Re: [PATCH] net/bnxt: reduce barriers in NEON vector Rx

[Ajit Khaparde <ajit.khaparde@broadcom.com>]

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On Sun, Jun 12, 2022 at 11:22 PM Ruifeng Wang <ruifeng.wang@arm.com> wrote:
>
> To read descriptors in expected order, barriers are inserted after each
> descriptor read. The excessive use of barriers is unnecessary and could
> cause performance drop.
>
> Removed barriers between descriptor reads. And changed counting of valid
> packets so as to handle discontinuous valid packets. Because out of
> order read could lead to valid descriptors that fetched being
> discontinuous.
>
> In VPP L3 routing test, 6% performance gain was observed. The test was
> done on a platform with ThunderX2 CPU and Broadcom PS225 NIC.
>
> Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>

Patch applied to dpdk-next-net-brcm. Thanks

>
> ---
>  drivers/net/bnxt/bnxt_rxtx_vec_neon.c | 47 ++++++++++++++-------------
>  1 file changed, 25 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/net/bnxt/bnxt_rxtx_vec_neon.c b/drivers/net/bnxt/bnxt_rxtx_vec_neon.c
> index 32f8e59b3a..6a4ece681b 100644
> --- a/drivers/net/bnxt/bnxt_rxtx_vec_neon.c
> +++ b/drivers/net/bnxt/bnxt_rxtx_vec_neon.c
> @@ -235,34 +235,32 @@ recv_burst_vec_neon(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
>                  * IO barriers are used to ensure consistent state.
>                  */
>                 rxcmp1[3] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 7]);
> -               rte_io_rmb();
> +               rxcmp1[2] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 5]);
> +               rxcmp1[1] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 3]);
> +               rxcmp1[0] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 1]);
> +
> +               /* Use acquire fence to order loads of descriptor words. */
> +               rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
>                 /* Reload lower 64b of descriptors to make it ordered after info3_v. */
>                 rxcmp1[3] = vreinterpretq_u32_u64(vld1q_lane_u64
>                                 ((void *)&cpr->cp_desc_ring[cons + 7],
>                                 vreinterpretq_u64_u32(rxcmp1[3]), 0));
> -               rxcmp[3] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 6]);
> -
> -               rxcmp1[2] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 5]);
> -               rte_io_rmb();
>                 rxcmp1[2] = vreinterpretq_u32_u64(vld1q_lane_u64
>                                 ((void *)&cpr->cp_desc_ring[cons + 5],
>                                 vreinterpretq_u64_u32(rxcmp1[2]), 0));
> -               rxcmp[2] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 4]);
> -
> -               t1 = vreinterpretq_u64_u32(vzip2q_u32(rxcmp1[2], rxcmp1[3]));
> -
> -               rxcmp1[1] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 3]);
> -               rte_io_rmb();
>                 rxcmp1[1] = vreinterpretq_u32_u64(vld1q_lane_u64
>                                 ((void *)&cpr->cp_desc_ring[cons + 3],
>                                 vreinterpretq_u64_u32(rxcmp1[1]), 0));
> -               rxcmp[1] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 2]);
> -
> -               rxcmp1[0] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 1]);
> -               rte_io_rmb();
>                 rxcmp1[0] = vreinterpretq_u32_u64(vld1q_lane_u64
>                                 ((void *)&cpr->cp_desc_ring[cons + 1],
>                                 vreinterpretq_u64_u32(rxcmp1[0]), 0));
> +
> +               rxcmp[3] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 6]);
> +               rxcmp[2] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 4]);
> +
> +               t1 = vreinterpretq_u64_u32(vzip2q_u32(rxcmp1[2], rxcmp1[3]));
> +
> +               rxcmp[1] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 2]);
>                 rxcmp[0] = vld1q_u32((void *)&cpr->cp_desc_ring[cons + 0]);
>
>                 t0 = vreinterpretq_u64_u32(vzip2q_u32(rxcmp1[0], rxcmp1[1]));
> @@ -278,16 +276,21 @@ recv_burst_vec_neon(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
>                  * bits and count the number of set bits in order to determine
>                  * the number of valid descriptors.
>                  */
> -               valid = vget_lane_u64(vreinterpret_u64_u16(vqmovn_u32(info3_v)),
> -                                     0);
> +               valid = vget_lane_u64(vreinterpret_u64_s16(vshr_n_s16
> +                               (vreinterpret_s16_u16(vshl_n_u16
> +                               (vqmovn_u32(info3_v), 15)), 15)), 0);
> +
>                 /*
>                  * At this point, 'valid' is a 64-bit value containing four
> -                * 16-bit fields, each of which is either 0x0001 or 0x0000.
> -                * Compute number of valid descriptors from the index of
> -                * the highest non-zero field.
> +                * 16-bit fields, each of which is either 0xffff or 0x0000.
> +                * Count the number of consecutive 1s from LSB in order to
> +                * determine the number of valid descriptors.
>                  */
> -               num_valid = (sizeof(uint64_t) / sizeof(uint16_t)) -
> -                               (__builtin_clzl(valid & desc_valid_mask) / 16);
> +               valid = ~(valid & desc_valid_mask);
> +               if (valid == 0)
> +                       num_valid = 4;
> +               else
> +                       num_valid = __builtin_ctzl(valid) / 16;
>
>                 if (num_valid == 0)
>                         break;
> --
> 2.25.1
>

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