Re: [dpdk-dev] [PATCH 1/2] net/mlx5: remove redundant operations

[Slava Ovsiienko <viacheslavo@nvidia.com>]

Hi, Ruifeng

> -----Original Message-----
> From: Ruifeng Wang <ruifeng.wang@arm.com>
> Sent: Tuesday, June 1, 2021 11:31
> To: Raslan Darawsheh <rasland@nvidia.com>; Matan Azrad
> <matan@nvidia.com>; Shahaf Shuler <shahafs@nvidia.com>; Slava Ovsiienko
> <viacheslavo@nvidia.com>
> Cc: dev@dpdk.org; jerinj@marvell.com; nd@arm.com;
> honnappa.nagarahalli@arm.com; Ruifeng Wang <ruifeng.wang@arm.com>;
> stable@dpdk.org
> Subject: [PATCH 1/2] net/mlx5: remove redundant operations
> 
> Some operations on mask are redundant and can be removed.
> The change yielded 1.6% performance gain on N1SDP.
> On ThunderX2, slight performance uplift was also observed.
> 
> Fixes: 570acdb1da8a ("net/mlx5: add vectorized Rx/Tx burst for ARM")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
>  drivers/net/mlx5/mlx5_rxtx_vec_neon.h | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
> b/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
> index 2234fbe6b2..98a75b09c6 100644
> --- a/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
> +++ b/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
> @@ -768,18 +768,11 @@ rxq_cq_process_v(struct mlx5_rxq_data *rxq,
> volatile struct mlx5_cqe *cq,
>  					  comp_mask), 0)) /
>  					  (sizeof(uint16_t) * 8);
>  		/* D.6 mask out entries after the compressed CQE. */
> -		mask = vcreate_u16(comp_idx <
> MLX5_VPMD_DESCS_PER_LOOP ?
> -				   -1UL >> (comp_idx * sizeof(uint16_t) * 8) :
> -				   0);
> -		invalid_mask = vorr_u16(invalid_mask, mask);
> +		invalid_mask = vorr_u16(invalid_mask, comp_mask);

Mmmm... I'm not sure we can drop the masking compressed (and following) CQE skip.
Let's consider the completion scenario (the series of 4 CQEs, each element is 64B long)

0: normal uncompressed CQE, ownership OK, format uncompressed, opcode OK, no error
1: compressed CQE, ownership OK, format compressed, opcode OK, no error
2: miniCQE array, format can be any!!, may be discovered as ownership OK, format uncompressed, opcode OK, no error
3: miniCQE array, format can be any!!, may be discovered as ownership OK, format uncompressed, opcode OK, no error

Obviously, we should unconditionally mask out 2 and 3, regardless of recognized their formats/opcode/error/etc.
I think we can get the diff above and skip diff below:

>  		/* D.7 count non-compressed valid CQEs. */
>  		n = __builtin_clzl(vget_lane_u64(vreinterpret_u64_u16(
>  				   invalid_mask), 0)) / (sizeof(uint16_t) * 8);
>  		nocmp_n += n;
> -		/* D.2 get the final invalid mask. */
> -		mask = vcreate_u16(n < MLX5_VPMD_DESCS_PER_LOOP ?
> -				   -1UL >> (n * sizeof(uint16_t) * 8) : 0);
> -		invalid_mask = vorr_u16(invalid_mask, mask);

and get the correct final invalid_mask - all compressed and invalid CQEs and following ones will be masked out.

With best regards,
Slava