This is a note to let you know that I've just added the patch titled
tcp: when scheduling TLP, time of RTO should account for current ACK
to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: tcp-when-scheduling-tlp-time-of-rto-should-account-for-current-ack.patch and it can be found in the queue-4.14 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From foo@baz Thu Dec 14 11:45:40 CET 2017
From: Neal Cardwell ncardwell@google.com Date: Fri, 17 Nov 2017 21:06:14 -0500 Subject: tcp: when scheduling TLP, time of RTO should account for current ACK
From: Neal Cardwell ncardwell@google.com
[ Upstream commit ed66dfaf236c04d414de1d218441296e57fb2bd2 ]
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@google.com Signed-off-by: Yuchung Cheng ycheng@google.com Signed-off-by: Eric Dumazet edumazet@google.com Acked-by: Soheil Hassas Yeganeh soheil@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/net/tcp.h | 2 +- net/ipv4/tcp_input.c | 2 +- net/ipv4/tcp_output.c | 8 +++++--- 3 files changed, 7 insertions(+), 5 deletions(-)
--- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -563,7 +563,7 @@ void tcp_push_one(struct sock *, unsigne 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);
--- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -3021,7 +3021,7 @@ 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); }
--- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2337,7 +2337,7 @@ 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; @@ -2345,7 +2345,7 @@ repair: return !tp->packets_out && tcp_send_head(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); @@ -2384,7 +2384,9 @@ bool tcp_schedule_loss_probe(struct sock }
/* 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));
Patches currently in stable-queue which might be from ncardwell@google.com are
queue-4.14/tcp-when-scheduling-tlp-time-of-rto-should-account-for-current-ack.patch queue-4.14/tcp-use-current-time-in-tcp_rcv_space_adjust.patch