rxrpc: Add a tracepoint to log which packets will be retransmitted

Add a tracepoint to log in rxrpc_resend() which packets will be
retransmitted.  Note that if a positive ACK comes in whilst we have dropped
the lock to retransmit another packet, the actual retransmission may not
happen, though some of the effects will (such as altering the congestion
management).
Signed-off-by: David Howells <dhowells@redhat.com>

include/trace/events/rxrpc.h

@@ -543,6 +543,33 @@ TRACE_EVENT(rxrpc_propose_ack,
 		      rxrpc_propose_ack_outcomes[__entry->outcome])
 	    );
 
+TRACE_EVENT(rxrpc_retransmit,
+	    TP_PROTO(struct rxrpc_call *call, rxrpc_seq_t seq, u8 annotation,
+		     s64 expiry),
+
+	    TP_ARGS(call, seq, annotation, expiry),
+
+	    TP_STRUCT__entry(
+		    __field(struct rxrpc_call *,	call		)
+		    __field(rxrpc_seq_t,		seq		)
+		    __field(u8,				annotation	)
+		    __field(s64,			expiry		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->call = call;
+		    __entry->seq = seq;
+		    __entry->annotation = annotation;
+		    __entry->expiry = expiry;
+			   ),
+
+	    TP_printk("c=%p q=%x a=%02x xp=%lld",
+		      __entry->call,
+		      __entry->seq,
+		      __entry->annotation,
+		      __entry->expiry)
+	    );
+
 #endif /* _TRACE_RXRPC_H */
 
 /* This part must be outside protection */

net/rxrpc/call_event.c

@@ -196,6 +196,8 @@ static void rxrpc_resend(struct rxrpc_call *call)
 
 		/* Okay, we need to retransmit a packet. */
 		call->rxtx_annotations[ix] = RXRPC_TX_ANNO_RETRANS | annotation;
+		trace_rxrpc_retransmit(call, seq, annotation | anno_type,
+				       ktime_to_ns(ktime_sub(skb->tstamp, max_age)));
 	}
 
 	resend_at = ktime_sub(ktime_add_ms(oldest, rxrpc_resend_timeout), now);