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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> 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.
> 
> Perf report show bpf_xdp_pointer() percentage utilization being reduced
> from 4,19% to 3,37% (on CPU E5-1650 @3.60GHz).
> 
> 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 <brouer@redhat.com>
> ---
>  net/core/filter.c |   12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 968139f4a1ac..a635f537d499 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3948,20 +3948,24 @@ 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))
> -		return ERR_PTR(-EINVAL);
> +	if (likely((offset < size))) /* linear area */
> +		goto out;

Hi Jesper,

please correct me if I am wrong but looking at the code, in this way
bpf_xdp_pointer() will return NULL (and not ERR_PTR(-EINVAL)) if:
- offset < size
- offset + len > xdp_get_buff_len()

doing so I would say bpf_xdp_copy_buf() will copy the full packet starting from
offset leaving some part of the auxiliary buffer possible uninitialized.
Do you think it is an issue?

Regards,
Lorenzo

>  
> -	if (offset < size) /* linear area */
> +	if (likely(!xdp_buff_has_frags(xdp)))
>  		goto out;
>  
> +	if (offset + len > xdp_get_buff_len(xdp))
> +		return ERR_PTR(-EINVAL);
> +
> +	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]);
> 
> 

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