net/mlx5: reduce Tx completion index memory loads

There is a non-optimal check if doorbell is needed present in the
mlx5_tx_handle_completion() function. Advancing a copy of the txq
consumer index and checking this copy with initial value causes
unnecessary memory loads and hurts the performance. It is better to
have a simple small boolean variable for this purpose. That allows
to eliminate all the excessive memory operations with the txq consumer
index and restore the performance of the tx completions.

Fixes: 1fd9af05e44e ("net/mlx5: update Tx error handling routine")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>

diff --git a/drivers/net/mlx5/mlx5_rxtx.c b/drivers/net/mlx5/mlx5_rxtx.c
index 5ac63da..f3bf763 100644 (file)
--- a/drivers/net/mlx5/mlx5_rxtx.c
+++ b/drivers/net/mlx5/mlx5_rxtx.c
@@ -2160,7 +2160,7 @@ mlx5_tx_handle_completion(struct mlx5_txq_data *restrict txq,
 {
        unsigned int count = MLX5_TX_COMP_MAX_CQE;
        volatile struct mlx5_cqe *last_cqe = NULL;
-       uint16_t ci = txq->cq_ci;
+       bool ring_doorbell = false;
        int ret;
 
        static_assert(MLX5_CQE_STATUS_HW_OWN < 0, "Must be negative value");
@@ -2168,8 +2168,8 @@ mlx5_tx_handle_completion(struct mlx5_txq_data *restrict txq,
        do {
                volatile struct mlx5_cqe *cqe;
 
-               cqe = &txq->cqes[ci & txq->cqe_m];
-               ret = check_cqe(cqe, txq->cqe_s, ci);
+               cqe = &txq->cqes[txq->cq_ci & txq->cqe_m];
+               ret = check_cqe(cqe, txq->cqe_s, txq->cq_ci);
                if (unlikely(ret != MLX5_CQE_STATUS_SW_OWN)) {
                        if (likely(ret != MLX5_CQE_STATUS_ERR)) {
                                /* No new CQEs in completion queue. */
@@ -2183,7 +2183,6 @@ mlx5_tx_handle_completion(struct mlx5_txq_data *restrict txq,
                         * here, before we might perform SQ reset.
                         */
                        rte_wmb();
-                       txq->cq_ci = ci;
                        ret = mlx5_tx_error_cqe_handle
                                (txq, (volatile struct mlx5_err_cqe *)cqe);
                        if (unlikely(ret < 0)) {
@@ -2199,16 +2198,18 @@ mlx5_tx_handle_completion(struct mlx5_txq_data *restrict txq,
                         * MLX5_CQE_SYNDROME_WR_FLUSH_ERR status.
                         * The send queue is supposed to be empty.
                         */
-                       ++ci;
-                       txq->cq_pi = ci;
+                       ring_doorbell = true;
+                       ++txq->cq_ci;
+                       txq->cq_pi = txq->cq_ci;
                        last_cqe = NULL;
                        continue;
                }
                /* Normal transmit completion. */
-               MLX5_ASSERT(ci != txq->cq_pi);
-               MLX5_ASSERT((txq->fcqs[ci & txq->cqe_m] >> 16) ==
+               MLX5_ASSERT(txq->cq_ci != txq->cq_pi);
+               MLX5_ASSERT((txq->fcqs[txq->cq_ci & txq->cqe_m] >> 16) ==
                            cqe->wqe_counter);
-               ++ci;
+               ring_doorbell = true;
+               ++txq->cq_ci;
                last_cqe = cqe;
                /*
                 * We have to restrict the amount of processed CQEs
@@ -2221,14 +2222,10 @@ mlx5_tx_handle_completion(struct mlx5_txq_data *restrict txq,
                if (likely(--count == 0))
                        break;
        } while (true);
-       if (likely(ci != txq->cq_ci)) {
-               /*
-                * Update completion queue consuming index
-                * and ring doorbell to notify hardware.
-                */
+       if (likely(ring_doorbell)) {
+               /* Ring doorbell to notify hardware. */
                rte_compiler_barrier();
-               txq->cq_ci = ci;
-               *txq->cq_db = rte_cpu_to_be_32(ci);
+               *txq->cq_db = rte_cpu_to_be_32(txq->cq_ci);
                mlx5_tx_comp_flush(txq, last_cqe, olx);
        }
 }