netdev - Re: [PATCH 00/14] bpf: Add trampoline and dispatcher to /proc/kallsyms

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Date:   Wed, 12 Feb 2020 10:31:25 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Andrii Nakryiko <andriin@...com>, Yonghong Song <yhs@...com>,
        Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH 00/14] bpf: Add trampoline and dispatcher to
 /proc/kallsyms

Em Wed, Feb 12, 2020 at 12:13:46PM +0100, Jiri Olsa escreveu:
> On Tue, Feb 11, 2020 at 04:32:23PM -0300, Arnaldo Carvalho de Melo wrote:
> > Historically vmlinux was preferred because it contains function sizes,
> > but with all these out of the blue symbols, we need to prefer starting
> > with /proc/kallsyms and, as we do now, continue getting updates via
> > PERF_RECORD_KSYMBOL.

> > Humm, but then trampolines don't generate that, right? Or does it? If it
> > doesn't, then we will know about just the trampolines in place when the
> > record/top session starts, reparsing /proc/kallsyms periodically seems
> > excessive?

> I plan to extend the KSYMBOL interface to contain trampolines/dispatcher
> data,

That seems like the sensible, without looking too much at all the
details, to do, yes.

> plus we could do some inteligent fallback to /proc/kallsyms in case
> vmlinux won't have anything

At this point what would be the good reason to prefer vmlinux instead of
going straight to using /proc/kallsyms?

We have support for taking a snapshot of it at 'perf top' start, i.e.
right at the point we need to resolve a kernel symbol, then we get
PERF_RECORD_KSYMBOL for things that gets in place after that.

And as well we save it to the build-id cache so that later, at 'perf
report/script' time we can resolve kernel symbols, etc.

vmlinux is just what is in there right before boot, after that, for
quite some time, _lots_ of stuff happens :-)

- Arnaldo