net/mlx4_en: Poll XDP TX completion queue in RX NAPI

Instead of having their own NAPIs, XDP TX completion queues get
polled within the corresponding RX NAPI.
This prevents any possible race on TX ring prod/cons indices,
between the context that issues the transmits (RX NAPI) and the
context that handles the completions (was previously done in
a separate NAPI).

This also improves performance, as it decreases the number
of NAPIs running on a CPU, saving the overhead of syncing
and switching between the contexts.

Performance tests:
Tested on ConnectX3Pro, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
Single queue no-RSS optimization ON.

XDP_TX packet rate:
-------------------------------------
     | Before    | After     | Gain |
IPv4 | 12.0 Mpps | 13.8 Mpps |  15% |
IPv6 | 12.0 Mpps | 13.8 Mpps |  15% |
-------------------------------------
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Cc: kernel-team@fb.com
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_cq.c

@@ -146,16 +146,25 @@ int mlx4_en_activate_cq(struct mlx4_en_priv *priv, struct mlx4_en_cq *cq,
 	if (err)
 		goto free_eq;
 
-	cq->mcq.comp  = cq->type != RX ? mlx4_en_tx_irq : mlx4_en_rx_irq;
 	cq->mcq.event = mlx4_en_cq_event;
 
-	if (cq->type != RX)
+	switch (cq->type) {
+	case TX:
+		cq->mcq.comp = mlx4_en_tx_irq;
 		netif_tx_napi_add(cq->dev, &cq->napi, mlx4_en_poll_tx_cq,
 				  NAPI_POLL_WEIGHT);
-	else
+		napi_enable(&cq->napi);
+		break;
+	case RX:
+		cq->mcq.comp = mlx4_en_rx_irq;
 		netif_napi_add(cq->dev, &cq->napi, mlx4_en_poll_rx_cq, 64);
-
-	napi_enable(&cq->napi);
+		napi_enable(&cq->napi);
+		break;
+	case TX_XDP:
+		/* nothing regarding napi, it's shared with rx ring */
+		cq->xdp_busy = false;
+		break;
+	}
 
 	return 0;
 
@@ -184,8 +193,10 @@ void mlx4_en_destroy_cq(struct mlx4_en_priv *priv, struct mlx4_en_cq **pcq)
 
 void mlx4_en_deactivate_cq(struct mlx4_en_priv *priv, struct mlx4_en_cq *cq)
 {
-	napi_disable(&cq->napi);
-	netif_napi_del(&cq->napi);
+	if (cq->type != TX_XDP) {
+		napi_disable(&cq->napi);
+		netif_napi_del(&cq->napi);
+	}
 
 	mlx4_cq_free(priv->mdev->dev, &cq->mcq);
 }

drivers/net/ethernet/mellanox/mlx4/en_netdev.c

@@ -1679,13 +1679,15 @@ int mlx4_en_start_port(struct net_device *dev)
 			if (t != TX_XDP) {
 				tx_ring->tx_queue = netdev_get_tx_queue(dev, i);
 				tx_ring->recycle_ring = NULL;
+
+				/* Arm CQ for TX completions */
+				mlx4_en_arm_cq(priv, cq);
+
 			} else {
 				mlx4_en_init_recycle_ring(priv, i);
+				/* XDP TX CQ should never be armed */
 			}
 
-			/* Arm CQ for TX completions */
-			mlx4_en_arm_cq(priv, cq);
-
 			/* Set initial ownership of all Tx TXBBs to SW (1) */
 			for (j = 0; j < tx_ring->buf_size; j += STAMP_STRIDE)
 				*((u32 *)(tx_ring->buf + j)) = 0xffffffff;

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -880,8 +880,10 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 	rcu_read_unlock();
 
 	if (likely(polled)) {
-		if (doorbell_pending)
-			mlx4_en_xmit_doorbell(priv->tx_ring[TX_XDP][cq->ring]);
+		if (doorbell_pending) {
+			priv->tx_cq[TX_XDP][cq_ring]->xdp_busy = true;
+			mlx4_en_xmit_doorbell(priv->tx_ring[TX_XDP][cq_ring]);
+		}
 
 		mlx4_cq_set_ci(&cq->mcq);
 		wmb(); /* ensure HW sees CQ consumer before we post new buffers */
@@ -912,16 +914,30 @@ int mlx4_en_poll_rx_cq(struct napi_struct *napi, int budget)
 	struct mlx4_en_cq *cq = container_of(napi, struct mlx4_en_cq, napi);
 	struct net_device *dev = cq->dev;
 	struct mlx4_en_priv *priv = netdev_priv(dev);
+	struct mlx4_en_cq *xdp_tx_cq = NULL;
+	bool clean_complete = true;
 	int done;
 
+	if (priv->tx_ring_num[TX_XDP]) {
+		xdp_tx_cq = priv->tx_cq[TX_XDP][cq->ring];
+		if (xdp_tx_cq->xdp_busy) {
+			clean_complete = mlx4_en_process_tx_cq(dev, xdp_tx_cq,
+							       budget);
+			xdp_tx_cq->xdp_busy = !clean_complete;
+		}
+	}
+
 	done = mlx4_en_process_rx_cq(dev, cq, budget);
 
 	/* If we used up all the quota - we're probably not done yet... */
-	if (done == budget) {
+	if (done == budget || !clean_complete) {
 		const struct cpumask *aff;
 		struct irq_data *idata;
 		int cpu_curr;
 
+		/* in case we got here because of !clean_complete */
+		done = budget;
+
 		INC_PERF_COUNTER(priv->pstats.napi_quota);
 
 		cpu_curr = smp_processor_id();

drivers/net/ethernet/mellanox/mlx4/en_tx.c

@@ -395,8 +395,8 @@ int mlx4_en_free_tx_buf(struct net_device *dev, struct mlx4_en_tx_ring *ring)
 	return cnt;
 }
 
-static bool mlx4_en_process_tx_cq(struct net_device *dev,
-				  struct mlx4_en_cq *cq, int napi_budget)
+bool mlx4_en_process_tx_cq(struct net_device *dev,
+			   struct mlx4_en_cq *cq, int napi_budget)
 {
 	struct mlx4_en_priv *priv = netdev_priv(dev);
 	struct mlx4_cq *mcq = &cq->mcq;
@@ -1176,6 +1176,7 @@ netdev_tx_t mlx4_en_xmit_frame(struct mlx4_en_rx_ring *rx_ring,
 
 tx_drop_count:
 	rx_ring->xdp_tx_full++;
+	*doorbell_pending = true;
 tx_drop:
 	return NETDEV_TX_BUSY;
 }

drivers/net/ethernet/mellanox/mlx4/mlx4_en.h

@@ -358,7 +358,10 @@ struct mlx4_en_cq {
 	struct mlx4_hwq_resources wqres;
 	int                     ring;
 	struct net_device      *dev;
-	struct napi_struct	napi;
+	union {
+		struct napi_struct napi;
+		bool               xdp_busy;
+	};
 	int size;
 	int buf_size;
 	int vector;
@@ -720,6 +723,8 @@ int mlx4_en_process_rx_cq(struct net_device *dev,
 			  int budget);
 int mlx4_en_poll_rx_cq(struct napi_struct *napi, int budget);
 int mlx4_en_poll_tx_cq(struct napi_struct *napi, int budget);
+bool mlx4_en_process_tx_cq(struct net_device *dev,
+			   struct mlx4_en_cq *cq, int napi_budget);
 u32 mlx4_en_free_tx_desc(struct mlx4_en_priv *priv,
 			 struct mlx4_en_tx_ring *ring,
 			 int index, u64 timestamp,