Re: [PATCH v3 01/15] stacktrace/x86: add function for detecting reliable stack traces

Miroslav Benes
2016-12-19T18:23:01Z

On Mon, 19 Dec 2016, Josh Poimboeuf wrote:

> On Mon, Dec 19, 2016 at 05:25:19PM +0100, Miroslav Benes wrote:
> > On Thu, 8 Dec 2016, Josh Poimboeuf wrote:
> >
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index 215612c..b4a6663 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -155,6 +155,7 @@ config X86
> > > select HAVE_PERF_REGS
> > > select HAVE_PERF_USER_STACK_DUMP
> > > select HAVE_REGS_AND_STACK_ACCESS_API
> > > + select HAVE_RELIABLE_STACKTRACE if X86_64 && FRAME_POINTER && STACK_VALIDATION
> >
> > Tests to measure possible performance penalty of frame pointers were done
> > by Mel Gorman. The outcome was quite clear. There IS a measurable
> > impact. The percentage depends on the workflow but I think it is safe to
> > say that FP usually takes 5-10 percents.
> >
> > If my understanding is correct there is no single culprit. Register
> > pressure is definitely not a problem. We ran simple benchmarks while
> > taking a register away from GCC (RBP or a common one). The impact is a
> > combination of more cacheline pressure, more memory accesses and the fact
> > that the kernel contains a lot of small functions.
> >
> > Thus, I think that DWARF should be the way to go here.
> >
> > Other than that the patch looks good to me.
>
> I agree that DWARF is generally a good idea, and I'm working toward it.
> However there's still quite a bit of work to get there.
>
> For this consistency model to work with DWARF on x86, we would need:
>
> 1) a reliable x86 DWARF unwinder with Linus's blessing
> 2) objtool DWARF support (I'm working on this at the moment)
> 3) probably some kind of runtime NMI stack checking feature to
> complement objtool, along with a lot of burn time to ensure there are
> no issues, particularly in entry code
> 4) port save_stack_trace_tsk_reliable() to work with DWARF

Yes, this is a lot of work to do.

> DWARF will be nice to have, but it's definitely not required before
> merging this consistency model.

Oh, I didn't mean it to be done before merging this patch set. Sorry for
the confusion. The point was that as long as the performance is involved
FP does not look that promising and DWARF could be better (but who knows,
right?).

> Also I doubt we'll ever be able to drop frame pointer support
> completely. Some embedded systems may not want the overhead of the
> DWARF metadata.

True. There should be a choice in this respect.

Regards,
Miroslav