[PATCH iwl-next 7/8] ixgbe: limit ITR decrease in latency mode to prevent ACK overdrive

[Aleksandr Loktionov <aleksandr.loktionov@intel.com>]

From: Alexander Duyck <alexander.h.duyck@intel.com>

When operating in latency mode and the computed ITR is lower than the
current setting, the algorithm can reduce the interrupt rate too
aggressively in a single step.  For a TCP workload this means the ACK
stream (a latency-sensitive, low-packet-rate workload) can drive the
moderation down to very high interrupt rates, starving CPU time from
the sender side.

After the speed-based ITR calculation is complete, check whether the
result is in latency mode and would decrease below the current setting.
If so, limit the decrease to at most IXGBE_ITR_ADAPTIVE_MIN_INC (2 us)
per update.  This ensures the number of interrupts grows by no more
than 2x per adjustment step for latency-class workloads, dialling in
smoothly rather than overshooting.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
---
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index aea76b3..ba7b013 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2888,6 +2888,17 @@ static void ixgbe_update_itr(struct ixgbe_q_vector *q_vector,
 		break;
 	}
 
+	/* In the case of a latency specific workload only allow us to
+	 * reduce the ITR by at most 2us. By doing this we should dial
+	 * in so that our number of interrupts is no more than 2x the number
+	 * of packets for the least busy workload. So for example in the case
+	 * of a TCP workload the ACK packets being received would set the
+	 * interrupt rate as they are a latency specific workload.
+	 */
+	if ((itr & IXGBE_ITR_ADAPTIVE_LATENCY) && itr < ring_container->itr)
+		itr = max_t(unsigned int, itr,
+			    ring_container->itr - IXGBE_ITR_ADAPTIVE_MIN_INC);
+
 clear_counts:
 	/* write back value */
 	ring_container->itr = itr;
-- 
2.52.0