linux-kernel - Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

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Date:   Thu, 13 Jul 2017 08:15:00 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On Wed, Jul 12, 2017 at 09:29:17PM -0700, Andi Kleen wrote:
> On Wed, Jul 12, 2017 at 05:47:59PM -0500, Josh Poimboeuf wrote:
> > On Wed, Jul 12, 2017 at 03:30:31PM -0700, Andi Kleen wrote:
> > > Josh Poimboeuf <jpoimboe@...hat.com> writes:
> > > >
> > > > The ORC data format does have a few downsides compared to DWARF.  The
> > > > ORC unwind tables take up ~1MB more memory than DWARF eh_frame tables.
> > > >
> > > Can we have an option to just use dwarf instead? For people
> > > who don't want to waste a MB+ to solve a problem that doesn't
> > > exist (as proven by many years of opensuse kernel experience)
> > > 
> > > As far as I can tell this whole thing has only downsides compared
> > > to the dwarf unwinder that was earlier proposed. I don't see
> > > a single advantage.
> > 
> > Improved speed, reliability, maintainability.  Are those not advantages?
> 
> Ok. We'll see how it works out.
> 
> The memory overhead is quite bad though. You're basically undoing many
> years of efforts to shrink kernel text. I hope this can be still
> done better.

If we're talking *text*, this further shrinks text size by 3% because
frame pointers can be disabled.

As far as the data size goes, is anyone *truly* impacted by that extra
1MB or so?  If you're enabling a DWARF/ORC unwinder, you're already
signing up for a few extra megs anyway.

I do have a vague idea about how to reduce the data size, if/when the
size becomes a problem.  Basically there's a *lot* of duplication in the
ORC data:

  $ tools/objtool/objtool orc dump vmlinux | wc -l
  311095

  $ tools/objtool/objtool orc dump vmlinux |cut -d' ' -f2- |sort |uniq |wc -l
  345

So that's over 300,000 6-byte entries, only 345 of which are unique.
There should be a way to compress that.  However, it will probably
require sacrificing some combination of speed and simplicity.

-- 
Josh