linux/net/ipv4/tcp_recovery.c
Priyaranjan Jha 1f2556916d tcp: higher throughput under reordering with adaptive RACK reordering wnd
Currently TCP RACK loss detection does not work well if packets are
being reordered beyond its static reordering window (min_rtt/4).Under
such reordering it may falsely trigger loss recoveries and reduce TCP
throughput significantly.

This patch improves that by increasing and reducing the reordering
window based on DSACK, which is now supported in major TCP implementations.
It makes RACK's reo_wnd adaptive based on DSACK and no. of recoveries.

- If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
  by srtt), since there is possibility that spurious retransmission was
  due to reordering delay longer than reo_wnd.

- Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
  no. of successful recoveries (accounts for full DSACK-based loss
  recovery undo). After that, reset it to default (min_rtt/4).

- At max, reo_wnd is incremented only once per rtt. So that the new
  DSACK on which we are reacting, is due to the spurious retx (approx)
  after the reo_wnd has been updated last time.

- reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
  absolute value to account for change in rtt.

In our internal testing, we observed significant increase in throughput,
in scenarios where reordering exceeds min_rtt/4 (previous static value).

Signed-off-by: Priyaranjan Jha <priyarjha@google.com>
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>
2017-11-05 23:15:42 +09:00

207 lines
6.7 KiB
C

// SPDX-License-Identifier: GPL-2.0
#include <linux/tcp.h>
#include <net/tcp.h>
static void tcp_rack_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
tcp_skb_mark_lost_uncond_verify(tp, skb);
if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
/* Account for retransmits that are lost again */
TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
tp->retrans_out -= tcp_skb_pcount(skb);
NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
tcp_skb_pcount(skb));
}
}
static bool tcp_rack_sent_after(u64 t1, u64 t2, u32 seq1, u32 seq2)
{
return t1 > t2 || (t1 == t2 && after(seq1, seq2));
}
/* RACK loss detection (IETF draft draft-ietf-tcpm-rack-01):
*
* Marks a packet lost, if some packet sent later has been (s)acked.
* The underlying idea is similar to the traditional dupthresh and FACK
* but they look at different metrics:
*
* dupthresh: 3 OOO packets delivered (packet count)
* FACK: sequence delta to highest sacked sequence (sequence space)
* RACK: sent time delta to the latest delivered packet (time domain)
*
* The advantage of RACK is it applies to both original and retransmitted
* packet and therefore is robust against tail losses. Another advantage
* is being more resilient to reordering by simply allowing some
* "settling delay", instead of tweaking the dupthresh.
*
* When tcp_rack_detect_loss() detects some packets are lost and we
* are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
* or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
* make us enter the CA_Recovery state.
*/
static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
{
struct tcp_sock *tp = tcp_sk(sk);
u32 min_rtt = tcp_min_rtt(tp);
struct sk_buff *skb, *n;
u32 reo_wnd;
*reo_timeout = 0;
/* To be more reordering resilient, allow min_rtt/4 settling delay
* (lower-bounded to 1000uS). We use min_rtt instead of the smoothed
* RTT because reordering is often a path property and less related
* to queuing or delayed ACKs.
*/
reo_wnd = 1000;
if ((tp->rack.reord || !tp->lost_out) && min_rtt != ~0U) {
reo_wnd = max((min_rtt >> 2) * tp->rack.reo_wnd_steps, reo_wnd);
reo_wnd = min(reo_wnd, tp->srtt_us >> 3);
}
list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
tcp_tsorted_anchor) {
struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
s32 remaining;
/* Skip ones marked lost but not yet retransmitted */
if ((scb->sacked & TCPCB_LOST) &&
!(scb->sacked & TCPCB_SACKED_RETRANS))
continue;
