linux-kernel - Re: [PATCH bpf-next v2 14/18] libbpf: add btf type hash lookup support

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Message-ID: <3339133.5fSG56mABF@7940hx>
Date: Wed, 16 Jul 2025 19:53:50 +0800
From: Menglong Dong <menglong.dong@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Menglong Dong <menglong8.dong@...il.com>, alexei.starovoitov@...il.com,
 rostedt@...dmis.org, jolsa@...nel.org, bpf@...r.kernel.org,
 Martin KaFai Lau <martin.lau@...ux.dev>,
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 linux-kernel@...r.kernel.org
Subject:
 Re: [PATCH bpf-next v2 14/18] libbpf: add btf type hash lookup support

On Wednesday, July 16, 2025 1:20 AM Andrii Nakryiko <andrii.nakryiko@...il.com> write:
> On Mon, Jul 14, 2025 at 9:41 PM Menglong Dong <menglong.dong@...ux.dev> wrote:
> >
> >
> > On 7/15/25 06:07, Andrii Nakryiko wrote:
> > > On Thu, Jul 3, 2025 at 5:22 AM Menglong Dong <menglong8.dong@...il.com> wrote:
> > >> For now, the libbpf find the btf type id by loop all the btf types and
> > >> compare its name, which is inefficient if we have many functions to
> > >> lookup.
> > >>
> > >> We add the "use_hash" to the function args of find_kernel_btf_id() to
> > >> indicate if we should lookup the btf type id by hash. The hash table will
> > >> be initialized if it has not yet.
> > > Or we could build hashtable-based index outside of struct btf for a
> > > specific use case, because there is no one perfect hashtable-based
> > > indexing that can be done generically (e.g., just by name, or
> > > name+kind, or kind+name, or some more complicated lookup key) and
> > > cover all potential use cases. I'd prefer not to get into a problem of
> > > defining and building indexes and leave it to callers (even if the
> > > caller is other part of libbpf itself).
> >
> >
> > I think that works. We can define a global hash table in libbpf.c,
> > and add all the btf type to it. I'll redesign this part, and make it
> > separate with the btf.
> 
> No global things, please. It can be held per-bpf_object, or even
> constructed on demand during attachment and then freed. No need for
> anything global.

Okay, the per-bpf_object is a good idea, and I'll try to implement
it this way.

Thanks!
Menglong Dong