net/sched: adjust device watchdog timer to detect stopped queue at right time

Applications are sensitive to long network latency, particularly
heartbeat monitoring ones. Longer the tx timeout recovery higher the
risk with such applications on a production machines. This patch
remedies, yet honoring device set tx timeout.

Modify watchdog next timeout to be shorter than the device specified.
Compute the next timeout be equal to device watchdog timeout less the
how long ago queue stop had been done. At next watchdog timeout tx
timeout handler is called into if still in stopped state. Either called
or not called, restore the watchdog timeout back to device specified.
Signed-off-by: Praveen Kumar Kannoju <praveen.kannoju@oracle.com>
Link: https://lore.kernel.org/r/20240508133617.4424-1-praveen.kannoju@oracle.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_generic.c

@@ -506,19 +506,22 @@ static void dev_watchdog(struct timer_list *t)
 			unsigned int timedout_ms = 0;
 			unsigned int i;
 			unsigned long trans_start;
+			unsigned long oldest_start = jiffies;
 
 			for (i = 0; i < dev->num_tx_queues; i++) {
 				struct netdev_queue *txq;
 
 				txq = netdev_get_tx_queue(dev, i);
 				trans_start = READ_ONCE(txq->trans_start);
-				if (netif_xmit_stopped(txq) &&
-				    time_after(jiffies, (trans_start +
-							 dev->watchdog_timeo))) {
+				if (!netif_xmit_stopped(txq))
+					continue;
+				if (time_after(jiffies, trans_start + dev->watchdog_timeo)) {
 					timedout_ms = jiffies_to_msecs(jiffies - trans_start);
 					atomic_long_inc(&txq->trans_timeout);
 					break;
 				}
+				if (time_after(oldest_start, trans_start))
+					oldest_start = trans_start;
 			}
 
 			if (unlikely(timedout_ms)) {
@@ -531,7 +534,7 @@ static void dev_watchdog(struct timer_list *t)
 				netif_unfreeze_queues(dev);
 			}
 			if (!mod_timer(&dev->watchdog_timer,
-				       round_jiffies(jiffies +
+				       round_jiffies(oldest_start +
 						     dev->watchdog_timeo)))
 				release = false;
 		}