netdev - Re: [PATCH net] tcp: avoid too many retransmit packets

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Message-ID: <CAGHK07CjoeiUgNHrtY1+Km92rWft8nSONUnkzmRK4gOWW8npyQ@mail.gmail.com>
Date: Wed, 10 Jul 2024 11:43:05 +1000
From: Jonathan Maxwell <jmaxwell37@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	Kuniyuki Iwashima <kuniyu@...zon.com>, eric.dumazet@...il.com, 
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net] tcp: avoid too many retransmit packets

On Wed, Jul 10, 2024 at 10:14 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> If a TCP socket is using TCP_USER_TIMEOUT, and the other peer
> retracted its window to zero, tcp_retransmit_timer() can
> retransmit a packet every two jiffies (2 ms for HZ=1000),
> for about 4 minutes after TCP_USER_TIMEOUT has 'expired'.
>
> The fix is to make sure tcp_rtx_probe0_timed_out() takes
> icsk->icsk_user_timeout into account.
>
> Before blamed commit, the socket would not timeout after
> icsk->icsk_user_timeout, but would use standard exponential
> backoff for the retransmits.
>
> Also worth noting that before commit e89688e3e978 ("net: tcp:
> fix unexcepted socket die when snd_wnd is 0"), the issue
> would last 2 minutes instead of 4.
>
> Fixes: b701a99e431d ("tcp: Add tcp_clamp_rto_to_user_timeout() helper to improve accuracy")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Jon Maxwell <jmaxwell37@...il.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>

Nice catch Eric. Thanks for fixing LGTM.

Reviewed-by: Jon Maxwell <jmaxwell37@...il.com>



> ---
>  net/ipv4/tcp_timer.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index db9d826560e57caf8274d1b7253c7af4dd7821a0..892c86657fbc243ce53a939157b77f1fe0410097 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -483,15 +483,26 @@ static bool tcp_rtx_probe0_timed_out(const struct sock *sk,
>                                      const struct sk_buff *skb,
>                                      u32 rtx_delta)
>  {
> +       const struct inet_connection_sock *icsk = inet_csk(sk);
> +       u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);
>         const struct tcp_sock *tp = tcp_sk(sk);
> -       const int timeout = TCP_RTO_MAX * 2;
> +       int timeout = TCP_RTO_MAX * 2;
>         s32 rcv_delta;
>
> +       if (user_timeout) {
> +               /* If user application specified a TCP_USER_TIMEOUT,
> +                * it does not want win 0 packets to 'reset the timer'
> +                * while retransmits are not making progress.
> +                */
> +               if (rtx_delta > user_timeout)
> +                       return true;
> +               timeout = min_t(u32, timeout, msecs_to_jiffies(user_timeout));
> +       }
>         /* Note: timer interrupt might have been delayed by at least one jiffy,
>          * and tp->rcv_tstamp might very well have been written recently.
>          * rcv_delta can thus be negative.
>          */
> -       rcv_delta = inet_csk(sk)->icsk_timeout - tp->rcv_tstamp;
> +       rcv_delta = icsk->icsk_timeout - tp->rcv_tstamp;
>         if (rcv_delta <= timeout)
>                 return false;
>
> --
> 2.45.2.993.g49e7a77208-goog
>