![Yuchung Cheng](/assets/img/avatar_default.png)
This patch adjusts the timeout formula to schedule the TCP loss probe (TLP). The previous formula uses 2*SRTT or 1.5*RTT + DelayACKMax if only one packet is in flight. It keeps a lower bound of 10 msec which is too large for short RTT connections (e.g. within a data-center). The new formula = 2*RTT + (inflight == 1 ? 200ms : 2ticks) which performs better for short and fast connections. Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
178 lines
5.4 KiB
C
178 lines
5.4 KiB
C
#include <linux/tcp.h>
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#include <net/tcp.h>
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int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOSS_DETECTION;
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static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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tcp_skb_mark_lost_uncond_verify(tp, skb);
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if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
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/* Account for retransmits that are lost again */
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TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
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tp->retrans_out -= tcp_skb_pcount(skb);
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NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
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tcp_skb_pcount(skb));
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}
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}
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static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
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{
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return t1 > t2 || (t1 == t2 && after(seq1, seq2));
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}
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/* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
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*
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* Marks a packet lost, if some packet sent later has been (s)acked.
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* The underlying idea is similar to the traditional dupthresh and FACK
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* but they look at different metrics:
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*
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* dupthresh: 3 OOO packets delivered (packet count)
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* FACK: sequence delta to highest sacked sequence (sequence space)
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* RACK: sent time delta to the latest delivered packet (time domain)
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*
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* The advantage of RACK is it applies to both original and retransmitted
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* packet and therefore is robust against tail losses. Another advantage
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* is being more resilient to reordering by simply allowing some
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* "settling delay", instead of tweaking the dupthresh.
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*
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* When tcp_rack_detect_loss() detects some packets are lost and we
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* are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
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* or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
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* make us enter the CA_Recovery state.
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*/
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static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct sk_buff *skb;
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u32 reo_wnd;
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*reo_timeout = 0;
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/* To be more reordering resilient, allow min_rtt/4 settling delay
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* (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
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* RTT because reordering is often a path property and less related
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* to queuing or delayed ACKs.
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*/
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reo_wnd = 1000;
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if ((tp->rack.reord || !tp->lost_out) && tcp_min_rtt(tp) != ~0U)
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reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd);
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tcp_for_write_queue(skb, sk) {
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struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
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if (skb == tcp_send_head(sk))
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break;
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/* Skip ones already (s)acked */
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if (!after(scb->end_seq, tp->snd_una) ||
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scb->sacked & TCPCB_SACKED_ACKED)
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continue;
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if (tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
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tp->rack.end_seq, scb->end_seq)) {
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/* Step 3 in draft-cheng-tcpm-rack-00.txt:
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* A packet is lost if its elapsed time is beyond
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* the recent RTT plus the reordering window.
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*/
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u32 elapsed = tcp_stamp_us_delta(tp->tcp_mstamp,
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skb->skb_mstamp);
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s32 remaining = tp->rack.rtt_us + reo_wnd - elapsed;
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if (remaining < 0) {
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tcp_rack_mark_skb_lost(sk, skb);
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continue;
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}
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/* Skip ones marked lost but not yet retransmitted */
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if ((scb->sacked & TCPCB_LOST) &&
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!(scb->sacked & TCPCB_SACKED_RETRANS))
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continue;
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/* Record maximum wait time (+1 to avoid 0) */
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*reo_timeout = max_t(u32, *reo_timeout, 1 + remaining);
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} else if (!(scb->sacked & TCPCB_RETRANS)) {
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/* Original data are sent sequentially so stop early
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* b/c the rest are all sent after rack_sent
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*/
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break;
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}
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}
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}
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void tcp_rack_mark_lost(struct sock *sk)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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u32 timeout;
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if (!tp->rack.advanced)
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return;
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/* Reset the advanced flag to avoid unnecessary queue scanning */
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tp->rack.advanced = 0;
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tcp_rack_detect_loss(sk, &timeout);
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if (timeout) {
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timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
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inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
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timeout, inet_csk(sk)->icsk_rto);
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}
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}
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/* Record the most recently (re)sent time among the (s)acked packets
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* This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
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* draft-cheng-tcpm-rack-00.txt
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*/
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void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
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u64 xmit_time)
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{
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u32 rtt_us;
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if (tp->rack.mstamp &&
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!tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
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end_seq, tp->rack.end_seq))
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return;
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rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
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if (sacked & TCPCB_RETRANS) {
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/* If the sacked packet was retransmitted, it's ambiguous
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* whether the retransmission or the original (or the prior
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* retransmission) was sacked.
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*
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* If the original is lost, there is no ambiguity. Otherwise
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* we assume the original can be delayed up to aRTT + min_rtt.
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* the aRTT term is bounded by the fast recovery or timeout,
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* so it's at least one RTT (i.e., retransmission is at least
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* an RTT later).
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*/
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if (rtt_us < tcp_min_rtt(tp))
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return;
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}
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tp->rack.rtt_us = rtt_us;
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tp->rack.mstamp = xmit_time;
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tp->rack.end_seq = end_seq;
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tp->rack.advanced = 1;
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}
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/* We have waited long enough to accommodate reordering. Mark the expired
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* packets lost and retransmit them.
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*/
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void tcp_rack_reo_timeout(struct sock *sk)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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u32 timeout, prior_inflight;
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prior_inflight = tcp_packets_in_flight(tp);
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tcp_rack_detect_loss(sk, &timeout);
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if (prior_inflight != tcp_packets_in_flight(tp)) {
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if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
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tcp_enter_recovery(sk, false);
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if (!inet_csk(sk)->icsk_ca_ops->cong_control)
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tcp_cwnd_reduction(sk, 1, 0);
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}
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tcp_xmit_retransmit_queue(sk);
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}
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if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
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tcp_rearm_rto(sk);
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}
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