Re: [dpdk-dev] [PATCH v1] net/i40e: remove the SMP barrier in HW scanning func

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

> -----Original Message-----
> From: Joyce Kong <joyce.kong@arm.com>
> Sent: Friday, June 4, 2021 3:34 PM
> To: Xing, Beilei <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>;
> ruifeng.wang@arm.com; honnappa.nagarahalli@arm.com
> Cc: dev@dpdk.org; nd@arm.com
> Subject: [PATCH v1] net/i40e: remove the SMP barrier in HW scanning func
> 
> Add the logic to determine how many DD bits have been set for contiguous
> packets, for removing the SMP barrier while reading descs.

I didn't understand this.
The current logic already guarantee the read out DD bits are from continue packets, as it read Rx descriptor in a reversed order from the ring.
So I didn't see the a new logic be added, would you describe more clear about the purpose of this patch?

> 
> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
>  drivers/net/i40e/i40e_rxtx.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c index
> 6c58decec..410a81f30 100644
> --- a/drivers/net/i40e/i40e_rxtx.c
> +++ b/drivers/net/i40e/i40e_rxtx.c
> @@ -452,7 +452,7 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue *rxq)
>  	uint16_t pkt_len;
>  	uint64_t qword1;
>  	uint32_t rx_status;
> -	int32_t s[I40E_LOOK_AHEAD], nb_dd;
> +	int32_t s[I40E_LOOK_AHEAD], var, nb_dd;
>  	int32_t i, j, nb_rx = 0;
>  	uint64_t pkt_flags;
>  	uint32_t *ptype_tbl = rxq->vsi->adapter->ptype_tbl; @@ -482,11 +482,14
> @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue *rxq)
>  					I40E_RXD_QW1_STATUS_SHIFT;
>  		}
> 
> -		rte_smp_rmb();

Any performance gain by removing this? and it is not necessary to be combined with below change, right?
 
> -
>  		/* Compute how many status bits were set */
> -		for (j = 0, nb_dd = 0; j < I40E_LOOK_AHEAD; j++)
> -			nb_dd += s[j] & (1 << I40E_RX_DESC_STATUS_DD_SHIFT);
> +		for (j = 0, nb_dd = 0; j < I40E_LOOK_AHEAD; j++) {
> +			var = s[j] & (1 << I40E_RX_DESC_STATUS_DD_SHIFT);
> +			if (var)
> +				nb_dd += 1;
> +			else
> +				break;
> +		}
> 
>  		nb_rx += nb_dd;
> 
> --
> 2.17.1