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:

quoted

On Sun, Jan 25, 2015 at 03:56:52PM +0000, Russell King - ARM Linux wrote:

quoted

On Sat, Jan 24, 2015 at 04:23:42PM -0600, Felipe Balbi wrote:

quoted

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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