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

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Message-ID: <f515305c3b250f9dbed003b98d78f72c3d72cc2c.camel@gmail.com>
Date: Sat, 22 Nov 2025 00:50:08 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Donglin Peng <dolinux.peng@...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, 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.