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

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Date:   Thu, 1 Jun 2017 15:50:05 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> 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 :-)

One trick I sometimes use is to only size some of the key builtin.o files.

> > 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.

That's not what I meant! The speedup comes from (hopefully) being able to disable 
CONFIG_FRAME_POINTER, which:

 - creates simpler/faster function prologues and epilogues - no managing of RBP 
   needed

 - gives one more generic purpose register to work from. This matters less on 
   64-bit kernels but it's a small effect.

I've seen numbers of 1-2% of instruction count reduction in common kernel 
workloads, which would be pretty significant on well cached workloads.

Thanks,

	Ingo