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 10:56 AM Yongseok Koh <yskoh@mellanox.com> wrote:

>
> > On Jan 9, 2019, at 1:52 AM, Olivier Matz <olivier.matz@6wind.com> wrote:
> >
> > On Wed, Jan 09, 2019 at 10:38:07AM +0100, David Marchand wrote:
> >> 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().
> >
> > Agree. Maybe rte_mbuf_default_buf_addr(m) ?
>
> I'm also okay to add. Will come up with a new patch.
>
> > Side note, is the assert() correct in the patch? I'd say there's a
> > difference of RTE_PKTMBUF_HEADROOM between the 2 values.
>
> Oops, my fault. Thanks for the catch, you saved a crash. :-)
>

Is this assert really necessary if we have a common macro ?
I was under the impression that this assert is there to catch misalignement
between the mbuf api and the driver.


-- 
David Marchand