[PATCH RFC] net/sched: adjust device watchdog timer to detect stopped queue at right time

[Praveen Kumar Kannoju]

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.

For example, mellanox driver with 15 sec watchdog timeout on its
interfaces will be called for handling timeout at random times as shown
below:

	mlx5_core 0000:af:00.0 enp175s0f0: TX timeout on queue: 40, SQ: 0x13106, CQ: 0x5b5, SQ Cons: 0x1 SQ Prod: 0x1, usecs since last trans: 28559000
	mlx5_core 0000:af:00.0 enp175s0f0: TX timeout on queue: 59, SQ: 0x132a4, CQ: 0x4f3, SQ Cons: 0x1 SQ Prod: 0x1, usecs since last trans: 21424000
	mlx5_core 0000:af:00.0 enp175s0f0: TX timeout on queue: 42, SQ: 0x12688, CQ: 0x5b5, SQ Cons: 0x1 SQ Prod: 0x1, usecs since last trans: 15919000

whereas with the proposed fix, timeout handler is always called at
appropriate time set into the watchdog by the driver as shown below:

	mlx5_core 0000:af:00.0 enp175s0f0: TX timeout on queue: 42, SQ: 0x122d6, CQ: 0x4af, SQ Cons: 0x4 SQ Prod: 0x4, usecs since last trans: 15137000
	mlx5_core 0000:af:00.0 enp175s0f0: TX timeout on queue: 33, SQ: 0x1410f, CQ: 0x48d, SQ Cons: 0xb SQ Prod: 0xb, usecs since last trans: 15568000
	mlx5_core 0000:af:00.0 enp175s0f0: TX timeout on queue: 33, SQ: 0x1424a, CQ: 0x599, SQ Cons: 0x8 SQ Prod: 0x8, usecs since last trans: 15539000

Signed-off-by: Praveen Kumar Kannoju <praveen.kannoju@xxxxxxxxxx>
---
 net/sched/sch_generic.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 4a2c763e2d11..64e31f8b4ac1 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -506,18 +506,25 @@ static void dev_watchdog(struct timer_list *t)
 			unsigned int timedout_ms = 0;
 			unsigned int i;
 			unsigned long trans_start;
+			unsigned long next_check = 0;
+			unsigned long current_jiffies;
 
 			for (i = 0; i < dev->num_tx_queues; i++) {
 				struct netdev_queue *txq;
+				current_jiffies = jiffies;
 
 				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))) {
-					timedout_ms = jiffies_to_msecs(jiffies - trans_start);
-					atomic_long_inc(&txq->trans_timeout);
-					break;
+				if (netif_xmit_stopped(txq)) {
+					if (time_after(current_jiffies, (trans_start +
+								   dev->watchdog_timeo))) {
+						timedout_ms = jiffies_to_msecs(current_jiffies -
+										trans_start);
+						atomic_long_inc(&txq->trans_timeout);
+						break;
+					}
+					next_check = trans_start + dev->watchdog_timeo -
+									current_jiffies;
 				}
 			}
 
@@ -530,9 +537,11 @@ static void dev_watchdog(struct timer_list *t)
 				dev->netdev_ops->ndo_tx_timeout(dev, i);
 				netif_unfreeze_queues(dev);
 			}
+			if (!next_check)
+				next_check = dev->watchdog_timeo;
 			if (!mod_timer(&dev->watchdog_timer,
 				       round_jiffies(jiffies +
-						     dev->watchdog_timeo)))
+						     next_check)))
 				release = false;
 		}
 	}
-- 
2.31.1