoint. Our team has been seeing good results for a
while with the simpler approach. What do folks think?


neal