netdev - Re: [RFC] store function address in BTF

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Message-ID: <YV1h+cBxmYi2hrTM@krava>
Date:   Wed, 6 Oct 2021 10:44:41 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, Daniel Xu <dxu@...uu.xyz>,
        Viktor Malik <vmalik@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [RFC] store function address in BTF

On Wed, Oct 06, 2021 at 10:41:41AM +0200, Jiri Olsa wrote:
> hi,
> I'm hitting performance issue and soft lock ups with the new version
> of the patchset and the reason seems to be kallsyms lookup that we
> need to do for each btf id we want to attach

ugh, I meant to sent this as reply to the patchset mentioned above,
nevermind, here's the patchset:
  https://lore.kernel.org/bpf/20210605111034.1810858-1-jolsa@kernel.org/

jirka

> 
> I tried to change kallsyms_lookup_name linear search into rbtree search,
> but it has its own pitfalls like duplicate function names and it still
> seems not to be fast enough when you want to attach like 30k functions
> 
> so I wonder we could 'fix this' by storing function address in BTF,
> which would cut kallsyms lookup completely, because it'd be done in
> compile time
> 
> my first thought was to add extra BTF section for that, after discussion
> with Arnaldo perhaps we could be able to store extra 8 bytes after
> BTF_KIND_FUNC record, using one of the 'unused' bits in btf_type to
> indicate that? or new BTF_KIND_FUNC2 type?
> 
> thoughts?
> 
> thanks,
> jirka