> 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 <brouer@xxxxxxxxxx>
> ---
> 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 <lorenzo@xxxxxxxxxx>
> 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]);
>
>
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