linux-kernel - Re: Additional debug info to aid cacheline analysis

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Date:   Fri, 30 Oct 2020 14:26:19 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Mark Wielaard <mark@...mp.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        linux-toolchains@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        "Phillips, Kim" <kim.phillips@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andi Kleen <andi@...stfloor.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: Additional debug info to aid cacheline analysis

Hello,

On Thu, Oct 8, 2020 at 6:38 PM Mark Wielaard <mark@...mp.org> wrote:
>
> Hi,
>
> On Thu, 2020-10-08 at 09:02 +0200, Peter Zijlstra wrote:
> > Some time ago, I had my intern pursue the other 2 approaches for
> > > symbolization. The one I see as most promising is by using the DWARF
> > > information (no BPF needed). The good news is that I believe we do not
> > > need more information than what is already there. We just need the
> > > compiler to generate valid DWARF at most optimization levels, which I
> > > believe is not the case for LLVM based compilers but maybe okay for
> > > GCC.
> >
> > Right, I think GCC improved a lot on this front over the past few years.
> > Also added Andi and Masami, who have worked on this or related topics.
>
> For GCC Alexandre Oliva did a really thorough write up of all the
> various optimization and their effect on debugging/DWARF:
> https://www.fsfla.org/~lxoliva/writeups/gOlogy/gOlogy.html

Thanks for the link.  Looks very nice.

>
> GCC using -fvar-tracking and -fvar-tracking-assignments is pretty good
> at keeping track of where variables are held (in memory or registers)
> when in the program, even through various optimizations.
>
> -fvar-tracking-assignments is the default with -g -O2.
> Except for the upstream linux kernel code. Most distros enable it
> again, but you do want to enable it by hand when building from the
> upstream linux git repo.

Please correct me if I'm wrong.  This seems to track local variables.
But I'm not sure it's enough for this purpose as we want to know
types of any memory references (not directly from a variable).

Let's say we have a variable like below:

  struct xxx a;

  a.b->c->d++;

And we have a sample where 'd' is updated, then how can we know
it's from the variable 'a'?  Maybe we don't need to know it, but we
should know it accesses the 'd' field in the struct 'c'.

Probably we can analyze the asm code and figure out it's from 'a'
and accessing 'd' at the moment.  I'm curious if there's a way in
the DWARF to help this kind of work.

Thanks
Namhyung