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tcp: move cwnd reduction after recovery state procesing 

Currently the cwnd is reduced and increased in various different
places. The reduction happens in various places in the recovery
state processing (tcp_fastretrans_alert) while the increase
happens afterward.

A better sequence is to identify lost packets and update
the congestion control state (icsk_ca_state) first. Then base
on the new state, up/down the cwnd in one central place. It's
more clear to reason cwnd changes.

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>
This commit is contained in:
Yuchung Cheng 2016-02-02 10:33:05 -08:00 committed by David S. Miller
parent e662ca40de
commit 31ba0c1072
1 changed files with 28 additions and 32 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

60
net/ipv4/tcp_input.c
View File

@ -2471,14 +2471,12 @@ static void tcp_init_cwnd_reduction(struct sock *sk)
tcp_ecn_queue_cwr(tp);
}
static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked,
int fast_rexmit, int flag)
static void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked,
int flag)
{
struct tcp_sock *tp = tcp_sk(sk);
int sndcnt = 0;
int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
int newly_acked_sacked = prior_unsacked -
(tp->packets_out - tp->sacked_out);
if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
return;
@ -2496,7 +2494,8 @@ static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked,
} else {
sndcnt = min(delta, newly_acked_sacked);
}
sndcnt = max(sndcnt, (fast_rexmit ? 1 : 0));
/* Force a fast retransmit upon entering fast recovery */
sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
}
@ -2541,7 +2540,7 @@ static void tcp_try_keep_open(struct sock *sk)
}
}
static void tcp_try_to_open(struct sock *sk, int flag, const int prior_unsacked)
static void tcp_try_to_open(struct sock *sk, int flag)
{
struct tcp_sock *tp = tcp_sk(sk);
@ -2555,8 +2554,6 @@ static void tcp_try_to_open(struct sock *sk, int flag, const int prior_unsacked)
if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
tcp_try_keep_open(sk);
} else {
tcp_cwnd_reduction(sk, prior_unsacked, 0, flag);
}
}
@ -2720,8 +2717,7 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
}
/* Undo during fast recovery after partial ACK. */
static bool tcp_try_undo_partial(struct sock *sk, const int acked,
const int prior_unsacked, int flag)
static bool tcp_try_undo_partial(struct sock *sk, const int acked)
{
struct tcp_sock *tp = tcp_sk(sk);
@ -2736,10 +2732,8 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked,
* can undo. Otherwise we clock out new packets but do not
* mark more packets lost or retransmit more.
*/
if (tp->retrans_out) {
tcp_cwnd_reduction(sk, prior_unsacked, 0, flag);
if (tp->retrans_out)
return true;
}
if (!tcp_any_retrans_done(sk))
tp->retrans_stamp = 0;
@ -2758,21 +2752,21 @@ static bool tcp_try_undo_partial(struct sock *sk, const int acked,
* taking into account both packets sitting in receiver's buffer and
* packets lost by network.
*
* Besides that it does CWND reduction, when packet loss is detected
* and changes state of machine.
* Besides that it updates the congestion state when packet loss or ECN
* is detected. But it does not reduce the cwnd, it is done by the
* congestion control later.
*
* It does _not_ decide what to send, it is made in function
* tcp_xmit_retransmit_queue().
*/
static void tcp_fastretrans_alert(struct sock *sk, const int acked,
const int prior_unsacked,
bool is_dupack, int flag, int *rexmit)
bool is_dupack, int *ack_flag, int *rexmit)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
int fast_rexmit = 0, flag = *ack_flag;
bool do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
(tcp_fackets_out(tp) > tp->reordering));
int fast_rexmit = 0;
if (WARN_ON(!tp->packets_out && tp->sacked_out))
tp->sacked_out = 0;
@ -2819,8 +2813,10 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
/* Use RACK to detect loss */
if (sysctl_tcp_recovery & TCP_RACK_LOST_RETRANS &&
tcp_rack_mark_lost(sk))
tcp_rack_mark_lost(sk)) {
flag |= FLAG_LOST_RETRANS;
*ack_flag |= FLAG_LOST_RETRANS;
}
/* E. Process state. */
switch (icsk->icsk_ca_state) {
@ -2829,7 +2825,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
if (tcp_is_reno(tp) && is_dupack)
tcp_add_reno_sack(sk);
} else {
if (tcp_try_undo_partial(sk, acked, prior_unsacked, flag))
if (tcp_try_undo_partial(sk, acked))
return;
/* Partial ACK arrived. Force fast retransmit. */
do_lost = tcp_is_reno(tp) ||
@ -2858,7 +2854,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
tcp_try_undo_dsack(sk);
if (!tcp_time_to_recover(sk, flag)) {
tcp_try_to_open(sk, flag, prior_unsacked);
tcp_try_to_open(sk, flag);
return;
}
@ -2880,7 +2876,6 @@ 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);
*rexmit = REXMIT_LOST;
}
@ -3306,9 +3301,6 @@ static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
/* Decide wheather to run the increase function of congestion control. */
static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
{
if (tcp_in_cwnd_reduction(sk))
return false;
/* If reordering is high then always grow cwnd whenever data is
* delivered regardless of its ordering. Otherwise stay conservative
* and only grow cwnd on in-order delivery (RFC5681). A stretched ACK w/
@ -3551,6 +3543,7 @@ 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 acked_sacked; /* Number of packets newly acked or sacked */
int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
sack_state.first_sackt.v64 = 0;
@ -3647,15 +3640,20 @@ 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, &rexmit);
tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
}
if (tp->tlp_high_seq)
tcp_process_tlp_ack(sk, ack, flag);
acked_sacked = prior_unsacked - (tp->packets_out - tp->sacked_out);
/* Advance cwnd if state allows */
if (tcp_may_raise_cwnd(sk, flag))
if (tcp_in_cwnd_reduction(sk)) {
/* Reduce cwnd if state mandates */
tcp_cwnd_reduction(sk, acked_sacked, flag);
} else if (tcp_may_raise_cwnd(sk, flag)) {
/* Advance cwnd if state allows */
tcp_cong_avoid(sk, ack, acked);
}
if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP)) {
struct dst_entry *dst = __sk_dst_get(sk);
@ -3672,8 +3670,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
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, &rexmit);
tcp_fastretrans_alert(sk, acked, 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.
@ -3696,8 +3693,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
if (TCP_SKB_CB(skb)->sacked) {
flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
&sack_state);
tcp_fastretrans_alert(sk, acked, prior_unsacked,
is_dupack, flag, &rexmit);
tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
tcp_xmit_recovery(sk, rexmit);
}