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

* Josh Poimboeuf <jpoimboe@redhat.com> 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

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