Re: [PATCH 2/2] perf/x86: Disable precise sampling for PERF_SAMPLE_STACK_USER

[Andi Kleen <ak@linux.intel.com>]

> That's what we already do, no? I have distinct memories of making the
> stack unwind use the NMI regs rather then the PEBS regs.
> 
> > In my opinion, it could even make the thing worse. User
> > requires to get precise samplings, but perf silently returns imprecise
> > records, this would mislead user. 
> 
> Mostly just the unwind might be off a little, the rest is accurate. This
> has been the case 'forever'. Performance analysis isn't for silly
> people, if they can't deal with a little fuzz then perhaps they're in
> the wrong business.

Is the main problem that the stack doesn't agree? Perhaps there
could be a check for regs->rsp == pebs->user rsp (if in user space)
to detect problematic samples.

The question is how to report it and who should do the checking. 

It may need new fields in the ABI either to communicate the extra PEBS RSP
or a bit to indicate that there might be a mismatch.

I guess checking in the kernel and reporting an error might be simpler
and maybe cleaner, but it would likely limit more advanced recovery
possibilities.

Are there other mismatches that break the unwinding? Perhaps the same
for RBP?

-Andi