linux-kernel - Re: perf not capturing stack traces

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Date:	Mon, 26 Jan 2015 13:54:06 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
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

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:
> > On Sun, Jan 25, 2015 at 03:56:52PM +0000, Russell King - ARM Linux wrote:
> > > On Sat, Jan 24, 2015 at 04:23:42PM -0600, Felipe Balbi wrote:
> > > > yeah, I'll try a few older kernels, also see if I can reproduce on other
> > > > boards.
> > > 
> > > Perf works for me with CONFIG_FRAME_POINTER=y, but that's only for kernel
> > > space, and for userspace where the programs have been built for ARM mode
> > > with frame pointers.
> > > 
> > > The kernel may work without CONFIG_FRAME_POINTER set, but I've never
> > > tested that, and I'd suggest that (given my experience looking at oops
> > > dumps) it's not all that reliable.
> > > 
> > > Lastly, userspace without frame pointers is pretty much hopeless.
> > 
> > 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'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.

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