Re: [dpdk-dev] [PATCH] net/mlx5: fix instruction hotspot on replenishing Rx buffer

[David Marchand <david.marchand@redhat.com>]

On Wed, Jan 9, 2019 at 9:54 AM Yongseok Koh <yskoh@mellanox.com> wrote:

> On replenishing Rx buffers for vectorized Rx, mbuf->buf_addr isn't needed
> to be accessed as it is static and easily calculated from the mbuf address.
> Accessing the mbuf content causes unnecessary load stall and it is worsened
> on ARM.
>
> Fixes: 545b884b1da3 ("net/mlx5: fix buffer address posting in SSE Rx")
> Cc: stable@dpdk.org
>
> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
> ---
>  drivers/net/mlx5/mlx5_rxtx_vec.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/mlx5/mlx5_rxtx_vec.h
> b/drivers/net/mlx5/mlx5_rxtx_vec.h
> index fda7004e2d..ced5547307 100644
> --- a/drivers/net/mlx5/mlx5_rxtx_vec.h
> +++ b/drivers/net/mlx5/mlx5_rxtx_vec.h
> @@ -102,8 +102,12 @@ mlx5_rx_replenish_bulk_mbuf(struct mlx5_rxq_data
> *rxq, uint16_t n)
>                 return;
>         }
>         for (i = 0; i < n; ++i) {
> -               wq[i].addr = rte_cpu_to_be_64((uintptr_t)elts[i]->buf_addr
> +
> -                                             RTE_PKTMBUF_HEADROOM);
> +               uintptr_t buf_addr =
> +                       (uintptr_t)elts[i] + sizeof(struct rte_mbuf) +
> +                       rte_pktmbuf_priv_size(rxq->mp) +
> RTE_PKTMBUF_HEADROOM;
> +
> +               assert(buf_addr == (uintptr_t)elts[i]->buf_addr);
> +               wq[i].addr = rte_cpu_to_be_64(buf_addr);
>                 /* If there's only one MR, no need to replace LKey in WQE.
> */
>                 if (unlikely(mlx5_mr_btree_len(&rxq->mr_ctrl.cache_bh) >
> 1))
>                         wq[i].lkey = mlx5_rx_mb2mr(rxq, elts[i]);
> --
> 2.11.0
>
>
How about having a macro / inline in the mbuf api to get this information
in a consistent/unique way ?
I can see we have this calculation at least in rte_pktmbuf_init() and
rte_pktmbuf_detach().


-- 
David Marchand