Re: Getting empty callchain from perf_callchain_kernel()

From: Josh Poimboeuf
Date: Wed Jun 12 2019 - 10:31:56 EST

On Wed, Jun 12, 2019 at 09:10:23AM -0400, Steven Rostedt wrote:
> On Tue, 11 Jun 2019 22:05:01 -0500
> Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > Right now, ftrace has a special hook in the ORC unwinder
> > (orc_ftrace_find). It would be great if we could get rid of that in
> > favor of the "always use frame pointers" approach. I'll hold off on
> > doing the kpatch/kprobe trampoline conversions in my patches since it
> > would conflict with yours.
> Basically, IIUC, what you are saying is that the ftrace trampoline
> should always store the %sp in %rb even when CONFIG_FRAME_POINTER is not
> enabled? And this can allow you to remove the ftrace specific code from
> the orc unwinder?

Basically, yes. Though the frame pointer encoding which Peter is adding
to the ftrace/kprobes trampolines might complicate things a bit.