linux-kernel - Re: PEBS level 2/3 breaks dwarf unwinding! [WAS: Re: Broken dwarf unwinding

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Message-ID: <6310843.N4ooJGWzKY@agathebauer>
Date:   Thu, 15 Nov 2018 10:10:13 +0100
From:   Milian Wolff <milian.wolff@...b.com>
To:     Travis Downs <travis.downs@...il.com>
Cc:     ak@...ux.intel.com, jolsa@...hat.com, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, namhyung@...nel.org,
        linux-perf-users@...r.kernel.org, acme@...nel.org
Subject: Re: PEBS level 2/3 breaks dwarf unwinding! [WAS: Re: Broken dwarf
 unwinding - wrong stack pointer register value?]

On Donnerstag, 15. November 2018 03:05:32 CET Travis Downs wrote:
> On Wed, Nov 14, 2018 at 8:20 AM Milian Wolff <milian.wolff@...b.com> wrote:
> > 3) I suggest we always keep the first frame and sample IP from the user
> > regs, i.e. the accurate PEBS/IBS IP. Then we add the following frames
> > from unwinding the ustack with the iregs.
> 
> Does this mean that the displayed unwind will sometimes be
> "impossible" to have actually be generated from a consistent execution
> of the user program?

Yes, that is exactly what I'm saying.

> For example, the top frame (from PEBS) and second frame (from iregs)
> may be inconsistent in that the latter function never calls the first.
> At this point it would be good to have an indication at the top frame
> is from a different source than the rest of the frames, lest the user
> pull is hair out trying to determine how function X seems to call
> function Y despite that not being the case in the source.

I agree. I personally like your suggested approach - only add an indication 
when the IP differs so much that it points to a different function. What do 
others say to this?

Cheers

-- 
Milian Wolff | milian.wolff@...b.com | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts
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