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rxrpc: Fix congestion management

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rxrpc has a problem in its congestion management in that it saves the
congestion window size (cwnd) from one call to another, but if this is 0 at
the time is saved, then the next call may not actually manage to ever
transmit anything.

To this end:

 (1) Don't save cwnd between calls, but rather reset back down to the
     initial cwnd and re-enter slow-start if data transmission is idle for
     more than an RTT.

 (2) Preserve ssthresh instead, as that is a handy estimate of pipe
     capacity.  Knowing roughly when to stop slow start and enter
     congestion avoidance can reduce the tendency to overshoot and drop
     larger amounts of packets when probing.

In future, cwind growth also needs to be constrained when the window isn't
being filled due to being application limited.

Reported-by: Simon Wilkinson <sxw@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
This commit is contained in:
David Howells 2022-10-03 18:49:11 +01:00
parent 6869ddb87d
commit 1fc4fa2ac9
11 changed files with 60 additions and 20 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/trace/events/rxrpc.h
View File

@ -193,6 +193,7 @@
EM(rxrpc_cong_new_low_nack, " NewLowN") \
EM(rxrpc_cong_no_change, " -") \
EM(rxrpc_cong_progress, " Progres") \
EM(rxrpc_cong_idle_reset, " IdleRes") \
EM(rxrpc_cong_retransmit_again, " ReTxAgn") \
EM(rxrpc_cong_rtt_window_end, " RttWinE") \
E_(rxrpc_cong_saw_nack, " SawNack")

9
net/rxrpc/ar-internal.h
View File

@ -332,7 +332,7 @@ struct rxrpc_peer {
u32 rto_j; /* Retransmission timeout in jiffies */
u8 backoff; /* Backoff timeout */
u8 cong_cwnd; /* Congestion window size */
u8 cong_ssthresh; /* Congestion slow-start threshold */
};
/*
@ -626,6 +626,7 @@ struct rxrpc_call {
u16 tx_backoff; /* Delay to insert due to Tx failure */
u8 tx_winsize; /* Maximum size of Tx window */
#define RXRPC_TX_MAX_WINDOW 128
ktime_t tx_last_sent; /* Last time a transmission occurred */
/* Received data tracking */
struct sk_buff_head recvmsg_queue; /* Queue of packets ready for recvmsg() */
@ -687,10 +688,10 @@ struct rxrpc_call {
* Summary of a new ACK and the changes it made to the Tx buffer packet states.
*/
struct rxrpc_ack_summary {
u16 nr_acks; /* Number of ACKs in packet */
u16 nr_new_acks; /* Number of new ACKs in packet */
u16 nr_rot_new_acks; /* Number of rotated new ACKs */
u8 ack_reason;
u8 nr_acks; /* Number of ACKs in packet */
u8 nr_new_acks; /* Number of new ACKs in packet */
u8 nr_rot_new_acks; /* Number of rotated new ACKs */
bool saw_nacks; /* Saw NACKs in packet */
bool new_low_nack; /* T if new low NACK found */
bool retrans_timeo; /* T if reTx due to timeout happened */

3
net/rxrpc/call_accept.c
View File

@ -324,7 +324,8 @@ static struct rxrpc_call *rxrpc_alloc_incoming_call(struct rxrpc_sock *rx,
call->security = conn->security;
call->security_ix = conn->security_ix;
call->peer = rxrpc_get_peer(conn->params.peer);
call->cong_cwnd = call->peer->cong_cwnd;
call->cong_ssthresh = call->peer->cong_ssthresh;
call->tx_last_sent = ktime_get_real();
return call;
}

5
net/rxrpc/call_object.c
View File

@ -166,7 +166,12 @@ struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *rx, gfp_t gfp,
call->rx_winsize = rxrpc_rx_window_size;
call->tx_winsize = 16;
if (RXRPC_TX_SMSS > 2190)
call->cong_cwnd = 2;
else if (RXRPC_TX_SMSS > 1095)
call->cong_cwnd = 3;
else
call->cong_cwnd = 4;
call->cong_ssthresh = RXRPC_TX_MAX_WINDOW;
call->rxnet = rxnet;

3
net/rxrpc/conn_client.c
View File

@ -363,7 +363,8 @@ static struct rxrpc_bundle *rxrpc_prep_call(struct rxrpc_sock *rx,
if (!cp->peer)
goto error;
call->cong_cwnd = cp->peer->cong_cwnd;
call->tx_last_sent = ktime_get_real();
call->cong_ssthresh = cp->peer->cong_ssthresh;
if (call->cong_cwnd >= call->cong_ssthresh)
call->cong_mode = RXRPC_CALL_CONGEST_AVOIDANCE;
else

