rxrpc: Implement slow-start

Implement RxRPC slow-start, which is similar to RFC 5681 for TCP.  A
tracepoint is added to log the state of the congestion management algorithm
and the decisions it makes.

Notes:

 (1) Since we send fixed-size DATA packets (apart from the final packet in
     each phase), counters and calculations are in terms of packets rather
     than bytes.

 (2) The ACK packet carries the equivalent of TCP SACK.

 (3) The FLIGHT_SIZE calculation in RFC 5681 doesn't seem particularly
     suited to SACK of a small number of packets.  It seems that, almost
     inevitably, by the time three 'duplicate' ACKs have been seen, we have
     narrowed the loss down to one or two missing packets, and the
     FLIGHT_SIZE calculation ends up as 2.

 (4) In rxrpc_resend(), if there was no data that apparently needed
     retransmission, we transmit a PING ACK to ask the peer to tell us what
     its Rx window state is.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 45 insertions, 0 deletions

diff --git a/include/trace/events/rxrpc.h b/include/trace/events/rxrpc.h
index 5647549..ada12d0 100644
--- a/include/trace/events/rxrpc.h
+++ b/include/trace/events/rxrpc.h
@@ -570,6 +570,51 @@ TRACE_EVENT(rxrpc_retransmit,
 		      __entry->expiry)
 	    );
 
+TRACE_EVENT(rxrpc_congest,
+	    TP_PROTO(struct rxrpc_call *call, struct rxrpc_ack_summary *summary,
+		     rxrpc_serial_t ack_serial, enum rxrpc_congest_change change),
+
+	    TP_ARGS(call, summary, ack_serial, change),
+
+	    TP_STRUCT__entry(
+		    __field(struct rxrpc_call *,		call		)
+		    __field(enum rxrpc_congest_change,		change		)
+		    __field(rxrpc_seq_t,			hard_ack	)
+		    __field(rxrpc_seq_t,			top		)
+		    __field(rxrpc_seq_t,			lowest_nak	)
+		    __field(rxrpc_serial_t,			ack_serial	)
+		    __field_struct(struct rxrpc_ack_summary,	sum		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->call	= call;
+		    __entry->change	= change;
+		    __entry->hard_ack	= call->tx_hard_ack;
+		    __entry->top	= call->tx_top;
+		    __entry->lowest_nak	= call->acks_lowest_nak;
+		    __entry->ack_serial	= ack_serial;
+		    memcpy(&__entry->sum, summary, sizeof(__entry->sum));
+			   ),
+
+	    TP_printk("c=%p %08x %s %08x %s cw=%u ss=%u nr=%u,%u nw=%u,%u r=%u b=%u u=%u d=%u l=%x%s%s%s",
+		      __entry->call,
+		      __entry->ack_serial,
+		      rxrpc_ack_names[__entry->sum.ack_reason],
+		      __entry->hard_ack,
+		      rxrpc_congest_modes[__entry->sum.mode],
+		      __entry->sum.cwnd,
+		      __entry->sum.ssthresh,
+		      __entry->sum.nr_acks, __entry->sum.nr_nacks,
+		      __entry->sum.nr_new_acks, __entry->sum.nr_new_nacks,
+		      __entry->sum.nr_rot_new_acks,
+		      __entry->top - __entry->hard_ack,
+		      __entry->sum.cumulative_acks,
+		      __entry->sum.dup_acks,
+		      __entry->lowest_nak, __entry->sum.new_low_nack ? "!" : "",
+		      rxrpc_congest_changes[__entry->change],
+		      __entry->sum.retrans_timeo ? " rTxTo" : "")
+	    );
+
 #endif /* _TRACE_RXRPC_H */
 
 /* This part must be outside protection */