On Thu, Jun 1, 2023 at 1:34 PM Lorenzo Bianconi <lorenzo@xxxxxxxxxx> wrote:
>
> > 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:
Fixed while applying. Thanks everyone.
> 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]);
> >
> >
