net: ena: limit refill Rx threshold to 256 to avoid latency issues

Currently Rx refill is done when the number of required descriptors is
above 1/8 queue size. With a default of 1024 entries per queue the
threshold is 128 descriptors.
There is intention to increase the queue size to 8196 entries.
In this case threshold of 1024 descriptors is too large and can hurt
latency.
Add another limitation to Rx threshold to be at most 256 descriptors.
Signed-off-by: NArthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/amazon/ena/ena_netdev.c

@@ -1122,7 +1122,9 @@ static int ena_clean_rx_irq(struct ena_ring *rx_ring, struct napi_struct *napi,
 	rx_ring->next_to_clean = next_to_clean;
 
 	refill_required = ena_com_free_desc(rx_ring->ena_com_io_sq);
-	refill_threshold = rx_ring->ring_size / ENA_RX_REFILL_THRESH_DIVIDER;
+	refill_threshold =
+		min_t(int, rx_ring->ring_size / ENA_RX_REFILL_THRESH_DIVIDER,
+		      ENA_RX_REFILL_THRESH_PACKET);
 
 	/* Optimization, try to batch new rx buffers */
 	if (refill_required > refill_threshold) {

drivers/net/ethernet/amazon/ena/ena_netdev.h

@@ -106,10 +106,11 @@
  */
 #define ENA_TX_POLL_BUDGET_DIVIDER	4
 
-/* Refill Rx queue when number of available descriptors is below
- * QUEUE_SIZE / ENA_RX_REFILL_THRESH_DIVIDER
+/* Refill Rx queue when number of required descriptors is above
+ * QUEUE_SIZE / ENA_RX_REFILL_THRESH_DIVIDER or ENA_RX_REFILL_THRESH_PACKET
  */
 #define ENA_RX_REFILL_THRESH_DIVIDER	8
+#define ENA_RX_REFILL_THRESH_PACKET	256
 
 /* Number of queues to check for missing queues per timer service */
 #define ENA_MONITORED_TX_QUEUES	4