linux-kernel - Re: Getting empty callchain from perf_callchain

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Message-ID: <20190612091023.6bccf262@gandalf.local.home>
Date:   Wed, 12 Jun 2019 09:10:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>
Subject: Re: Getting empty callchain from perf_callchain_kernel()

On Tue, 11 Jun 2019 22:05:01 -0500
Josh Poimboeuf <jpoimboe@...hat.com> 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?

-- Steve


> 
> 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...