netdev - Re: [PATCH RFC bpf-next 01/20] trait: limited KV store for packet metadata

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Message-Id: <D89ZNSJCPNUA.20V16A9FXJ54J@arthurfabre.com>
Date: Fri, 07 Mar 2025 12:14:20 +0100
From: "Arthur Fabre" <arthur@...hurfabre.com>
To: "Alexei Starovoitov" <alexei.starovoitov@...il.com>
Cc: "Network Development" <netdev@...r.kernel.org>, "bpf"
 <bpf@...r.kernel.org>, "Jakub Sitnicki" <jakub@...udflare.com>, "Jesper
 Dangaard Brouer" <hawk@...nel.org>, "Yan Zhai" <yan@...udflare.com>,
 <jbrandeburg@...udflare.com>, Toke Høiland-Jørgensen
 <thoiland@...hat.com>, <lbiancon@...hat.com>, "Arthur Fabre"
 <afabre@...udflare.com>
Subject: Re: [PATCH RFC bpf-next 01/20] trait: limited KV store for packet
 metadata

On Fri Mar 7, 2025 at 7:36 AM CET, Alexei Starovoitov wrote:
> On Wed, Mar 5, 2025 at 6:33 AM <arthur@...hurfabre.com> wrote:
> >
> > +struct __trait_hdr {
> > +       /* Values are stored ordered by key, immediately after the header.
> > +        *
> > +        * The size of each value set is stored in the header as two bits:
> > +        *  - 00: Not set.
> > +        *  - 01: 2 bytes.
> > +        *  - 10: 4 bytes.
> > +        *  - 11: 8 bytes.
>
> ...
>
> > +        *  - hweight(low) + hweight(high)<<1 is offset.
>
> the comment doesn't match the code
>
> > +        */
> > +       u64 high;
> > +       u64 low;
>
> ...
>
> > +static __always_inline int __trait_total_length(struct __trait_hdr h)
> > +{
> > +       return (hweight64(h.low) << 1) + (hweight64(h.high) << 2)
> > +               // For size 8, we only get 4+2=6. Add another 2 in.
> > +               + (hweight64(h.high & h.low) << 1);
> > +}
>
> This is really cool idea, but 2 byte size doesn't feel that useful.
> How about:
> - 00: Not set.
> - 01: 4 bytes.
> - 10: 8 bytes.
> - 11: 16 bytes.
>
> 4 byte may be useful for ipv4, 16 for ipv6, and 8 is just a good number.
> And compute the same way with 3 popcount with extra +1 to shifts.

I chose the sizes arbitrarily, happy to change them.

16 is also useful for UUIDs, for tracing.

Size 0 could store bools / flags. Keys could be set without a value, 
and users could check if the key is set or not.
That replaces single bits of the mark today, for example a
"route locally" key.

That only leaves one other size, maybe 4 for smaller values?

If we want more sizes, we could also:

- Add another u64 word to the header, so we have 3 bits per key. It
  uses more room, and we need more popcnts, but most modern x86 CPUs can
  do 3 popcnts in parallel so it could be ok.

- Let users set consecutive keys to one big value. Instead of supporting
  size 16, we let them set two 8 byte KVs in one trait_set() call and
  provide a 16 byte value. Eg:

	trait_set_batch(u64 key_from, u64_key_to, size, ...);

  It's easy to implement, but it makes the API more complicated.