linux-kernel - Re: [RFC PATCH v7 5/7] libbpf: Implement BTF type sorting validation for binary search optimization

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Message-ID: <CAErzpmtaHHuukkep26GTc+r0aTGHNt0LXiJ2aYKz50dD+Acw7A@mail.gmail.com>
Date: Sat, 22 Nov 2025 23:59:10 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>, ast@...nel.org, zhangxiaoqin@...omi.com, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	Donglin Peng <pengdonglin@...omi.com>, Alan Maguire <alan.maguire@...cle.com>, 
	Song Liu <song@...nel.org>
Subject: Re: [RFC PATCH v7 5/7] libbpf: Implement BTF type sorting validation
 for binary search optimization

On Sat, Nov 22, 2025 at 4:50 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Sat, 2025-11-22 at 15:19 +0800, Donglin Peng wrote:
>
> [...]
>
> > > - find_bpffs_btf_enums() - this function does a linear scan over all
> > >   types in module BTFs.
> >
> > I think putting names ahead is helpful here, because there is a check
> > (info->cmd_t && info->map_t && info->prog_t && info->attach_t) to
> > return early. but I think it can be converted to use btf_find_by_name_kind.
>
> Oh, sorry, I somehow missed the early exit here.
> But as you say, it is a combination of 4 by-name lookups, essentially.
> Thus can be converted to btf_find_by_name_kind() trivially.
>
> > > - find_btf_percpu_datasec() - this function looks for a DATASEC with
> > >   name ".data..percpu" and returns as soon as the match is found.
> > >
> > > Of the 4 functions above only find_btf_percpu_datasec() will return
> > > early if BTF type with specified name is found. And it can be
> > > converted to use btf_find_by_name_kind().
> >
> > Thanks. I’ve looked into find_btf_percpu_datasec and we can’t use
> > btf_find_by_name_kind here because the search scope differs. For
> > a module BTF, find_btf_percpu_datasec only searches within the
> > module’s own BTF, whereas btf_find_by_name_kind prioritizes
> > searching the base BTF first. Thus, placing named types ahead is
> > more effective here. Besides, I found that the '.data..percpu' named
> > type will be placed at [1] for vmlinux BTF because the prefix '.' is
> > smaller than any letter, so the linear search only requires one loop to
> > locate it. However, if we put named types at the end, it will need more
> > than 60,000 loops..
>
> But this can be easily fixed if a variant of btf_find_by_name_kind()
> is provided that looks for a match only in a specific BTF. Or accepts
> a start id parameter.

Yes, but I'm not sure it's necessary to add a new parameter to
btf_find_by_name_kind for just one use case.