linux-kernel - Re: Getting empty callchain from perf_callchain

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Date:   Thu, 23 May 2019 12:27:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kairui Song <kasong@...hat.com>
Cc:     Alexei Starovoitov <ast@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Fri, May 24, 2019 at 12:41:59AM +0800, Kairui Song wrote:
>  On Thu, May 23, 2019 at 11:24 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > On Thu, May 23, 2019 at 10:50:24PM +0800, Kairui Song wrote:
> > > > > Hi Josh, this still won't fix the problem.
> > > > >
> > > > > Problem is not (or not only) with ___bpf_prog_run, what actually went
> > > > > wrong is with the JITed bpf code.
> > > >
> > > > There seem to be a bunch of issues.  My patch at least fixes the failing
> > > > selftest reported by Alexei for ORC.
> > > >
> > > > How can I recreate your issue?
> > >
> > > Hmm, I used bcc's example to attach bpf to trace point, and with that
> > > fix stack trace is still invalid.
> > >
> > > CMD I used with bcc:
> > > python3 ./tools/stackcount.py t:sched:sched_fork
> >
> > I've had problems in the past getting bcc to build, so I was hoping it
> > was reproducible with a standalone selftest.
> >
> > > And I just had another try applying your patch, self test is also failing.
> >
> > Is it the same selftest reported by Alexei?
> >
> >   test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap err -1 errno 2
> >
> > > I'm applying on my local master branch, a few days older than
> > > upstream, I can update and try again, am I missing anything?
> >
> > The above patch had some issues, so with some configs you might see an
> > objtool warning for ___bpf_prog_run(), in which case the patch doesn't
> > fix the test_stacktrace_map selftest.
> >
> > Here's the latest version which should fix it in all cases (based on
> > tip/master):
> >
> >   https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/commit/?h=bpf-orc-fix
> 
> Hmm, I still get the failure:
> test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap
> err -1 errno 2
> 
> And I didn't see how this will fix the issue. As long as ORC need to
> unwind through the JITed code it will fail. And that will happen
> before reaching ___bpf_prog_run.

Ok, I was able to recreate by doing

  echo 1 > /proc/sys/net/core/bpf_jit_enable

first.  I'm guessing you have CONFIG_BPF_JIT_ALWAYS_ON.

-- 
Josh