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

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Message-ID: <a254810a42510ad3adc00d27d1fd456710c7faa9.camel@gmail.com>
Date: Mon, 24 Nov 2025 10:16:48 -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 23:45 +0800, Donglin Peng wrote:
> On Sat, Nov 22, 2025 at 5:05 PM Eduard Zingerman <eddyz87@...il.com> wrote:
> > 
> > On Sat, 2025-11-22 at 00:50 -0800, Eduard Zingerman wrote:
> > 
> > [...]
> > 
> > > > 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.
> > 
> > Also, I double checked, and for my vmlinux the id for '.data..percpu'
> > section is 110864, the last id of all. So, having all anonymous types
> > in front does not change status-quo compared to current implementation.
> 
> Yes. If types are sorted alphabetically, the '.data..percpu' section will
> have ID 1 in vmlinux BTF. In this case, linear search performance is
> optimal when named types are placed ahead of anonymous types.
> 
> I would like to understand the benefits of having anonymous types at the
> front of named types.

This will allow using strcmp() instead of btf_compare_type_names(),
which we have to copy both in kernel and in libbpf. Reducing the code
size and cognitive load.