From ed66dfaf236c04d414de1d218441296e57fb2bd2 Mon Sep 17 00:00:00 2001 From: Neal Cardwell Date: Fri, 17 Nov 2017 21:06:14 -0500 Subject: [PATCH] tcp: when scheduling TLP, time of RTO should account for current ACK 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 Signed-off-by: Yuchung Cheng Signed-off-by: Eric Dumazet Acked-by: Soheil Hassas Yeganeh Signed-off-by: David S. Miller --- include/net/tcp.h | 2 +- net/ipv4/tcp_input.c | 2 +- net/ipv4/tcp_output.c | 8 +++++--- 3 files changed, 7 insertions(+), 5 deletions(-) diff --git a/include/net/tcp.h b/include/net/tcp.h index 85ea578..4e09398 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -539,7 +539,7 @@ void tcp_push_one(struct sock *, unsigned int mss_now); void tcp_send_ack(struct sock *sk); void tcp_send_delayed_ack(struct sock *sk); void tcp_send_loss_probe(struct sock *sk); -bool tcp_schedule_loss_probe(struct sock *sk); +bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto); void tcp_skb_collapse_tstamp(struct sk_buff *skb, const struct sk_buff *next_skb); diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index f844c06..734cfc8f 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2964,7 +2964,7 @@ void tcp_rearm_rto(struct sock *sk) /* Try to schedule a loss probe; if that doesn't work, then schedule an RTO. */ static void tcp_set_xmit_timer(struct sock *sk) { - if (!tcp_schedule_loss_probe(sk)) + if (!tcp_schedule_loss_probe(sk, true)) tcp_rearm_rto(sk); } diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 540b7d9..a4d214c 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2391,7 +2391,7 @@ repair: /* Send one loss probe per tail loss episode. */ if (push_one != 2) - tcp_schedule_loss_probe(sk); + tcp_schedule_loss_probe(sk, false); is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tp->snd_cwnd); tcp_cwnd_validate(sk, is_cwnd_limited); return false; @@ -2399,7 +2399,7 @@ repair: return !tp->packets_out && !tcp_write_queue_empty(sk); } -bool tcp_schedule_loss_probe(struct sock *sk) +bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto) { struct inet_connection_sock *icsk = inet_csk(sk); struct tcp_sock *tp = tcp_sk(sk); @@ -2440,7 +2440,9 @@ bool tcp_schedule_loss_probe(struct sock *sk) } /* If the RTO formula yields an earlier time, then use that time. */ - rto_delta_us = tcp_rto_delta_us(sk); /* How far in future is RTO? */ + rto_delta_us = advancing_rto ? + jiffies_to_usecs(inet_csk(sk)->icsk_rto) : + tcp_rto_delta_us(sk); /* How far in future is RTO? */ if (rto_delta_us > 0) timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us)); -- 2.7.4