From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754641AbZBNWEd (ORCPT ); Sat, 14 Feb 2009 17:04:33 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752959AbZBNWEX (ORCPT ); Sat, 14 Feb 2009 17:04:23 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:56451 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752573AbZBNWEW (ORCPT ); Sat, 14 Feb 2009 17:04:22 -0500 Date: Sat, 14 Feb 2009 23:03:26 +0100 From: Ingo Molnar To: Sam Ravnborg Cc: Anton Vorontsov , Steven Rostedt , Paul Mackerras , Benjamin Herrenschmidt , linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Subject: Re: [PATCH 1/3] Makefile: Include arch Makefiles as late as possible Message-ID: <20090214220326.GA5200@elte.hu> References: <20090204150755.GA24163@oksana.dev.rtsoft.ru> <20090204150835.GA30027@oksana.dev.rtsoft.ru> <20090204212612.GN22608@elte.hu> <20090214195702.GB1241@uranus.ravnborg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090214195702.GB1241@uranus.ravnborg.org> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Sam Ravnborg wrote: > On Wed, Feb 04, 2009 at 10:26:12PM +0100, Ingo Molnar wrote: > > > > * Anton Vorontsov wrote: > > > > > This patch gives arches more freedom on overwriting CFLAGS, specifically > > > on PowerPC we want to remove -fno-omit-frame-pointer flag. > > > > > > Signed-off-by: Anton Vorontsov > > > --- > > > Makefile | 4 ++-- > > > 1 files changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/Makefile b/Makefile > > > index 7715b2c..d1ba93f 100644 > > > --- a/Makefile > > > +++ b/Makefile > > > @@ -525,8 +525,6 @@ else > > > KBUILD_CFLAGS += -O2 > > > endif > > > > > > -include $(srctree)/arch/$(SRCARCH)/Makefile > > > - > > > ifneq (CONFIG_FRAME_WARN,0) > > > KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN}) > > > endif > > > @@ -555,6 +553,8 @@ ifdef CONFIG_DEBUG_SECTION_MISMATCH > > > KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once) > > > endif > > > > > > +include $(srctree)/arch/$(SRCARCH)/Makefile > > > + > > > # arch Makefile may override CC so keep this after arch Makefile is included > > > NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include) > > > CHECKFLAGS += $(NOSTDINC_FLAGS) > > > -- > > > > this patch is really for Sam to judge - Cc:-ed him. > > If we move the include further down then the following: > > # Force gcc to behave correct even for buggy distributions > # Arch Makefiles may override this setting > KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector) > > will most likely fail. ah, ok. (I long ago made the mental note of "dont change the toplevel Makefile if you can avoid it" - this reinforces that.) > If popwerpc needs to get rid of "-fno-omit-frame-pointer" then > we need a way to express this at KConfig level and NOT by doing > some tricks with CFLAGS. Here is what we have in the toplevel Makefile at the moment: ifdef CONFIG_FRAME_POINTER KBUILD_CFLAGS += -fno-omit-frame-pointer -fno-optimize-sibling-calls else KBUILD_CFLAGS += -fomit-frame-pointer endif My original suggestion (more than a week ago) was to make PPC always select FRAME_POINTERS. It was pointed out that -fno-omit-frame-pointers (i.e.: generate frame pointers) not only makes the code less optimal on PPC, but it can also be miscompiled. But instrumentation really needs to know whether __builtin_return_address(1) [etc] is reliable, whether stack tracing is fast - and other details - and PPC is the odd one out. So the question is: even with FRAME_POINTERS disabled on PPC, is __builtin_return_address(1)/(2) reliable, and is save_stack_trace() fast? (i.e. can it walk down the stack frame efficiently, or does it have to scan the full kernel stack) I.e. does PPC have all the material advantages of frame pointers? Ingo