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

From	Ingo Molnar <mingo@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling taken at 'callq' instruction
Date	2015-12-01 17:20 +0100
Message-ID	<qAVOO-3v2-29@gated-at.bofh.it> (permalink)
References	(5 earlier) <qwvzB-by-39@gated-at.bofh.it> <qzmJt-pZ-31@gated-at.bofh.it> <qAsWv-1JR-35@gated-at.bofh.it> <qANxU-6yD-27@gated-at.bofh.it> <qAODF-7cY-15@gated-at.bofh.it>
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* Peter Zijlstra <peterz@infradead.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?

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

Thanks,

	Ingo
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Thread

Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling  taken at 'callq' instruction Ingo Molnar <mingo@kernel.org> - 2015-12-01 08:30 +0100
  Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling  taken at 'callq' instruction Peter Zijlstra <peterz@infradead.org> - 2015-12-01 09:40 +0100
    Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling  taken at 'callq' instruction Ingo Molnar <mingo@kernel.org> - 2015-12-01 17:20 +0100
      Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling  taken at 'callq' instruction Peter Zijlstra <peterz@infradead.org> - 2015-12-01 18:30 +0100
        Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling  taken at 'callq' instruction Ingo Molnar <mingo@kernel.org> - 2015-12-02 11:00 +0100

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