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

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Date:   Fri, 17 Mar 2023 03:51:15 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Ian Rogers <irogers@...gle.com>, 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 02:51:52PM -0700, Andrii Nakryiko wrote:
> Yep, Meta is also capturing stack traces with build ID as well, if
> possible. Build IDs help with profiling short-lived processes which
> exit before the profiling session is done and user-space tooling is
> able to collect /proc/<pid>/maps contents (which is what Ian is
> referring to here). But also build ID allows to offload more of the
> expensive stack symbolization process (converting raw memory addresses
> into human readable function+offset+file path+line numbers
> information) to dedicated remote servers, by allowing to cache and
> reuse preprocessed DWARF/ELF information based on build ID.
> 
> I believe perf tool is also using build ID, so any tool relying on
> perf capturing full and complete profiling data for system-wide
> performance analysis would benefit as well.
> 
> Generally speaking, there is a whole ecosystem built on top of
> assumption that binaries have build ID and profiling tooling is able
> to provide more value if those build IDs are more reliably collected.
> Which ultimately benefits the entire open-source ecosystem by allowing
> people to spot issues (not necessarily just performance, it could be
> correctness issues as well) more reliably, fix them, and benefit every
> user.

But build IDs are _generally_ available.  The only problem (AIUI)
is when you're trying to examine the contents of one container from
another container.  And to solve that problem, you're imposing a cost
on everybody else with (so far) pretty vague justifications.  I really
don't like to see you growing struct file for this (nor struct inode,
nor struct vm_area_struct).  It's all quite unsatisfactory and I don't
have a good suggestion.