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

[Ingo Molnar <mingo@kernel.org>]

* 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