Re: [dpdk-dev] [PATCH v3 2/2] net/i40e: replace SMP barrier with thread fence

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

> -----Original Message-----
> From: Joyce Kong <joyce.kong@arm.com>
> Sent: Tuesday, July 6, 2021 2:54 PM
> To: Xing, Beilei <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>;
> ruifeng.wang@arm.com; honnappa.nagarahalli@arm.com; Richardson, Bruce
> <bruce.richardson@intel.com>; Zhang, Helin <helin.zhang@intel.com>
> Cc: dev@dpdk.org; stable@dpdk.org; nd@arm.com
> Subject: [PATCH v3 2/2] net/i40e: replace SMP barrier with thread fence
> 
> Simply replace the SMP barrier with atomic thread fence for i40e hw ring sacn,
> if there is no synchronization point.
> 
> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
>  drivers/net/i40e/i40e_rxtx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c index
> 9aaabfd92..86e2f083e 100644
> --- a/drivers/net/i40e/i40e_rxtx.c
> +++ b/drivers/net/i40e/i40e_rxtx.c
> @@ -482,7 +482,8 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue *rxq)
>  					I40E_RXD_QW1_STATUS_SHIFT;
>  		}
> 
> -		rte_smp_rmb();
> +		/* This barrier is to order loads of different words in the descriptor */
> +		rte_atomic_thread_fence(__ATOMIC_ACQUIRE);

Now for x86, you actually replace a compiler barrier with a memory fence, this may have potential performance impact which need additional resource to investigate 
So there are 2 options:
1. if you want this patch be merged into DPDK 21.08, please change this for ARM only.
2. you can wait for our update for x86 but I guess it will miss 21.08.

What do you think?

Btw for patch 1/2, I think I can merge it independently right?


> 
>  		/* Compute how many status bits were set */
>  		for (j = 0, nb_dd = 0; j < I40E_LOOK_AHEAD; j++) {
> --
> 2.17.1