sfc: By default refill RX rings as soon as space for a batch

Previously we refilled with much larger batches, which caused large latency
spikes.  We now have many more much much smaller spikes!
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/rx.c

@@ -76,7 +76,7 @@ static int rx_alloc_method = RX_ALLOC_METHOD_AUTO;
 /* This is the percentage fill level below which new RX descriptors
  * will be added to the RX descriptor ring.
  */
-static unsigned int rx_refill_threshold = 90;
+static unsigned int rx_refill_threshold;
 
 /*
  * RX maximum head room required.
@@ -363,8 +363,7 @@ void efx_fast_push_rx_descriptors(struct efx_rx_queue *rx_queue)
 	}
 
 	space = rx_queue->max_fill - fill_level;
-	if (space < EFX_RX_BATCH)
-		goto out;
+	EFX_BUG_ON_PARANOID(space < EFX_RX_BATCH);
 
 	netif_vdbg(rx_queue->efx, rx_status, rx_queue->efx->net_dev,
 		   "RX queue %d fast-filling descriptor ring from"
@@ -676,7 +675,7 @@ int efx_probe_rx_queue(struct efx_rx_queue *rx_queue)
 void efx_init_rx_queue(struct efx_rx_queue *rx_queue)
 {
 	struct efx_nic *efx = rx_queue->efx;
-	unsigned int max_fill, trigger;
+	unsigned int max_fill, trigger, max_trigger;
 
 	netif_dbg(rx_queue->efx, drv, rx_queue->efx->net_dev,
 		  "initialising RX queue %d\n", efx_rx_queue_index(rx_queue));
@@ -689,7 +688,14 @@ void efx_init_rx_queue(struct efx_rx_queue *rx_queue)
 
 	/* Initialise limit fields */
 	max_fill = efx->rxq_entries - EFX_RXD_HEAD_ROOM;
-	trigger = max_fill * min(rx_refill_threshold, 100U) / 100U;
+	max_trigger = max_fill - EFX_RX_BATCH;
+	if (rx_refill_threshold != 0) {
+		trigger = max_fill * min(rx_refill_threshold, 100U) / 100U;
+		if (trigger > max_trigger)
+			trigger = max_trigger;
+	} else {
+		trigger = max_trigger;
+	}
 
 	rx_queue->max_fill = max_fill;
 	rx_queue->fast_fill_trigger = trigger;
@@ -739,5 +745,5 @@ MODULE_PARM_DESC(rx_alloc_method, "Allocation method used for RX buffers");
 
 module_param(rx_refill_threshold, uint, 0444);
 MODULE_PARM_DESC(rx_refill_threshold,
-		 "RX descriptor ring fast/slow fill threshold (%)");
+		 "RX descriptor ring refill threshold (%)");