From 5598a97bcf374991575c1409cc179a46c37c576e Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Wed, 18 May 2022 17:34:10 -0700 Subject: [PATCH] tcp: improve PRR loss recovery This patch improves TCP PRR loss recovery behavior for a corner case. Previously during PRR conservation-bound mode, it strictly sends the amount equals to the amount newly acked or s/acked. The patch changes s.t. PRR may send additional amount that was banked previously (e.g. application-limited) in the conservation-bound mode, similar to the slow-start mode. This unifies and simplifies the algorithm further and may improve the recovery latency. This change still follow the general packet conservation design principle and always keep inflight/cwnd below the slow start threshold set by the congestion control module. PRR is described in RFC 6937. We'll include this change in the latest revision rfc6937-bis as well. Reported-by: Neal Cardwell Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Link: https://lore.kernel.org/r/20220519003410.2531936-1-ycheng@google.com Signed-off-by: Jakub Kicinski --- net/ipv4/tcp_input.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 97cfcd85f84e6..3231af73e4302 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2620,12 +2620,12 @@ void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost, u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered + tp->prior_cwnd - 1; sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out; - } else if (flag & FLAG_SND_UNA_ADVANCED && !newly_lost) { - sndcnt = min_t(int, delta, - max_t(int, tp->prr_delivered - tp->prr_out, - newly_acked_sacked) + 1); } else { - sndcnt = min(delta, newly_acked_sacked); + sndcnt = max_t(int, tp->prr_delivered - tp->prr_out, + newly_acked_sacked); + if (flag & FLAG_SND_UNA_ADVANCED && !newly_lost) + sndcnt++; + sndcnt = min(delta, sndcnt); } /* Force a fast retransmit upon entering fast recovery */ sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1)); -- 2.39.5