lacp: short timeout erroneously declares link-flapping

Panasas was seeing a higher-than-expected number of link-flap events.
After joint debugging with the switch vendor, we determined there were
problems on both sides; either of which might cause the occasional
event, but together caused lots of them.

On the switch side, an internal queuing issue was causing LACP PDUs --
which should be sent every second, in short-timeout mode -- to sometimes
be sent slightly later than they should have been. In some cases, two
successive PDUs were late, but we never saw three late PDUs in a row.

On the FreeBSD side, we saw a link-flap event every time there were two
late PDUs, while the spec says that it takes *three* seconds of downtime
to trigger that event. It turns out that if a PDU was received shortly
before the timer code was run, it would decrement less than a full
second after the PDU arrived. Then two delayed PDUs would cause two
additional decrements, causing it to reach zero less than three seconds
after the most-recent on-time PDU.

The solution is to note the time a PDU arrives, and only decrement if at
least a full second has elapsed since then.

Reported by:	Greg Foster <gfoster@panasas.com>
Reviewed by:	gallatin
Tested by:	Greg Foster <gfoster@panasas.com>
MFC after:	3 days
Sponsored by:	Panasas
Differential Revision:	https://reviews.freebsd.org/D35070

(cherry picked from commit 00a80538)

sys/net/ieee8023ad_lacp.c

@@ -48,6 +48,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/lock.h>
 #include <sys/rwlock.h>
 #include <sys/taskqueue.h>
+#include <sys/time.h>
 
 #include <net/if.h>
 #include <net/if_var.h>
@@ -1704,6 +1705,7 @@ lacp_sm_rx(struct lacp_port *lp, const struct lacpdu *du)
 	 * EXPIRED, DEFAULTED, CURRENT -> CURRENT
 	 */
 
+	microuptime(&lp->lp_last_lacpdu_rx);
 	lacp_sm_rx_update_selected(lp, du);
 	lacp_sm_rx_update_ntt(lp, du);
 	lacp_sm_rx_record_pdu(lp, du);
@@ -1913,14 +1915,26 @@ static void
 lacp_run_timers(struct lacp_port *lp)
 {
 	int i;
+	struct timeval time_diff;
 
 	for (i = 0; i < LACP_NTIMER; i++) {
 		KASSERT(lp->lp_timer[i] >= 0,
 		    ("invalid timer value %d", lp->lp_timer[i]));
 		if (lp->lp_timer[i] == 0) {
 			continue;
-		} else if (--lp->lp_timer[i] <= 0) {
-			if (lacp_timer_funcs[i]) {
+		} else {
+			if (i == LACP_TIMER_CURRENT_WHILE) {
+				microuptime(&time_diff);
+				timevalsub(&time_diff, &lp->lp_last_lacpdu_rx);
+				if (time_diff.tv_sec) {
+					/* At least one sec has elapsed since last LACP packet. */
+					--lp->lp_timer[i];
+				}
+			} else {
+				--lp->lp_timer[i];
+			}
+
+			if ((lp->lp_timer[i] <= 0) && (lacp_timer_funcs[i])) {
 				(*lacp_timer_funcs[i])(lp);
 			}
 		}

sys/net/ieee8023ad_lacp.h

@@ -215,6 +215,7 @@ struct lacp_port {
 #define	lp_key		lp_actor.lip_key
 #define	lp_systemid	lp_actor.lip_systemid
 	struct timeval		lp_last_lacpdu;
+	struct timeval		lp_last_lacpdu_rx;
 	int			lp_lacpdu_sent;
 	enum lacp_mux_state	lp_mux_state;
 	enum lacp_selected	lp_selected;