Subject: Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 1 Jun 2023 22:34:24 +0200

> Currently we observed a significant performance degradation in
> samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
> added in commit 772251742262 ("samples/bpf: fixup some tools to be able
> to support xdp multibuffer").
> 
> This patch reduce the overhead by avoiding to read/load shared_info
> (sinfo) memory area, when XDP packet don't have any frags. This improves
> performance because sinfo is located in another cacheline.
> 
> Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
> and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
> potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
> to avoid accessing sinfo in no-frags case.
> 
> The likely/unlikely instrumentation lays out asm code such that sinfo
> access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
> The generated asm code is more compact towards the no-frags case.
> 
> The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
> should also take effect for that.
> 
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> ---
>  net/core/filter.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 968139f4a1ac..961db5bd2f94 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3948,20 +3948,21 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
>  
>  void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
>  {
> -	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>  	u32 size = xdp->data_end - xdp->data;
> +	struct skb_shared_info *sinfo;
>  	void *addr = xdp->data;
>  	int i;
>  
>  	if (unlikely(offset > 0xffff || len > 0xffff))
>  		return ERR_PTR(-EFAULT);
>  
> -	if (offset + len > xdp_get_buff_len(xdp))
> +	if (unlikely(offset + len > xdp_get_buff_len(xdp)))
>  		return ERR_PTR(-EINVAL);
>  
> -	if (offset < size) /* linear area */
> +	if (likely((offset < size))) /* linear area */

nit: you can drop a round bracket here. Other than that:

Acked-by: Lorenzo Bianconi <[email-masked]>

>  		goto out;
>  
> +	sinfo = xdp_get_shared_info_from_buff(xdp);
>  	offset -= size;
>  	for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
>  		u32 frag_size = skb_frag_size(&sinfo->frags[i]);
> 
>