Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

[Ingo Molnar]

* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> > > The results wouldn't be 100% accurate, but they could end up being useful 
> > > over time.
> > 
> > And to expound further on the bad idea, maybe the "bad" addresses could be 
> > filtered out somehow in post-processing (insert lots of hand waving).
> 
> And some details on the post-processing: in most cases it should be possible to 
> determine which of the found stack addresses are valid by looking at the call 
> instructions immediately preceding the stack text addresses, and making sure the 
> call target points to the same function as the previously found address.  But of 
> course that wouldn't work for indirect calls.

I believe this is similar to how OProfile did graph/dwarf profiling, by saving a 
copy of the stack and post-processing it.

By my best recollection (but I haven't used OProfile that much) it was both a 
performance nightmare, was limited (because it only saved a part of the stack), 
and was rather fragile as well, because it depended on the task VM being 
post-processable.

I think the highest quality implementation is to generate the call trace either in 
hardware (LBR), or as close to the event as possible: generate the kernel call 
chain in the PMI context, and the user-space call chain before user-space executes 
again (at the latest). Call chain generation should be roughly O(chain_depth), 
which both FP and ORC ensures.

Thanks,

	Ingo