linux-kernel - Re: perf not capturing stack traces

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Message-ID: <20150126121621.GG15598@arm.com>
Date:	Mon, 26 Jan 2015 12:16:21 +0000
From:	Will Deacon <will.deacon@....com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	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 12:12:43PM +0000, 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?

Did he state that? I thought he was just saying that he couldn't unwind
userspace when *not* using frame pointers, which requires you to link
a recent perf with a bleeding-edge libunwind.

Will
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