net/mlx4_en: Avoid false sharing in mlx4_en_en_process_tx_cq()

mlx4_en_process_tx_cq() carefully fetches and writes ring->last_nr_txbb
and ring->cons only one time to avoid false sharing

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
index b96627c..d9aaeb2 100644 (file)
--- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
@@ -387,6 +387,8 @@ static bool mlx4_en_process_tx_cq(struct net_device *dev,
        u64 timestamp = 0;
        int done = 0;
        int budget = priv->tx_work_limit;
+       u32 last_nr_txbb;
+       u32 ring_cons;
 
        if (!priv->port_up)
                return true;
@@ -394,7 +396,9 @@ static bool mlx4_en_process_tx_cq(struct net_device *dev,
        prefetchw(&ring->tx_queue->dql.limit);
        index = cons_index & size_mask;
        cqe = mlx4_en_get_cqe(buf, index, priv->cqe_size) + factor;
-       ring_index = ring->cons & size_mask;
+       last_nr_txbb = ACCESS_ONCE(ring->last_nr_txbb);
+       ring_cons = ACCESS_ONCE(ring->cons);
+       ring_index = ring_cons & size_mask;
        stamp_index = ring_index;
 
        /* Process all completed CQEs */
@@ -419,19 +423,19 @@ static bool mlx4_en_process_tx_cq(struct net_device *dev,
                new_index = be16_to_cpu(cqe->wqe_index) & size_mask;
 
                do {
-                       txbbs_skipped += ring->last_nr_txbb;
-                       ring_index = (ring_index + ring->last_nr_txbb) & size_mask;
+                       txbbs_skipped += last_nr_txbb;
+                       ring_index = (ring_index + last_nr_txbb) & size_mask;
                        if (ring->tx_info[ring_index].ts_requested)
                                timestamp = mlx4_en_get_cqe_ts(cqe);
 
                        /* free next descriptor */
-                       ring->last_nr_txbb = mlx4_en_free_tx_desc(
+                       last_nr_txbb = mlx4_en_free_tx_desc(
                                        priv, ring, ring_index,
-                                       !!((ring->cons + txbbs_skipped) &
+                                       !!((ring_cons + txbbs_skipped) &
                                        ring->size), timestamp);
 
                        mlx4_en_stamp_wqe(priv, ring, stamp_index,
-                                         !!((ring->cons + txbbs_stamp) &
+                                         !!((ring_cons + txbbs_stamp) &
                                                ring->size));
                        stamp_index = ring_index;
                        txbbs_stamp = txbbs_skipped;
@@ -452,7 +456,11 @@ static bool mlx4_en_process_tx_cq(struct net_device *dev,
        mcq->cons_index = cons_index;
        mlx4_cq_set_ci(mcq);
        wmb();
-       ring->cons += txbbs_skipped;
+
+       /* we want to dirty this cache line once */
+       ACCESS_ONCE(ring->last_nr_txbb) = last_nr_txbb;
+       ACCESS_ONCE(ring->cons) = ring_cons + txbbs_skipped;
+
        netdev_tx_completed_queue(ring->tx_queue, packets, bytes);
 
        /*