rxrpc: Add keepalive for a call

We need to transmit a packet every so often to act as a keepalive for the
peer (which has a timeout from the last time it received a packet) and also
to prevent any intervening firewalls from closing the route.

Do this by resetting a timer every time we transmit a packet.  If the timer
ever expires, we transmit a PING ACK packet and thereby also elicit a PING
RESPONSE ACK from the other side - which prevents our last-rx timeout from
expiring.

The timer is set to 1/6 of the last-rx timeout so that we can detect the
other side going away if it misses 6 replies in a row.

This is particularly necessary for servers where the processing of the
service function may take a significant amount of time.

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

include/trace/events/rxrpc.h

@@ -141,6 +141,7 @@ enum rxrpc_timer_trace {
 	rxrpc_timer_exp_ack,
 	rxrpc_timer_exp_hard,
 	rxrpc_timer_exp_idle,
+	rxrpc_timer_exp_keepalive,
 	rxrpc_timer_exp_lost_ack,
 	rxrpc_timer_exp_normal,
 	rxrpc_timer_exp_ping,
@@ -152,6 +153,7 @@ enum rxrpc_timer_trace {
 	rxrpc_timer_set_for_ack,
 	rxrpc_timer_set_for_hard,
 	rxrpc_timer_set_for_idle,
+	rxrpc_timer_set_for_keepalive,
 	rxrpc_timer_set_for_lost_ack,
 	rxrpc_timer_set_for_normal,
 	rxrpc_timer_set_for_ping,
@@ -162,6 +164,7 @@ enum rxrpc_timer_trace {
 enum rxrpc_propose_ack_trace {
 	rxrpc_propose_ack_client_tx_end,
 	rxrpc_propose_ack_input_data,
+	rxrpc_propose_ack_ping_for_keepalive,
 	rxrpc_propose_ack_ping_for_lost_ack,
 	rxrpc_propose_ack_ping_for_lost_reply,
 	rxrpc_propose_ack_ping_for_params,
@@ -311,6 +314,7 @@ enum rxrpc_congest_change {
 	EM(rxrpc_timer_exp_ack,			"ExpAck") \
 	EM(rxrpc_timer_exp_hard,		"ExpHrd") \
 	EM(rxrpc_timer_exp_idle,		"ExpIdl") \
+	EM(rxrpc_timer_exp_keepalive,		"ExpKA ") \
 	EM(rxrpc_timer_exp_lost_ack,		"ExpLoA") \
 	EM(rxrpc_timer_exp_normal,		"ExpNml") \
 	EM(rxrpc_timer_exp_ping,		"ExpPng") \
@@ -321,6 +325,7 @@ enum rxrpc_congest_change {
 	EM(rxrpc_timer_set_for_ack,		"SetAck") \
 	EM(rxrpc_timer_set_for_hard,		"SetHrd") \
 	EM(rxrpc_timer_set_for_idle,		"SetIdl") \
+	EM(rxrpc_timer_set_for_keepalive,	"KeepAl") \
 	EM(rxrpc_timer_set_for_lost_ack,	"SetLoA") \
 	EM(rxrpc_timer_set_for_normal,		"SetNml") \
 	EM(rxrpc_timer_set_for_ping,		"SetPng") \
@@ -330,6 +335,7 @@ enum rxrpc_congest_change {
 #define rxrpc_propose_ack_traces \
 	EM(rxrpc_propose_ack_client_tx_end,	"ClTxEnd") \
 	EM(rxrpc_propose_ack_input_data,	"DataIn ") \
+	EM(rxrpc_propose_ack_ping_for_keepalive, "KeepAlv") \
 	EM(rxrpc_propose_ack_ping_for_lost_ack,	"LostAck") \
 	EM(rxrpc_propose_ack_ping_for_lost_reply, "LostRpl") \
 	EM(rxrpc_propose_ack_ping_for_params,	"Params ") \

net/rxrpc/ar-internal.h

@@ -519,6 +519,7 @@ struct rxrpc_call {
 	unsigned long		ack_lost_at;	/* When ACK is figured as lost */
 	unsigned long		resend_at;	/* When next resend needs to happen */
 	unsigned long		ping_at;	/* When next to send a ping */
+	unsigned long		keepalive_at;	/* When next to send a keepalive ping */
 	unsigned long		expect_rx_by;	/* When we expect to get a packet by */
 	unsigned long		expect_req_by;	/* When we expect to get a request DATA packet by */
 	unsigned long		expect_term_by;	/* When we expect call termination by */

net/rxrpc/call_event.c

@@ -366,6 +366,15 @@ recheck_state:
 		set_bit(RXRPC_CALL_EV_ACK_LOST, &call->events);
 	}
 
+	t = READ_ONCE(call->keepalive_at);
+	if (time_after_eq(now, t)) {
+		trace_rxrpc_timer(call, rxrpc_timer_exp_keepalive, now);
+		cmpxchg(&call->keepalive_at, t, now + MAX_JIFFY_OFFSET);
+		rxrpc_propose_ACK(call, RXRPC_ACK_PING, 0, 0, true, true,
+				  rxrpc_propose_ack_ping_for_keepalive);
+		set_bit(RXRPC_CALL_EV_PING, &call->events);
+	}
+
 	t = READ_ONCE(call->ping_at);
 	if (time_after_eq(now, t)) {
 		trace_rxrpc_timer(call, rxrpc_timer_exp_ping, now);
@@ -423,6 +432,7 @@ recheck_state:
 	set(call->ack_at);
 	set(call->ack_lost_at);
 	set(call->resend_at);
+	set(call->keepalive_at);
 	set(call->ping_at);
 
 	now = jiffies;

net/rxrpc/output.c

@@ -32,6 +32,24 @@ struct rxrpc_abort_buffer {
 	__be32 abort_code;
 };
 
+/*
+ * Arrange for a keepalive ping a certain time after we last transmitted.  This
+ * lets the far side know we're still interested in this call and helps keep
+ * the route through any intervening firewall open.
+ *
+ * Receiving a response to the ping will prevent the ->expect_rx_by timer from
+ * expiring.
+ */
+static void rxrpc_set_keepalive(struct rxrpc_call *call)
+{
+	unsigned long now = jiffies, keepalive_at = call->next_rx_timo / 6;
+
+	keepalive_at += now;
+	WRITE_ONCE(call->keepalive_at, keepalive_at);
+	rxrpc_reduce_call_timer(call, keepalive_at, now,
+				rxrpc_timer_set_for_keepalive);
+}
+
 /*
  * Fill out an ACK packet.
  */
@@ -205,6 +223,8 @@ int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping,
 				call->ackr_seen = top;
 			spin_unlock_bh(&call->lock);
 		}
+
+		rxrpc_set_keepalive(call);
 	}
 
 out:
@@ -388,6 +408,9 @@ done:
 			}
 		}
 	}
+
+	rxrpc_set_keepalive(call);
+
 	_leave(" = %d [%u]", ret, call->peer->maxdata);
 	return ret;