rxrpc: Fix the excessive initial retransmission timeout

rxrpc currently uses a fixed 4s retransmission timeout until the RTT is
sufficiently sampled.  This can cause problems with some fileservers with
calls to the cache manager in the afs filesystem being dropped from the
fileserver because a packet goes missing and the retransmission timeout is
greater than the call expiry timeout.

Fix this by:

 (1) Copying the RTT/RTO calculation code from Linux's TCP implementation
     and altering it to fit rxrpc.

 (2) Altering the various users of the RTT to make use of the new SRTT
     value.

 (3) Replacing the use of rxrpc_resend_timeout to use the calculated RTO
     value instead (which is needed in jiffies), along with a backoff.

Notes:

 (1) rxrpc provides RTT samples by matching the serial numbers on outgoing
     DATA packets that have the RXRPC_REQUEST_ACK set and PING ACK packets
     against the reference serial number in incoming REQUESTED ACK and
     PING-RESPONSE ACK packets.

 (2) Each packet that is transmitted on an rxrpc connection gets a new
     per-connection serial number, even for retransmissions, so an ACK can
     be cross-referenced to a specific trigger packet.  This allows RTT
     information to be drawn from retransmitted DATA packets also.

 (3) rxrpc maintains the RTT/RTO state on the rxrpc_peer record rather than
     on an rxrpc_call because many RPC calls won't live long enough to
     generate more than one sample.

 (4) The calculated SRTT value is in units of 8ths of a microsecond rather
     than nanoseconds.

The (S)RTT and RTO values are displayed in /proc/net/rxrpc/peers.

Fixes: 17926a79320a ([AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both"")
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index 3eb1ab40ca5c..9fe264bec70c 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -7,6 +7,7 @@
 
 #include <linux/atomic.h>
 #include <linux/seqlock.h>
+#include <linux/win_minmax.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
 #include <net/sock.h>
@@ -311,11 +312,14 @@ struct rxrpc_peer {
 #define RXRPC_RTT_CACHE_SIZE 32
 	spinlock_t		rtt_input_lock;	/* RTT lock for input routine */
 	ktime_t			rtt_last_req;	/* Time of last RTT request */
-	u64			rtt;		/* Current RTT estimate (in nS) */
-	u64			rtt_sum;	/* Sum of cache contents */
-	u64			rtt_cache[RXRPC_RTT_CACHE_SIZE]; /* Determined RTT cache */
-	u8			rtt_cursor;	/* next entry at which to insert */
-	u8			rtt_usage;	/* amount of cache actually used */
+	unsigned int		rtt_count;	/* Number of samples we've got */
+
+	u32			srtt_us;	/* smoothed round trip time << 3 in usecs */
+	u32			mdev_us;	/* medium deviation			*/
+	u32			mdev_max_us;	/* maximal mdev for the last rtt period	*/
+	u32			rttvar_us;	/* smoothed mdev_max			*/
+	u32			rto_j;		/* Retransmission timeout in jiffies */
+	u8			backoff;	/* Backoff timeout */
 
 	u8			cong_cwnd;	/* Congestion window size */
 };
@@ -1041,7 +1045,6 @@ extern unsigned long rxrpc_idle_ack_delay;
 extern unsigned int rxrpc_rx_window_size;
 extern unsigned int rxrpc_rx_mtu;
 extern unsigned int rxrpc_rx_jumbo_max;
-extern unsigned long rxrpc_resend_timeout;
 
 extern const s8 rxrpc_ack_priority[];
 
@@ -1069,8 +1072,6 @@ void rxrpc_send_keepalive(struct rxrpc_peer *);
  * peer_event.c
  */
 void rxrpc_error_report(struct sock *);
-void rxrpc_peer_add_rtt(struct rxrpc_call *, enum rxrpc_rtt_rx_trace,
-			rxrpc_serial_t, rxrpc_serial_t, ktime_t, ktime_t);
 void rxrpc_peer_keepalive_worker(struct work_struct *);
 
 /*
@@ -1103,6 +1104,14 @@ void rxrpc_notify_socket(struct rxrpc_call *);
 int rxrpc_recvmsg(struct socket *, struct msghdr *, size_t, int);
 
 /*
+ * rtt.c
+ */
+void rxrpc_peer_add_rtt(struct rxrpc_call *, enum rxrpc_rtt_rx_trace,
+			rxrpc_serial_t, rxrpc_serial_t, ktime_t, ktime_t);
+unsigned long rxrpc_get_rto_backoff(struct rxrpc_peer *, bool);
+void rxrpc_peer_init_rtt(struct rxrpc_peer *);
+
+/*
  * rxkad.c
  */
 #ifdef CONFIG_RXKAD