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

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Date:	Thu, 28 Jan 2016 12:50:49 +0100
From:	Torsten Duwe <duwe@....de>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Anton Blanchard <anton@...ba.org>, amodra@...il.com,
	Jiri Kosina <jkosina@...e.cz>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: 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