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tcp: retransmit after recovery processing and congestion control
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The retransmission and F-RTO transmission currently happen inside
recovery state processing (tcp_fastretrans_alert) but before
congestion control.  This refactoring moves the logic after both
s.t. we can determine how much to send (cwnd) before deciding what to
send.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Yuchung Cheng authored and David S. Miller committed Feb 7, 2016
1 parent 3575dbf commit e662ca4
Showing 1 changed file with 46 additions and 12 deletions.
58 changes: 46 additions & 12 deletions net/ipv4/tcp_input.c
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Original file line number Diff line number Diff line change
Expand Up @@ -126,6 +126,10 @@ int sysctl_tcp_invalid_ratelimit __read_mostly = HZ/2;
#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
#define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))

#define REXMIT_NONE 0 /* no loss recovery to do */
#define REXMIT_LOST 1 /* retransmit packets marked lost */
#define REXMIT_NEW 2 /* FRTO-style transmit of unsent/new packets */

/* Adapt the MSS value used to make delayed ack decision to the
* real world.
*/
Expand Down Expand Up @@ -2662,7 +2666,8 @@ static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
/* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
* recovered or spurious. Otherwise retransmits more on partial ACKs.
*/
static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
int *rexmit)
{
struct tcp_sock *tp = tcp_sk(sk);
bool recovered = !before(tp->snd_una, tp->high_seq);
Expand All @@ -2684,10 +2689,15 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
tp->frto = 0; /* Step 3.a. loss was real */
} else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
tp->high_seq = tp->snd_nxt;
__tcp_push_pending_frames(sk, tcp_current_mss(sk),
TCP_NAGLE_OFF);
if (after(tp->snd_nxt, tp->high_seq))
return; /* Step 2.b */
/* Step 2.b. Try send new data (but deferred until cwnd
* is updated in tcp_ack()). Otherwise fall back to
* the conventional recovery.
*/
if (tcp_send_head(sk) &&
after(tcp_wnd_end(tp), tp->snd_nxt)) {
*rexmit = REXMIT_NEW;
return;
}
tp->frto = 0;
}
}
Expand All @@ -2706,7 +2716,7 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
else if (flag & FLAG_SND_UNA_ADVANCED)
tcp_reset_reno_sack(tp);
}
tcp_xmit_retransmit_queue(sk);
*rexmit = REXMIT_LOST;
}

/* Undo during fast recovery after partial ACK. */
Expand Down Expand Up @@ -2756,7 +2766,7 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked,
*/
static void tcp_fastretrans_alert(struct sock *sk, const int acked,
const int prior_unsacked,
bool is_dupack, int flag)
bool is_dupack, int flag, int *rexmit)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
Expand Down Expand Up @@ -2831,7 +2841,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
}
break;
case TCP_CA_Loss:
tcp_process_loss(sk, flag, is_dupack);
tcp_process_loss(sk, flag, is_dupack, rexmit);
if (icsk->icsk_ca_state != TCP_CA_Open &&
!(flag & FLAG_LOST_RETRANS))
return;
Expand Down Expand Up @@ -2871,7 +2881,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
if (do_lost)
tcp_update_scoreboard(sk, fast_rexmit);
tcp_cwnd_reduction(sk, prior_unsacked, fast_rexmit, flag);
tcp_xmit_retransmit_queue(sk);
*rexmit = REXMIT_LOST;
}

/* Kathleen Nichols' algorithm for tracking the minimum value of
Expand Down Expand Up @@ -3506,6 +3516,27 @@ static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
icsk->icsk_ca_ops->in_ack_event(sk, flags);
}

/* Congestion control has updated the cwnd already. So if we're in
* loss recovery then now we do any new sends (for FRTO) or
* retransmits (for CA_Loss or CA_recovery) that make sense.
*/
static void tcp_xmit_recovery(struct sock *sk, int rexmit)
{
struct tcp_sock *tp = tcp_sk(sk);

if (rexmit == REXMIT_NONE)
return;

if (unlikely(rexmit == 2)) {
__tcp_push_pending_frames(sk, tcp_current_mss(sk),
TCP_NAGLE_OFF);
if (after(tp->snd_nxt, tp->high_seq))
return;
tp->frto = 0;
}
tcp_xmit_retransmit_queue(sk);
}

/* This routine deals with incoming acks, but not outgoing ones. */
static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
{
Expand All @@ -3520,6 +3551,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
int prior_packets = tp->packets_out;
const int prior_unsacked = tp->packets_out - tp->sacked_out;
int acked = 0; /* Number of packets newly acked */
int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */

sack_state.first_sackt.v64 = 0;

Expand Down Expand Up @@ -3616,7 +3648,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
if (tcp_ack_is_dubious(sk, flag)) {
is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
tcp_fastretrans_alert(sk, acked, prior_unsacked,
is_dupack, flag);
is_dupack, flag, &rexmit);
}
if (tp->tlp_high_seq)
tcp_process_tlp_ack(sk, ack, flag);
Expand All @@ -3634,13 +3666,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
if (icsk->icsk_pending == ICSK_TIME_RETRANS)
tcp_schedule_loss_probe(sk);
tcp_update_pacing_rate(sk);
tcp_xmit_recovery(sk, rexmit);
return 1;

no_queue:
/* If data was DSACKed, see if we can undo a cwnd reduction. */
if (flag & FLAG_DSACKING_ACK)
tcp_fastretrans_alert(sk, acked, prior_unsacked,
is_dupack, flag);
is_dupack, flag, &rexmit);
/* If this ack opens up a zero window, clear backoff. It was
* being used to time the probes, and is probably far higher than
* it needs to be for normal retransmission.
Expand All @@ -3664,7 +3697,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
&sack_state);
tcp_fastretrans_alert(sk, acked, prior_unsacked,
is_dupack, flag);
is_dupack, flag, &rexmit);
tcp_xmit_recovery(sk, rexmit);
}

SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
Expand Down

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