linux-kernel - Re: Getting empty callchain from perf_callchain

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Date:   Tue, 11 Jun 2019 22:05:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kairui Song <kasong@...hat.com>, Alexei Starovoitov <ast@...com>,
        Song Liu <songliubraving@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Getting empty callchain from perf_callchain_kernel()

On Fri, May 24, 2019 at 10:53:19AM +0200, Peter Zijlstra wrote:
> > For ORC, I'm thinking we may be able to just require that all generated
> > code (BPF and others) always use frame pointers.  Then when ORC doesn't
> > recognize a code address, it could try using the frame pointer as a
> > fallback.
> 
> Yes, this seems like a sensible approach. We'd also have to audit the
> ftrace and kprobe trampolines, IIRC they only do framepointer setup for
> CONFIG_FRAME_POINTER currently, which should be easy to fix (after the
> patches I have to fix the FP generation in the first place:
> 
>   https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=x86/wip

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.

Though, hm, because of pt_regs I guess ORC would need to be able to
decode an encoded frame pointer?  I was hoping we could leave those
encoded frame pointers behind in CONFIG_FRAME_POINTER-land forever...

Here are my latest BPF unwinder patches in case anybody wants a sneak
peek:

  https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=bpf-orc-fix

-- 
Josh