linux-kernel - Re: [PATCHv3 bpf-next 0/9] mm/bpf/perf: Store build id in file object

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Message-ID: <CAP-5=fVYriALLwF2FU1ZUtLuHndnvPw=3SctVqY6Uwex8JfscA@mail.gmail.com>
Date:   Thu, 16 Mar 2023 10:50:16 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hao Luo <haoluo@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        bpf@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-perf-users@...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>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Namhyung Kim <namhyung@...il.com>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCHv3 bpf-next 0/9] mm/bpf/perf: Store build id in file object

On Thu, Mar 16, 2023 at 10:35 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Mar 16, 2023 at 06:01:40PM +0100, Jiri Olsa wrote:
> > hi,
> > this patchset adds build id object pointer to struct file object.
> >
> > We have several use cases for build id to be used in BPF programs
> > [2][3].
>
> Yes, you have use cases, but you never answered the question I asked:
>
> Is this going to be enabled by every distro kernel, or is it for special
> use-cases where only people doing a very specialised thing who are
> willing to build their own kernels will use it?
>
> Saying "hubble/tetragon" doesn't answer that question.  Maybe it does
> to you, but I have no idea what that software is.
>
> Put it another way: how does this make *MY* life better?  Literally me.
> How will it affect my life?

So at Google we use build IDs for all profiling, I believe Meta is the
same but obviously I can't speak for them. For BPF program stack
traces, using build ID + offset stack traces is preferable to perf's
whole system synthesis of mmap events based on data held in
/proc/pid/maps. Individual stack traces are larger, but you avoid the
ever growing problem of coming up with some initial virtual memory
state that will allow you to identify samples.

This doesn't answer the question about how this will help you, but I
expect over time you will see scalability issues and also want to use
tools assuming build IDs are present and cheap to access.

Thanks,
Ian