linux-kernel - Re: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for CALLER

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Date:	Wed, 4 Feb 2009 18:07:55 +0300
From:	Anton Vorontsov <avorontsov@...mvista.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Steven Rostedt <srostedt@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: On PowerPC we don't need frame pointers for
	CALLER_ADDRs

On Tue, Feb 03, 2009 at 07:19:55PM +0300, Anton Vorontsov wrote:
> On Tue, Feb 03, 2009 at 05:06:45PM +0100, Ingo Molnar wrote:
> > 
> > * Anton Vorontsov <avorontsov@...mvista.com> wrote:
> > 
> > > According to this discussion:
> > > 
> > > http://lkml.org/lkml/2008/7/25/338
> > > http://lkml.org/lkml/2008/7/26/72
> > > 
> > > Frame pointers do nothing useful on PowerPC, so lib/Kconfig.debug
> > > makes CONFIG_FRAME_POINTER unselectable on PPC targets. But ftrace.h
> > > requires CONFIG_FRAME_POINTER for CALLER_ADDR macros. [...]
> > 
> > hm, why not add PPC to FRAME_POINTERS list of architectures, and select it 
> > from the powerpc arch Kconfig? Does that cause complications somewhere?
> 
> -fno-omit-frame-pointers makes the code worse w/o any actual
> benefit that we would use. Plus, there is a long standing bug in
> gcc that makes -fno-omit-frame-pointer generate wrong code for PPC
> targets:
> 
> http://lkml.org/lkml/2008/9/2/25
> 
> That is, the only tracer that needs[1] -fno-omit-frame-pointer is
> "FUNCTION_TRCER", but we workaround the issue via -mno-sched-epilog,
> quoting arch/powerpc/Makefile:
> 
> # Work around a gcc code-gen bug with -fno-omit-frame-pointer.
> ifeq ($(CONFIG_FUNCTION_TRACER),y)
> KBUILD_CFLAGS           += -mno-sched-epilog
> endif

Thinking about it more... we can workaround the bug the other way,
and then permit CONFIG_FRAME_POINTER on PowerPC.

Patches are coming...

-- 
Anton Vorontsov
email: cbouatmailru@...il.com
irc://irc.freenode.net/bd2
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