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

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Message-ID: <CAEf4BzZCPcq0eo=1SN-r=k5QF1XE5hihEYHYYdi37aiV7VXwVQ@mail.gmail.com>
Date: Tue, 15 Jul 2025 10:20:52 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong.dong@...ux.dev>
Cc: Menglong Dong <menglong8.dong@...il.com>, alexei.starovoitov@...il.com, 
	rostedt@...dmis.org, jolsa@...nel.org, bpf@...r.kernel.org, 
	Menglong Dong <dongml2@...natelecom.cn>, 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 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.

>
> Thanks!
> Menglong Dong
>
> >> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> >> ---
> >>   tools/lib/bpf/btf.c      | 102 +++++++++++++++++++++++++++++++++++++++
> >>   tools/lib/bpf/btf.h      |   6 +++
> >>   tools/lib/bpf/libbpf.c   |  37 +++++++++++---
> >>   tools/lib/bpf/libbpf.map |   3 ++
> >>   4 files changed, 140 insertions(+), 8 deletions(-)
> >>
> > [...]
> >