Disable sibling calls with DEBUG_KERNEL to get better backtraces

Debug patch. Probably not for mainline.

Signed-off-by: Andi Kleen <ak@suse.de>

---
 arch/x86_64/Makefile |    4 ++++
 1 files changed, 4 insertions(+)

Index: linux/arch/x86_64/Makefile
===================================================================
--- linux.orig/arch/x86_64/Makefile
+++ linux/arch/x86_64/Makefile
@@ -30,6 +30,10 @@ CHECKFLAGS      += -D__x86_64__ -m64
 cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
 cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)
 cflags-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=generic)
+
+# not optimizing sibling calls gives better backtraces
+cflags-$(CONFIG_DEBUG_KERNEL) += -fno-optimize-sibling-calls
+
 CFLAGS += $(cflags-y)
 
 CFLAGS += -m64