Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758361AbbDWV6D (ORCPT ); Thu, 23 Apr 2015 17:58:03 -0400 Received: from elastic.org ([69.20.226.105]:53908 "EHLO elastic.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758272AbbDWV57 (ORCPT ); Thu, 23 Apr 2015 17:57:59 -0400 X-Greylist: delayed 1900 seconds by postgrey-1.27 at vger.kernel.org; Thu, 23 Apr 2015 17:57:58 EDT From: "Frank Ch. Eigler" To: linux-kernel@vger.kernel.org Cc: Josh Stone , Jakub Jelinek , Josh Boyer , Greg Kroah-Hartman , Linus Torvalds , Andrew Morton , Markus Trippelsdorf , =?UTF-8?q?Michel=20D=C3=A4nzer?= , "Frank Ch. Eigler" Subject: [PATCH] Kbuild: Add an option to enable GCC VTA Date: Thu, 23 Apr 2015 17:25:29 -0400 Message-Id: <1429824329-40690-1-git-send-email-fche@redhat.com> X-Mailer: git-send-email 2.1.0 In-Reply-To: <1416595200-22359-1-git-send-email-jistone@redhat.com> References: <1416595200-22359-1-git-send-email-jistone@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4328 Lines: 107 From: Josh Stone Due to isolated gcc codegen issues, gcc -fvar-tracking-assignments was unconditionally disabled in commit 2062afb4f804 ("Fix gcc-4.9.0 miscompilation of load_balance() in scheduler"). However, this reduces the debuginfo coverage for variable locations, especially in inline functions. VTA is certainly not perfect either in those cases, but it is much better than without. With compiler versions that have fixed the codegen bugs, we would prefer to have the better details for SystemTap, and surely other debuginfo consumers like perf will benefit as well. This patch simply makes CONFIG_DEBUG_INFO_VTA an option. I considered Frank and Linus's discussion of a cc-option-like -fcompare-debug test, but I'm convinced that a narrow test of an arch-specific codegen issue is not really useful. GCC has their own regression tests for this, so I'd suggest GCC_COMPARE_DEBUG=-fvar-tracking-assignments-toggle is more useful for kernel developers to test confidence. In fact, I ran into a couple more issues when testing for this patch[1], although neither of those had any codegen impact. [1] https://bugzilla.redhat.com/show_bug.cgi?id=1140872 With gcc-4.9.2-1.fc22, I can now build v3.18-rc5 with Fedora's i686 and x86_64 configs, and this is completely clean with GCC_COMPARE_DEBUG. Cc: Jakub Jelinek Cc: Josh Boyer Cc: Greg Kroah-Hartman Cc: Linus Torvalds Cc: Andrew Morton Cc: Markus Trippelsdorf Cc: Michel Dänzer Signed-off-by: Josh Stone Signed-off-by: Frank Ch. Eigler --- Makefile | 8 ++++++-- lib/Kconfig.debug | 21 ++++++++++++++++++++- 2 files changed, 26 insertions(+), 3 deletions(-) diff --git a/Makefile b/Makefile index 6cc5b2434224..c8e1fcfdb41a 100644 --- a/Makefile +++ b/Makefile @@ -704,8 +704,6 @@ KBUILD_CFLAGS += -fomit-frame-pointer endif endif -KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments) - ifdef CONFIG_DEBUG_INFO ifdef CONFIG_DEBUG_INFO_SPLIT KBUILD_CFLAGS += $(call cc-option, -gsplit-dwarf, -g) @@ -718,6 +716,12 @@ ifdef CONFIG_DEBUG_INFO_DWARF4 KBUILD_CFLAGS += $(call cc-option, -gdwarf-4,) endif +ifdef CONFIG_DEBUG_INFO_VTA +KBUILD_CFLAGS += $(call cc-option, -fvar-tracking-assignments) +else +KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments) +endif + ifdef CONFIG_DEBUG_INFO_REDUCED KBUILD_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \ $(call cc-option,-fno-var-tracking) diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 17670573dda8..e8d072d2b402 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -165,7 +165,26 @@ config DEBUG_INFO_DWARF4 Generate dwarf4 debug info. This requires recent versions of gcc and gdb. It makes the debug information larger. But it significantly improves the success of resolving - variables in gdb on optimized code. + variables in gdb on optimized code. The gcc docs also + recommend enabling -fvar-tracking-assignments for maximum + benefit. (see DEBUG_INFO_VTA) + +config DEBUG_INFO_VTA + bool "Enable var-tracking-assignments for debuginfo" + depends on DEBUG_INFO + help + Enable gcc -fvar-tracking-assignments for improved debug + information on variable locations in optimized code. Per + gcc, DEBUG_INFO_DWARF4 is recommended for best use of VTA, + and allows maximal access to local variables in tracers + and debuggers like perf, systemtap, kgdb, and crash. + + VTA has been implicated in codegen bugs (gcc PR61801, + PR61904, both fixed in 2014-08), so this flag may be used + to exclude this rare class of problem. One can also set + GCC_COMPARE_DEBUG=-fvar-tracking-assignments-toggle in the + environment to automatically compile everything both ways, + generating an error if anything differs. config GDB_SCRIPTS bool "Provide GDB scripts for kernel debugging" -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/