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Re: [PATCH 7/7] DWARF: add the config option

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  Re: [PATCH 7/7] DWARF: add the config option Peter Zijlstra <peterz@infradead.org> - 2017-05-23 09:10 +0200
    Re: [PATCH 7/7] DWARF: add the config option Ingo Molnar <mingo@kernel.org> - 2017-05-23 09:30 +0200

#1647696 — Re: [PATCH 7/7] DWARF: add the config option

On Wed, May 10, 2017 at 10:32:06AM +0200, Jiri Slaby wrote:
> > But it does hurt, in the sense that the complicated format of DWARF CFI
> > means the unwinder has to jump through a lot more hoops to read it.
> 
> Why that matters, actually? Unwinder is nothing to be performance
> oriented. And if somebody is doing a lot of unwinding during runtime,
> they can switch to in-this-case-faster FP unwinder.

perf (and ftrace) like the unwinder to be considered performance
oriented.

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#1647723

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, May 10, 2017 at 10:32:06AM +0200, Jiri Slaby wrote:
> > > But it does hurt, in the sense that the complicated format of DWARF CFI
> > > means the unwinder has to jump through a lot more hoops to read it.
> > 
> > Why that matters, actually? Unwinder is nothing to be performance
> > oriented. And if somebody is doing a lot of unwinding during runtime,
> > they can switch to in-this-case-faster FP unwinder.
> 
> perf (and ftrace) like the unwinder to be considered performance
> oriented.

Yes, and given how critical debugging is there's a kind of useful synergy here: 
overall the perf unwinder is run about 10 orders of magnitude more often than the 
debugging unwinder, so it's a very big help in shaking out unwinder/debug-info 
bugs and increasing robustness overall.

The 'price' for using the unwinder in perf is that it has to be fast.

Thanks,

	Ingo

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