netdev - Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp

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Date: Mon, 5 Jun 2023 13:41:39 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: Jesper Dangaard Brouer <brouer@...hat.com>, Tariq Toukan <ttoukan.linux@...il.com>, 
	Daniel Borkmann <borkmann@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, 
	Andrii Nakryiko <andrii.nakryiko@...il.com>, bpf <bpf@...r.kernel.org>, 
	Tariq Toukan <tariqt@...dia.com>, Gal Pressman <gal@...dia.com>, 
	Network Development <netdev@...r.kernel.org>, Eelco Chaudron <echaudro@...hat.com>, 
	Andy Gospodarek <andrew.gospodarek@...adcom.com>
Subject: Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid
 reading sinfo

On Thu, Jun 1, 2023 at 1:34 PM Lorenzo Bianconi <lorenzo@...nel.org> 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@...hat.com>
> > ---
> >  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@...nel.org>
>
> >               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]);
> >
> >