Subject: Re: [PATCH v8 bpf-next 01/14] xdp: introduce mb in xdp_buff/xdp_frame

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 29 Apr 2021 15:36:29 +0200

On Thu,  8 Apr 2021 14:50:53 +0200
Lorenzo Bianconi <[email-masked]> wrote:

> Introduce multi-buffer bit (mb) in xdp_frame/xdp_buffer data structure
> in order to specify if this is a linear buffer (mb = 0) or a multi-buffer
> frame (mb = 1). In the latter case the shared_info area at the end of the
> first buffer will be properly initialized to link together subsequent
> buffers.
> 
> Signed-off-by: Lorenzo Bianconi <[email-masked]>
> ---
>  include/net/xdp.h | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index a5bc214a49d9..842580a61563 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -73,7 +73,10 @@ struct xdp_buff {
>  	void *data_hard_start;
>  	struct xdp_rxq_info *rxq;
>  	struct xdp_txq_info *txq;
> -	u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
> +	u32 frame_sz:31; /* frame size to deduce data_hard_end/reserved
> +			  * tailroom
> +			  */
> +	u32 mb:1; /* xdp non-linear buffer */
>  };
>  
>  static __always_inline void
> @@ -81,6 +84,7 @@ xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
>  {
>  	xdp->frame_sz = frame_sz;
>  	xdp->rxq = rxq;
> +	xdp->mb = 0;
>  }
>  
>  static __always_inline void
> @@ -116,7 +120,8 @@ struct xdp_frame {
>  	u16 len;
>  	u16 headroom;
>  	u32 metasize:8;
> -	u32 frame_sz:24;
> +	u32 frame_sz:23;
> +	u32 mb:1; /* xdp non-linear frame */
>  	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
>  	 * while mem info is valid on remote CPU.
>  	 */

So, it seems that these bitfield's are the root-cause of the
performance regression.  Credit to Alexei whom wisely already point
this out[1] in V2 ;-)

[1] https://lore.kernel.org/netdev/[email-masked]/

> @@ -179,6 +184,7 @@ void xdp_convert_frame_to_buff(struct xdp_frame *frame, struct xdp_buff *xdp)
>  	xdp->data_end = frame->data + frame->len;
>  	xdp->data_meta = frame->data - frame->metasize;
>  	xdp->frame_sz = frame->frame_sz;
> +	xdp->mb = frame->mb;
>  }
>  
>  static inline
> @@ -205,6 +211,7 @@ int xdp_update_frame_from_buff(struct xdp_buff *xdp,
>  	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
>  	xdp_frame->metasize = metasize;
>  	xdp_frame->frame_sz = xdp->frame_sz;
> +	xdp_frame->mb = xdp->mb;
>  
>  	return 0;
>  }

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer