linux-kernel - Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling taken at 'callq' instruction

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Message-ID: <20151201172136.GP3816@twins.programming.kicks-ass.net>
Date:	Tue, 1 Dec 2015 18:21:36 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"Wangnan (F)" <wangnan0@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Milian Wolff <milian.wolff@...b.com>,
	linux-kernel@...r.kernel.org, pi3orama <pi3orama@....com>,
	lizefan 00213767 <lizefan@...wei.com>
Subject: Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling
 taken at 'callq' instruction

On Tue, Dec 01, 2015 at 05:11:25PM +0100, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > so I think the problem here is that the RSP does not match up to the RIP. We 
> > > can either pass along the original RIP+RSP, or the fixed up one - but what we 
> > > do currently is that we pass along only half of it - which corrupts dwarf 
> > > unwinding state that doesn't tolerate such errors.
> > 
> > Still not sure what that gets you. Then you get a sample at a known wrong 
> > location, why would you want that?
> 
> Well, we'd at least get a valid call trace - which the 'mixed' one isn't? I.e. 
> this only matters with --call-graph.
> 
> But yeah, with my suggestion we'd essentially fall back from cycles:pp to 
> cycles:p, ideally we'd want to have real_rsp. Does the hardware provide that?

No, no real_rsp.

> User-space cannot compute that reliably I think, what if the 'real' instruction 
> was manipulating RSP in more complex ways than doing a CALL?

I'm not really too aware of these asm details :/ 

Jiri, what is in PERF_SAMPLE_REGS_USER.IP ? from a quick reading that is
whatever is returned from task_pt_regs(current), not the
perf_data.regs.ip field which contains the corrected IP.

Should the uwinder then not use PERF_SAMPLE_REGS_USER.{IP,SP} for a
consistent unwind?
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