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

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Message-ID: <20170601121721.lezoecnyah3aic6a@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Jun 2017 14:17:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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>, Jiri Slaby <jslaby@...e.cz>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 00/10] x86: undwarf unwinder

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.