Re: [PATCH net] net: fix the RTO timer retransmitting skb every 1ms if linear option is enabled

[Simon Horman <horms@kernel.org>]

On Thu, Aug 10, 2023 at 07:21:48PM +0800, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> In the real workload, I encountered an issue which could cause the RTO
> timer to retransmit the skb per 1ms with linear option enabled. The amount
> of lost-retransmitted skbs can go up to 1000+ instantly.
> 
> The root cause is that if the icsk_rto happens to be zero in the 6th round
> (which is the TCP_THIN_LINEAR_RETRIES value), then it will alway be zero

nit: alway -> always

     checkpatch.pl --codespell is your friend

> due to the changed calculation method in tcp_retransmit_timer() as follows:
> 
> icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
> 
> Above line could be converted to
> icsk->icsk_rto = min(0 << 1, TCP_RTO_MAX) = 0
> 
> Therefore, the timer expires so quickly without any doubt.
> 
> I read through the RFC 6298 and found that the RTO value can be rounded
> up to a certain value, in Linux, say TCP_RTO_MIN as default, which is
> regarded as the lower bound in this patch as suggested by Eric.
> 
> Fixes: 36e31b0af587 ("net: TCP thin linear timeouts")
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
>  net/ipv4/tcp_timer.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index d45c96c7f5a4..b2b25861355c 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -599,7 +599,9 @@ void tcp_retransmit_timer(struct sock *sk)
>  	    tcp_stream_is_thin(tp) &&
>  	    icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
>  		icsk->icsk_backoff = 0;
> -		icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
> +		icsk->icsk_rto = clamp(__tcp_set_rto(tp),
> +					    tcp_rto_min(sk),
> +					    TCP_RTO_MAX);

nit: this indentation looks a bit odd.

		icsk->icsk_rto = clamp(__tcp_set_rto(tp),
				       tcp_rto_min(sk),
				       TCP_RTO_MAX);

>  	} else if (sk->sk_state != TCP_SYN_SENT ||
>  		   icsk->icsk_backoff >
>  		   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
> -- 
> 2.37.3
> 
>