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

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Message-ID: <20170601140518.2fsgz2kp26owulcg@treble>
Date:   Thu, 1 Jun 2017 09:05:18 -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 03:50:05PM +0200, Ingo Molnar wrote:
> * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Thu, Jun 01, 2017 at 08:08:24AM +0200, Ingo Molnar wrote:
> > > 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.

Ah, you meant runtime performance with FP disabled.  I also dedicated a
whole sentence to that one :-)

  Unlike frame pointers, the debuginfo is out-of-band, so it has no
  effect on runtime performance.

I'll try to flesh that out and maybe come up with some numbers.

-- 
Josh