netdev - Re: [PATCH v2 net 1/5] tcp: feed correct number of pkts acked to cc modules also in recovery

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Date:   Fri, 9 Mar 2018 22:22:21 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
        netdev@...r.kernel.org
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 net 1/5] tcp: feed correct number of pkts acked to cc
 modules also in recovery

Hello!

On 03/09/2018 05:10 PM, Ilpo Järvinen wrote:

> A miscalculation for the number of acknowledged packets occurs during
> RTO recovery whenever SACK is not enabled and a cumulative ACK covers
> any non-retransmitted skbs. The reason is that pkts_acked value
> calculated in tcp_clean_rtx_queue is not correct for slow start after
> RTO as it may include segments that were not lost and therefore did
> not need retransmissions in the slow start following the RTO. Then
> tcp_slow_start will add the excess into cwnd bloating it and
> triggering a burst.
> 
> Instead, we want to pass only the number of retransmitted segments
> that were covered by the cumulative ACK (and potentially newly sent
> data segments too if the cumulative ACK covers that far).
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> ---
>  net/ipv4/tcp_input.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 9a1b3c1..0305f6d 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
[...]
> @@ -3068,8 +3072,11 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
>  			last_in_flight = TCP_SKB_CB(skb)->tx.in_flight;
>  			if (before(start_seq, reord))
>  				reord = start_seq;
> -			if (!after(scb->end_seq, tp->high_seq))
> +			if (!after(scb->end_seq, tp->high_seq)) {
>  				flag |= FLAG_ORIG_SACK_ACKED;
> +			} else {
> +				newdata_acked += acked_pcount;
> +			}

   Why add {} where they're not needed?

[...]

MBR, Sergei