linux-kernel - Re: perf not capturing stack traces

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Date:	Mon, 26 Jan 2015 11:33:29 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Will Deacon <will.deacon@....com>, Felipe Balbi <balbi@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tony Lindgren <tony@...mide.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel Mailing List 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: perf not capturing stack traces

Em Mon, Jan 26, 2015 at 01:54:06PM +0000, Russell King - ARM Linux escreveu:
> On Mon, Jan 26, 2015 at 09:12:43AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Mon, Jan 26, 2015 at 10:27:11AM +0000, Will Deacon escreveu:
> > > FWIW, perf can now use libunwind for unwinding the userspace side of
> > > things, so it's not quite as bad as it used to be. For the kernel side,
> > > if the unwinder isn't working properly it would be nice to know *why*,
> > > but I agree that it tends to be far flakier than the frame-pointer method.

> > Any idea why, with userspace using frame pointers, perf doesn't go all
> > the way from kernel to userspace main() (or whatever is the endpoint),
> > as Russel stated?
>            ^ *growl*

I misunderstood, as corrected on another message, sorry.
 
> I've rebuilt userspace code which I've been working on in with a bunch of
> flags which makes it use frame pointers in ARM mode, and perf does seem
> to be capable of that; in that case, perf_callchain_user() can walk the
> linked set of frames.
> 
> However, if glibc is built for thumb2 or doesn't contain frame pointers,
> userspace tracing pretty much stops after you hit the first function in
> userspace.

Right.

- Arnaldo
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