linux-kernel - Re: [RFC PATCH 00/10] x86: undwarf unwinder

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Message-ID: <20170601115819.3twoowcnvtrfzjzr@treble>
Date:   Thu, 1 Jun 2017 06:58:20 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ingo Molnar <mingo@...nel.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>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 00/10] x86: undwarf unwinder

On Thu, Jun 01, 2017 at 08:08:24AM +0200, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > Here's the contents of the undwarf.txt file which explains the 'why' in
> > more detail:
> 
> Ok, so the code quality looks pretty convincing to me - the new core 'undwarf' 
> unwinder code is a _lot_ more readable than any of the Dwarf based attempts 
> before.
> 
> That we control the debug info generation at build time is icing on the cake to 
> me.
> 
> One thing I'd like to see on the list of benefits side of the equation is a size 
> comparison of kernel .text, with frame pointers vs. undwarf, on 64-bit kernels.

Ok, will do a text size comparison.  The only difficulty I encountered
there is that the 'size' tool considers the .undwarf section to be text
for some reason.  So the "text" size grew considerably :-)

> Being able to generate more optimal code in the hottest code paths of the kernel 
> is the _real_, primary upstream kernel benefit of a different debuginfo method - 
> which has to be weighed against the pain of introducing a new unwinder. But this 
> submission does not talk about that aspect at all, which should be fixed I think.

Actually I devoted an entire one-sentence paragraph to performance in
the documentation:

  The simpler debuginfo format also enables the unwinder to be relatively
  fast, which is important for perf and lockdep.

But I'll try to highlight that a little more.

-- 
Josh