Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel

On Thu, Jan 28, 2016 at 03:26:59PM +1100, Michael Ellerman wrote:

That raises an interesting question, how does it work *without* DYNAMIC_FTRACE?

It looks like you haven't updated that version of _mcount at all? Or maybe I'm
missing an #ifdef somewhere?

You didn't, I did. I haven't considered that combination.

It doesn't look like that will work right with the -mprofile-kernel ABI. And
indeed it doesn't boot.

The lean _mcount should handle it and boot, had I not misplaced it in
the #ifdefs, but then of course profiling wouldn't work.

So we'll need to work that out. I guess the minimum would be to disable
-mprofile-kernel if DYNAMIC_FTRACE is disabled.

I feel that supporting all combinations of ABIv1/ABIv2, FTRACE/DYNAMIC_FTRACE,
-p/-mprofile-kernel will get us into #ifdef hell, and at least one kernel
developer will go insane. That will probably be the one porting this
to ppc64be (ABIv1).

Frankly I think we'd be happy to *only* support DYNAMIC_FTRACE, but the generic
code doesn't let us do that at the moment.

Seconded.
I'll have a look at the Kconfigs.

But it's better than the previous version which didn't boot :)

That's your fault, you picked the wrong compiler ;-)

Also ftracetest fails at step 8:
  ...
  [8] ftrace - function graph filters with stack tracer
  Unable to handle kernel paging request for data at address 0xd0000000033d7f70

[...]

That doesn't happen without your series applied, though that doesn't 100% mean
it's your bug. I haven't had time to dig any deeper.

Will check as well...

	Torsten