Message-ID: <CAEf4BzaL51nf_rKF7-pUWHeCiWm37fuFGfku4Z0kmXxmdHRAVA@mail.gmail.com>
Date:   Mon, 2 Nov 2020 22:31:28 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 08/11] libbpf: support BTF dedup of split BTFs

On Mon, Nov 2, 2020 at 9:59 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Nov 2, 2020, at 9:25 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> >
> > On Mon, Nov 2, 2020 at 6:49 PM Song Liu <songliubraving@...com> wrote:
> >>
> >>
> >>
> >>> On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@...nel.org> wrote:
> >>>
> >>> Add support for deduplication split BTFs. When deduplicating split BTF, base
> >>> BTF is considered to be immutable and can't be modified or adjusted. 99% of
> >>> BTF deduplication logic is left intact (module some type numbering adjustments).
> >>> There are only two differences.
> >>>
> >>> First, each type in base BTF gets hashed (expect VAR and DATASEC, of course,
> >>> those are always considered to be self-canonical instances) and added into
> >>> a table of canonical table candidates. Hashing is a shallow, fast operation,
> >>> so mostly eliminates the overhead of having entire base BTF to be a part of
> >>> BTF dedup.
> >>>
> >>> Second difference is very critical and subtle. While deduplicating split BTF
> >>> types, it is possible to discover that one of immutable base BTF BTF_KIND_FWD
> >>> types can and should be resolved to a full STRUCT/UNION type from the split
> >>> BTF part.  This is, obviously, can't happen because we can't modify the base
> >>> BTF types anymore. So because of that, any type in split BTF that directly or
> >>> indirectly references that newly-to-be-resolved FWD type can't be considered
> >>> to be equivalent to the corresponding canonical types in base BTF, because
> >>> that would result in a loss of type resolution information. So in such case,
> >>> split BTF types will be deduplicated separately and will cause some
> >>> duplication of type information, which is unavoidable.
> >>>
> >>> With those two changes, the rest of the algorithm manages to deduplicate split
> >>> BTF correctly, pointing all the duplicates to their canonical counter-parts in
> >>> base BTF, but also is deduplicating whatever unique types are present in split
> >>> BTF on their own.
> >>>
> >>> Also, theoretically, split BTF after deduplication could end up with either
> >>> empty type section or empty string section. This is handled by libbpf
> >>> correctly in one of previous patches in the series.
> >>>
> >>> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> >>
> >> Acked-by: Song Liu <songliubraving@...com>
> >>
> >> With some nits:
> >>
> >>> ---
> >>
> >> [...]
> >>
> >>>
> >>>      /* remap string offsets */
> >>>      err = btf_for_each_str_off(d, strs_dedup_remap_str_off, d);
> >>> @@ -3553,6 +3582,63 @@ static bool btf_compat_fnproto(struct btf_type *t1, struct btf_type *t2)
> >>>      return true;
> >>> }
> >>>
> >>
> >> An overview comment about bpf_deup_prep() will be great.
> >
> > ok
> >
> >>
> >>> +static int btf_dedup_prep(struct btf_dedup *d)
> >>> +{
> >>> +     struct btf_type *t;
> >>> +     int type_id;
> >>> +     long h;
> >>> +
> >>> +     if (!d->btf->base_btf)
> >>> +             return 0;
> >>> +
> >>> +     for (type_id = 1; type_id < d->btf->start_id; type_id++)
> >>> +     {
> >>
> >> Move "{" to previous line?
> >
> > yep, my bad
> >
> >>
> >>> +             t = btf_type_by_id(d->btf, type_id);
> >>> +
> >>> +             /* all base BTF types are self-canonical by definition */
> >>> +             d->map[type_id] = type_id;
> >>> +
> >>> +             switch (btf_kind(t)) {
> >>> +             case BTF_KIND_VAR:
> >>> +             case BTF_KIND_DATASEC:
> >>> +                     /* VAR and DATASEC are never hash/deduplicated */
> >>> +                     continue;
> >>
> >> [...]
> >>
> >>>      /* we are going to reuse hypot_map to store compaction remapping */
> >>>      d->hypot_map[0] = 0;
> >>> -     for (i = 1; i <= d->btf->nr_types; i++)
> >>> -             d->hypot_map[i] = BTF_UNPROCESSED_ID;
> >>> +     /* base BTF types are not renumbered */
> >>> +     for (id = 1; id < d->btf->start_id; id++)
> >>> +             d->hypot_map[id] = id;
> >>> +     for (i = 0, id = d->btf->start_id; i < d->btf->nr_types; i++, id++)
> >>> +             d->hypot_map[id] = BTF_UNPROCESSED_ID;
> >>
> >> We don't really need i in the loop, shall we just do
> >>        for (id = d->btf->start_id; id < d->btf->start_id + d->btf->nr_types; id++)
> >> ?
> >>
> >
> > I prefer the loop with i iterating over the count of types, it seems
> > more "obviously correct". For simple loop like this I could do
> >
> > for (i = 0; i < d->btf->nr_types; i++)
> >    d->hypot_map[d->start_id + i] = ...;
> >
> > But for the more complicated one below I found that maintaining id as
> > part of the for loop control block is a bit cleaner. So I just stuck
> > to the consistent pattern across all of them.
>
> How about
>
>         for (i = 0; i < d->btf->nr_types; i++) {
>                 id = d->start_id + i;
>                 ...
> ?

this would be excessive for that single-line for loop. I'd really like
to keep it consistent and confined within the for () block.

>
> I would expect for loop with two loop variable to do some tricks, like two
> termination conditions, or another conditional id++ somewhere in the loop.

Libbpf already uses such two variable loops for things like iterating
over btf_type's members, enums, func args, etc. So it's not an
entirely alien construct. I really appreciate you trying to keep the
code as simple and clean as possible, but I think it's pretty
straightforward in this case and there's no need to simplify it
further.

>
> Thanks,
> Song
>

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