netdev - Re: [PATCH net] tcp: when scheduling TLP, time of RTO should account for current ACK

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Date:   Sun, 19 Nov 2017 12:26:08 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     ncardwell@...gle.com
Cc:     netdev@...r.kernel.org, ycheng@...gle.com, edumazet@...gle.com
Subject: Re: [PATCH net] tcp: when scheduling TLP, time of RTO should
 account for current ACK

From: Neal Cardwell <ncardwell@...gle.com>
Date: Fri, 17 Nov 2017 21:06:14 -0500

> Fix the TLP scheduling logic so that when scheduling a TLP probe, we
> ensure that the estimated time at which an RTO would fire accounts for
> the fact that ACKs indicating forward progress should push back RTO
> times.
> 
> After the following fix:
> 
> df92c8394e6e ("tcp: fix xmit timer to only be reset if data ACKed/SACKed")
> 
> we had an unintentional behavior change in the following kind of
> scenario: suppose the RTT variance has been very low recently. Then
> suppose we send out a flight of N packets and our RTT is 100ms:
> 
> t=0: send a flight of N packets
> t=100ms: receive an ACK for N-1 packets
> 
> The response before df92c8394e6e that was:
>   -> schedule a TLP for now + RTO_interval
> 
> The response after df92c8394e6e is:
>   -> schedule a TLP for t=0 + RTO_interval
> 
> Since RTO_interval = srtt + RTT_variance, this means that we have
> scheduled a TLP timer at a point in the future that only accounts for
> RTT_variance. If the RTT_variance term is small, this means that the
> timer fires soon.
> 
> Before df92c8394e6e this would not happen, because in that code, when
> we receive an ACK for a prefix of flight, we did:
> 
>     1) Near the top of tcp_ack(), switch from TLP timer to RTO
>        at write_queue_head->paket_tx_time + RTO_interval:
>             if (icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
>                    tcp_rearm_rto(sk);
> 
>     2) In tcp_clean_rtx_queue(), update the RTO to now + RTO_interval:
>             if (flag & FLAG_ACKED) {
>                    tcp_rearm_rto(sk);
> 
>     3) In tcp_ack() after tcp_fastretrans_alert() switch from RTO
>        to TLP at now + RTO_interval:
>             if (icsk->icsk_pending == ICSK_TIME_RETRANS)
>                    tcp_schedule_loss_probe(sk);
> 
> In df92c8394e6e we removed that 3-phase dance, and instead directly
> set the TLP timer once: we set the TLP timer in cases like this to
> write_queue_head->packet_tx_time + RTO_interval. So if the RTT
> variance is small, then this means that this is setting the TLP timer
> to fire quite soon. This means if the ACK for the tail of the flight
> takes longer than an RTT to arrive (often due to delayed ACKs), then
> the TLP timer fires too quickly.
> 
> Fixes: df92c8394e6e ("tcp: fix xmit timer to only be reset if data ACKed/SACKed")
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied and queued up for -stable, thanks.