rxrpc: Generate rtt_min

Generate rtt_min as this is required by RACK-TLP.

Signed-off-by: David Howells <[e-mail]>
cc: Marc Dionne <[e-mail]>
cc: [e-mail]
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

lib/win_minmax.c

···
 
 	return minmax_subwin_update(m, win, &val);
 }
+EXPORT_SYMBOL(minmax_running_min);

net/rxrpc/ar-internal.h

···
 	spinlock_t		rtt_input_lock;	/* RTT lock for input routine */
 	ktime_t			rtt_last_req;	/* Time of last RTT request */
 	unsigned int		rtt_count;	/* Number of samples we've got */
+	unsigned int		rtt_taken;	/* Number of samples taken (wrapping) */
+	struct minmax		min_rtt;	/* Estimated minimum RTT */
 
 	u32			srtt_us;	/* smoothed round trip time << 3 in usecs */
 	u32			mdev_us;	/* medium deviation			*/

net/rxrpc/rtt.c

···
 	peer->rto_us = rxrpc_bound_rto(rto);
 }
 
-static void rxrpc_ack_update_rtt(struct rxrpc_peer *peer, long rtt_us)
+static void rxrpc_update_rtt_min(struct rxrpc_peer *peer, ktime_t resp_time, long rtt_us)
+{
+	/* Window size 5mins in approx usec (ipv4.sysctl_tcp_min_rtt_wlen) */
+	u32 wlen_us = 5ULL * NSEC_PER_SEC / 1024;
+
+	minmax_running_min(&peer->min_rtt, wlen_us, resp_time / 1024,
+			   (u32)rtt_us ? : jiffies_to_usecs(1));
+}
+
+static void rxrpc_ack_update_rtt(struct rxrpc_peer *peer, ktime_t resp_time, long rtt_us)
 {
 	if (rtt_us < 0)
 		return;
 
-	//rxrpc_update_rtt_min(peer, rtt_us);
+	/* Update RACK min RTT [RFC8985 6.1 Step 1]. */
+	rxrpc_update_rtt_min(peer, resp_time, rtt_us);
+
 	rxrpc_rtt_estimator(peer, rtt_us);
 	rxrpc_set_rto(peer);
 
-	/* RFC6298: only reset backoff on valid RTT measurement. */
+	/* Only reset backoff on valid RTT measurement [RFC6298]. */
 	peer->backoff = 0;
 }
 
···
 		return;
 
 	spin_lock(&peer->rtt_input_lock);
-	rxrpc_ack_update_rtt(peer, rtt_us);
+	rxrpc_ack_update_rtt(peer, resp_time, rtt_us);
 	if (peer->rtt_count < 3)
 		peer->rtt_count++;
+	peer->rtt_taken++;
 	spin_unlock(&peer->rtt_input_lock);
 
 	trace_rxrpc_rtt_rx(call, why, rtt_slot, send_serial, resp_serial,