netdev - Re: [PATCH v2 net] net: fix the RTO timer retransmitting skb every 1ms if linear option is enabled

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Message-ID: <CANn89i+RkWM6ZV4xzfTcPNG9o2J7bggTmzuOZi=MWMHxqdkauw@mail.gmail.com>
Date: Tue, 15 Aug 2023 08:59:32 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, 
	pabeni@...hat.com, apetlund@...ula.no, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH v2 net] net: fix the RTO timer retransmitting skb every
 1ms if linear option is enabled

On Fri, Aug 11, 2023 at 4:38 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.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 always be zero
> 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@...gle.com>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>