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Date:   Wed, 12 Jun 2019 09:50:08 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kairui Song <kasong@redhat.com>, Alexei Starovoitov <ast@fb.com>,
        Song Liu <songliubraving@fb.com>,
        lkml <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: Getting empty callchain from perf_callchain_kernel()
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On Wed, Jun 12, 2019 at 10:54:23AM +0200, Peter Zijlstra wrote:
> On Tue, Jun 11, 2019 at 10:05:01PM -0500, Josh Poimboeuf wrote:
> > 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...
> 
> Ah, I see.. could a similar approach work for the kprobe trampolines
> perhaps?

If you mean requiring that kprobes trampolines always use frame
pointers, I think it should work.

> > 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
> 
> On a quick read-through, that looks good to me. A minor nit:
> 
> 			/* mov dst_reg, %r11 */
> 			EMIT_mov(dst_reg, AUX_REG);
> 
> The disparity between %r11 and AUX_REG is jarring. I understand the
> whole bpf register mapping thing, but it is just weird when reading
> this.

True, but there are several cases where the r11 is hard-coded in the
instruction encoding itself, like:

			/* mov imm32, %r11 */
			EMIT3_off32(0x49, 0xC7, 0xC3, imm32);

If the code were more decoupled, like if it had helpers where you could
always just pass AUX_REG, and the code never had to know what the value
of AUX_REG is, then using "AUX_REG" in the comments would make sense.

But since there are inconsistencies, with hard-coded register mapping
knowledge in many places, I find it easier to follow what's going on
when the specific register name is always shown in the comments.

> Other than that, the same note as before, the 32bit JIT still seems
> buggered, but I'm not sure you (or anybody else) cares enough about that
> to fix it though. It seems to use ebp as its own frame pointer, which
> completely defeats an unwinder.

I'm still trying to decide if I care about 32-bit.  It does indeed use
ebp everywhere.  But I'm not sure if I want to poke the beehive...  Also
factoring into the equation is the fact that I'll be on PTO next week
:-)  If I have time in the next couple days then I may take a look.

-- 
Josh