Messages in this thread | | | Date | Tue, 1 Dec 2015 18:21:36 +0100 | From | Peter Zijlstra <> | Subject | Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling taken at 'callq' instruction |
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On Tue, Dec 01, 2015 at 05:11:25PM +0100, Ingo Molnar wrote: > > * 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?
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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