Messages in this thread | | | Date | Tue, 1 Dec 2015 17:11:25 +0100 | From | Ingo Molnar <> | Subject | Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling taken at 'callq' instruction |
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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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