> On Nov 2, 2020, at 10:31 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Mon, Nov 2, 2020 at 9:59 PM Song Liu <songliubraving@xxxxxx> wrote:
>>
>>
>>
>>> On Nov 2, 2020, at 9:25 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>>>
>>> On Mon, Nov 2, 2020 at 6:49 PM Song Liu <songliubraving@xxxxxx> wrote:
>>>>
>>>>
>>>>
>>>>> On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>
>>>> Acked-by: Song Liu <songliubraving@xxxxxx>
>>>>
>>>> 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.
No problem. It was just a nitpick. The loop is totally fine as is.
Thanks,
Song