if (!tcp_rack_sent_after(tp->rack.mstamp, skb->skb_mstamp,
tp->rack.end_seq, scb->end_seq))
break;
/* A packet is lost if it has not been s/acked beyond
* the recent RTT plus the reordering window.
*/
remaining = tp->rack.rtt_us + reo_wnd -
tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp);
if (remaining < 0) {
tcp_rack_mark_skb_lost(sk, skb);
list_del_init(&skb->tcp_tsorted_anchor);
} else {
/* Record maximum wait time (+1 to avoid 0) */
*reo_timeout = max_t(u32, *reo_timeout, 1 + remaining);
}
}
}
void tcp_rack_mark_lost(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout;
if (!tp->rack.advanced)
return;
/* Reset the advanced flag to avoid unnecessary queue scanning */
tp->rack.advanced = 0;
tcp_rack_detect_loss(sk, &timeout);
if (timeout) {
timeout = usecs_to_jiffies(timeout) + TCP_TIMEOUT_MIN;
inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
timeout, inet_csk(sk)->icsk_rto);
}
}
/* Record the most recently (re)sent time among the (s)acked packets
* This is "Step 3: Advance RACK.xmit_time and update RACK.RTT" from
* draft-cheng-tcpm-rack-00.txt
*/
void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
u64 xmit_time)
{
u32 rtt_us;
if (tp->rack.mstamp &&
!tcp_rack_sent_after(xmit_time, tp->rack.mstamp,
end_seq, tp->rack.end_seq))
return;
rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, xmit_time);
if (sacked & TCPCB_RETRANS) {
/* If the sacked packet was retransmitted, it's ambiguous
* whether the retransmission or the original (or the prior
* retransmission) was sacked.
*
* If the original is lost, there is no ambiguity. Otherwise
* we assume the original can be delayed up to aRTT + min_rtt.
* the aRTT term is bounded by the fast recovery or timeout,
* so it's at least one RTT (i.e., retransmission is at least
* an RTT later).
*/
if (rtt_us < tcp_min_rtt(tp))
return;
}
tp->rack.rtt_us = rtt_us;
tp->rack.mstamp = xmit_time;
tp->rack.end_seq = end_seq;
tp->rack.advanced = 1;
}
/* We have waited long enough to accommodate reordering. Mark the expired
* packets lost and retransmit them.
*/
void tcp_rack_reo_timeout(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout, prior_inflight;
prior_inflight = tcp_packets_in_flight(tp);
tcp_rack_detect_loss(sk, &timeout);
if (prior_inflight != tcp_packets_in_flight(tp)) {
if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
tcp_enter_recovery(sk, false);
if (!inet_csk(sk)->icsk_ca_ops->cong_control)
tcp_cwnd_reduction(sk, 1, 0);
}
tcp_xmit_retransmit_queue(sk);
}
if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
tcp_rearm_rto(sk);
}
/* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
*
* If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
* by srtt), since there is possibility that spurious retransmission was
* due to reordering delay longer than reo_wnd.
*
* Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
* no. of successful recoveries (accounts for full DSACK-based loss
* recovery undo). After that, reset it to default (min_rtt/4).
*
* At max, reo_wnd is incremented only once per rtt. So that the new
* DSACK on which we are reacting, is due to the spurious retx (approx)
* after the reo_wnd has been updated last time.
*
* reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
* absolute value to account for change in rtt.
*/
void tcp_rack_update_reo_wnd(struct sock *sk, struct rate_sample *rs)
{
struct tcp_sock *tp = tcp_sk(sk);
if (sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_STATIC_REO_WND ||
!rs->prior_delivered)
return;
/* Disregard DSACK if a rtt has not passed since we adjusted reo_wnd */
if (before(rs->prior_delivered, tp->rack.last_delivered))
tp->rack.dsack_seen = 0;
/* Adjust the reo_wnd if update is pending */
if (tp->rack.dsack_seen) {
tp->rack.reo_wnd_steps = min_t(u32, 0xFF,
tp->rack.reo_wnd_steps + 1);
tp->rack.dsack_seen = 0;
tp->rack.last_delivered = tp->delivered;
tp->rack.reo_wnd_persist = TCP_RACK_RECOVERY_THRESH;
} else if (!tp->rack.reo_wnd_persist) {
tp->rack.reo_wnd_steps = 1;
}
}