2
net/rxrpc/conn_object.c
View File

@ -207,7 +207,7 @@ void rxrpc_disconnect_call(struct rxrpc_call *call)
{
struct rxrpc_connection *conn = call->conn;
call->peer->cong_cwnd = call->cong_cwnd;
call->peer->cong_ssthresh = call->cong_ssthresh;
if (!hlist_unhashed(&call->error_link)) {
spin_lock_bh(&call->peer->lock);

21
net/rxrpc/input.c
View File

@ -58,6 +58,25 @@ static void rxrpc_congestion_management(struct rxrpc_call *call,
summary->cumulative_acks = cumulative_acks;
summary->dup_acks = call->cong_dup_acks;
/* If we haven't transmitted anything for >1RTT, we should reset the
* congestion management state.
*/
if ((call->cong_mode == RXRPC_CALL_SLOW_START ||
call->cong_mode == RXRPC_CALL_CONGEST_AVOIDANCE) &&
ktime_before(ktime_add_us(call->tx_last_sent,
call->peer->srtt_us >> 3),
ktime_get_real())
) {
change = rxrpc_cong_idle_reset;
summary->mode = RXRPC_CALL_SLOW_START;
if (RXRPC_TX_SMSS > 2190)
summary->cwnd = 2;
else if (RXRPC_TX_SMSS > 1095)
summary->cwnd = 3;
else
summary->cwnd = 4;
}
switch (call->cong_mode) {
case RXRPC_CALL_SLOW_START:
if (summary->saw_nacks)
@ -205,7 +224,7 @@ static bool rxrpc_rotate_tx_window(struct rxrpc_call *call, rxrpc_seq_t to,
if (call->acks_lowest_nak == call->acks_hard_ack) {
call->acks_lowest_nak = to;
} else if (before_eq(call->acks_lowest_nak, to)) {
} else if (after(to, call->acks_lowest_nak)) {
summary->new_low_nack = true;
call->acks_lowest_nak = to;
}

1
net/rxrpc/output.c
View File

@ -501,6 +501,7 @@ dont_set_request_ack:
done:
if (ret >= 0) {
call->tx_last_sent = txb->last_sent;
if (txb->wire.flags & RXRPC_REQUEST_ACK) {
call->peer->rtt_last_req = txb->last_sent;
if (call->peer->rtt_count > 1) {

7
net/rxrpc/peer_object.c
View File

@ -227,12 +227,7 @@ struct rxrpc_peer *rxrpc_alloc_peer(struct rxrpc_local *local, gfp_t gfp)
rxrpc_peer_init_rtt(peer);
if (RXRPC_TX_SMSS > 2190)
peer->cong_cwnd = 2;
else if (RXRPC_TX_SMSS > 1095)
peer->cong_cwnd = 3;
else
peer->cong_cwnd = 4;
peer->cong_ssthresh = RXRPC_TX_MAX_WINDOW;
trace_rxrpc_peer(peer->debug_id, rxrpc_peer_new, 1, here);
}

4
net/rxrpc/proc.c
View File

@ -217,7 +217,7 @@ static int rxrpc_peer_seq_show(struct seq_file *seq, void *v)
seq_puts(seq,
"Proto Local "
" Remote "
" Use CW MTU LastUse RTT RTO\n"
" Use SST MTU LastUse RTT RTO\n"
);
return 0;
}
@ -235,7 +235,7 @@ static int rxrpc_peer_seq_show(struct seq_file *seq, void *v)
lbuff,
rbuff,
refcount_read(&peer->ref),
peer->cong_cwnd,
peer->cong_ssthresh,
peer->mtu,
now - peer->last_tx_at,
peer->srtt_us >> 3,

22
net/rxrpc/sendmsg.c
View File

@ -22,11 +22,27 @@
*/
static bool rxrpc_check_tx_space(struct rxrpc_call *call, rxrpc_seq_t *_tx_win)
{
unsigned int win_size =
min_t(unsigned int, call->tx_winsize,
call->cong_cwnd + call->cong_extra);
unsigned int win_size;
rxrpc_seq_t tx_win = smp_load_acquire(&call->acks_hard_ack);
/* If we haven't transmitted anything for >1RTT, we should reset the
* congestion management state.
*/
if (ktime_before(ktime_add_us(call->tx_last_sent,
call->peer->srtt_us >> 3),
ktime_get_real())) {
if (RXRPC_TX_SMSS > 2190)
win_size = 2;
else if (RXRPC_TX_SMSS > 1095)
win_size = 3;
else
win_size = 4;
win_size += call->cong_extra;
} else {
win_size = min_t(unsigned int, call->tx_winsize,
call->cong_cwnd + call->cong_extra);
}
if (_tx_win)
*_tx_win = tx_win;
return call->tx_top - tx_win < win_size;