netdev - Re: [PATCH net-next 3/3] net: tcp: check timeout by icsk->icsk_timeout in tcp_retransmit

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Date: Thu, 27 Jul 2023 21:31:24 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: menglong8.dong@...il.com
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	dsahern@...nel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Menglong Dong <imagedong@...cent.com>, Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next 3/3] net: tcp: check timeout by
 icsk->icsk_timeout in tcp_retransmit_timer()

On Thu, Jul 27, 2023 at 2:52 PM <menglong8.dong@...il.com> wrote:
>
> From: Menglong Dong <imagedong@...cent.com>
>
> In tcp_retransmit_timer(), a window shrunk connection will be regarded
> as timeout if 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX'. This is not
> right all the time.
>
> The retransmits will become zero-window probes in tcp_retransmit_timer()
> if the 'snd_wnd==0'. Therefore, the icsk->icsk_rto will come up to
> TCP_RTO_MAX sooner or later.
>
> However, the timer is not precise enough, as it base on timer wheel.
> Sorry that I am not good at timer, but I know the concept of time-wheel.
> The longer of the timer, the rougher it will be. So the timeout is not
> triggered after TCP_RTO_MAX, but 122877ms as I tested.
>
> Therefore, 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX' is always true
> once the RTO come up to TCP_RTO_MAX.
>
> Fix this by replacing the 'tcp_jiffies32' with '(u32)icsk->icsk_timeout',
> which is exact the timestamp of the timeout.
>
> Signed-off-by: Menglong Dong <imagedong@...cent.com>
> ---
>  net/ipv4/tcp_timer.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index 470f581eedd4..3a20db15a186 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -511,7 +511,11 @@ void tcp_retransmit_timer(struct sock *sk)
>                                             tp->snd_una, tp->snd_nxt);
>                 }
>  #endif
> -               if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX) {
> +               /* It's a little rough here, we regard any valid packet that
> +                * update tp->rcv_tstamp as the reply of the retransmitted
> +                * packet.
> +                */
> +               if ((u32)icsk->icsk_timeout - tp->rcv_tstamp > TCP_RTO_MAX) {
>                         tcp_write_err(sk);
>                         goto out;
>                 }


Hmm, this looks like a net candidate, since this is unrelated to the
other patches ?

Neal, what do you think ?