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

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Date:   Thu, 1 Jun 2017 07:52:04 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 00/10] x86: undwarf unwinder

On Thu, Jun 01, 2017 at 02:33:20PM +0200, Jiri Slaby wrote:
> On 06/01/2017, 02:17 PM, Peter Zijlstra wrote:
> > On Thu, Jun 01, 2017 at 06:58:20AM -0500, Josh Poimboeuf 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 relative to a DWARF unwinder. It doesn't appear to be possible to
> > get anywhere near a frame-pointer unwinder due to having to do this
> > log(n) lookup for every single frame.
> 
> This is ~ 20 times faster than my DWARF unwinder by a quick measurement
> (20000 calls to save_stack_trace via single vfs_write).

Wow!  Thanks for quantifying that.  Looks like the lookup is indeed the
bottleneck as expected.

-- 
Josh