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

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Date:   Wed, 6 Oct 2021 07:53:31 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <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 6, 2021 at 1:44 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> 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?

That would be on top of your next patch set?
Please post it first.