netdev - Re: [PATCH RFC net-next] tcp: improve SACK RTT for CC

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Date:	Sun, 12 Apr 2015 11:37:28 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Kenneth Klette Jonassen <kennetkl@....uio.no>
Cc:	netdev@...r.kernel.org, ycheng@...gle.com
Subject: Re: [PATCH RFC net-next] tcp: improve SACK RTT for CC

On Sun, 2015-04-12 at 19:51 +0200, Kenneth Klette Jonassen wrote:
> tcp_sacktag_one() always picks the earliest sequence SACKed for RTT.
> This might not make sense for congestion control in cases where:
> 
>   1. ACKs are lost, i.e. a SACK subsequent to a lost SACK covers both
>      a new and an old segment at the receiver.
>   2. The receiver disregards the RFC 5681 recommendation to immediately
>      ACK out-of-order segments.
> 
> Give congestion control a RTT for the latest segment SACKed, which is the
> most accurate RTT estimate, but preserve the conservative RTT for RTO.
> 
> Also remove the dependency on skb_mstamp_get() in tcp_sacktag_one() which
> potentially contributes to variance between RTT signals.
> 
> This is a request for comments. For full effect, a later patch would move
> the call to pkts_acked() outside the scope of if (flag & FLAG_ACKED). As is,
> this hook only gets called when sequentially acknowledging new data.
> ---
>  net/ipv4/tcp_input.c | 90 ++++++++++++++++++++++++++++++----------------------
>  1 file changed, 52 insertions(+), 38 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index a7ef679..8f0bd3c 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1130,7 +1130,16 @@ static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
>  struct tcp_sacktag_state {
>  	int	reord;
>  	int	fack_count;
> -	long	rtt_us; /* RTT measured by SACKing never-retransmitted data */
> +	/* Timestamp for earliest never-retransmitted segment that was
> +	 * SACKed and its usec delta to the corresponding latest segment.
> +	 * RTO needs the earliest RTT to be conservative against receivers
> +	 * that might delay SACKing (RFC 5681 does not require ACKing
> +	 * out-of-order segments immediately), but congestion control should
> +	 * still get an accurate delay signal. Even without delayed SACKing,
> +	 * this helps mitigate the effect of ACK loss on delay-based CC.
> +	 */
> +	struct skb_mstamp first_ackt;
> +	u32	last_ack_us_delta;
>  	int	flag;
>  };
>  
> @@ -1233,14 +1242,13 @@ static u8 tcp_sacktag_one(struct sock *sk,
>  							   state->reord);
>  				if (!after(end_seq, tp->high_seq))
>  					state->flag |= FLAG_ORIG_SACK_ACKED;
> -				/* Pick the earliest sequence sacked for RTT */
> -				if (state->rtt_us < 0) {
> -					struct skb_mstamp now;
>  
> -					skb_mstamp_get(&now);
> -					state->rtt_us = skb_mstamp_us_delta(&now,
> -								xmit_time);
> -				}
> +				if (state->first_ackt.v64 == 0)
> +					state->first_ackt.v64 = xmit_time->v64;
> +				else
> +					state->last_ack_us_delta =
> +						 skb_mstamp_us_delta(xmit_time,
> +								     &state->first_ackt);

I do not think you need to perform this skb_mstamp_us_delta() at this
point.

Instead of storing a state->last_ack_us_delta, I would store
state->last_ackt

And later doing a single skb_mstamp_us_delta()



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