netdev - Re: [PATCH v4 bpf-next 06/13] bpf: introduce bpf_xdp_get_frags_{count, total

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Date:   Fri, 2 Oct 2020 18:25:46 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        sameehj@...zon.com, dsahern@...nel.org, brouer@...hat.com,
        echaudro@...hat.com
Subject: Re: [PATCH v4 bpf-next 06/13] bpf: introduce
 bpf_xdp_get_frags_{count, total_size} helpers

> Lorenzo Bianconi wrote:
> > From: Sameeh Jubran <sameehj@...zon.com>
> > 
> > Introduce the two following bpf helpers in order to provide some
> > metadata about a xdp multi-buff fame to bpf layer:
> > 
> > - bpf_xdp_get_frags_count()
> >   get the number of fragments for a given xdp multi-buffer.
> 
> Same comment as in the cover letter can you provide a use case
> for how/where I would use xdp_get_frags_count()? Is it just for
> debug? If its just debug do we really want a uapi helper for it.

I have no a strong opinion on it, I guess we can just drop this helper,
but I am not the original author of the patch :)

> 
> > 
> > * bpf_xdp_get_frags_total_size()
> >   get the total size of fragments for a given xdp multi-buffer.
> 
> This is awkward IMO. If total size is needed it should return total size
> in all cases not just in the mb case otherwise programs will have two
> paths the mb path and the non-mb path. And if you have mixed workload
> the branch predictor will miss? Plus its extra instructions to load.

ack, I am fine to make the helper reporing to total size instead of paged ones
(we can compute it if we really need it)

> 
> And if its useful for something beyond just debug and its going to be
> read every packet or something I think we should put it in the metadata
> so that its not hidden behind a helper which likely will show up as
> overhead on a 40+gbps nic. The use case I have in mind is counting
> bytes maybe sliced by IP or protocol. Here you will always read it
> and I don't want code with a if/else stuck in the middle when if
> we do it right we have a single read.

do you mean xdp_frame or data_meta area? As explained in the cover-letter we
choose this approach to save space in xdp_frame.

> 
> > 
> > Signed-off-by: Sameeh Jubran <sameehj@...zon.com>
> > Co-developed-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > ---
> >  include/uapi/linux/bpf.h       | 14 ++++++++++++
> >  net/core/filter.c              | 42 ++++++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 14 ++++++++++++
> >  3 files changed, 70 insertions(+)
> > 
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 4f556cfcbfbe..0715995eb18c 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3668,6 +3668,18 @@ union bpf_attr {
> >   * 	Return
> >   * 		The helper returns **TC_ACT_REDIRECT** on success or
> >   * 		**TC_ACT_SHOT** on error.
> > + *
> > + * int bpf_xdp_get_frags_count(struct xdp_buff *xdp_md)
> > + *	Description
> > + *		Get the number of fragments for a given xdp multi-buffer.
> > + *	Return
> > + *		The number of fragments
> > + *
> > + * int bpf_xdp_get_frags_total_size(struct xdp_buff *xdp_md)
> > + *	Description
> > + *		Get the total size of fragments for a given xdp multi-buffer.
> 
> Why just fragments? Will I have to also add the initial frag0 to it
> or not. I think the description is a bit ambiguous.
> 
> > + *	Return
> > + *		The total size of fragments for a given xdp multi-buffer.
> >   */
> 
> [...]
> 
> > +const struct bpf_func_proto bpf_xdp_get_frags_count_proto = {
> > +	.func		= bpf_xdp_get_frags_count,
> > +	.gpl_only	= false,
> > +	.ret_type	= RET_INTEGER,
> > +	.arg1_type	= ARG_PTR_TO_CTX,
> > +};
> > +
> > +BPF_CALL_1(bpf_xdp_get_frags_total_size, struct  xdp_buff*, xdp)
> > +{
> > +	struct skb_shared_info *sinfo;
> > +	int nfrags, i, size = 0;
> > +
> > +	if (likely(!xdp->mb))
> > +		return 0;
> > +
> > +	sinfo = xdp_get_shared_info_from_buff(xdp);
> > +	nfrags = min_t(u8, sinfo->nr_frags, MAX_SKB_FRAGS);
> > +
> > +	for (i = 0; i < nfrags; i++)
> > +		size += skb_frag_size(&sinfo->frags[i]);
> 
> Wont the hardware just know this? I think walking the frag list
> just to get the total seems wrong. The hardware should have a
> total_len field somewhere we can just read no? If mvneta doesn't
> know the total length that seems like a driver limitation and we
> shouldn't encode it in the helper.

I have a couple of patches to improve this (not posted yet):
- https://github.com/LorenzoBianconi/bpf-next/commit/ff9b3a74a105b64947931f83fe86a4b8b1808103
- https://github.com/LorenzoBianconi/bpf-next/commit/712e67333cbc5f6304122b1009cdae1e18e6eb26

Regards,
Lorenzo

> 
> > +
> > +	return size;
> > +}
> 

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