Re: [RFT/PATCH v2 6/6] x86-64: Turn off -pg and turn on -foptimize-sibling-calls for vDSO

Ingo Molnar
2011-04-07T08:03:23Z

* Andy Lutomirski <luto@mit.edu> wrote:

> The vDSO isn't part of the kernel, so profiling and kernel
> backtraces don't really matter.
>
> Signed-off-by: Andy Lutomirski <luto@mit.edu>
> ---
> arch/x86/vdso/Makefile | 15 ++++++++++++++-
> 1 files changed, 14 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile
> index a651861..bef0bc9 100644
> --- a/arch/x86/vdso/Makefile
> +++ b/arch/x86/vdso/Makefile
> @@ -37,11 +37,24 @@ $(obj)/%.so: OBJCOPYFLAGS := -S
> $(obj)/%.so: $(obj)/%.so.dbg FORCE
> $(call if_changed,objcopy)
>
> +#
> +# Don't omit frame pointers for ease of userspace debugging, but do
> +# optimize sibling calls.
> +#
> CFL := $(PROFILING) -mcmodel=small -fPIC -O2 -fasynchronous-unwind-tables -m64 \
> - $(filter -g%,$(KBUILD_CFLAGS)) $(call cc-option, -fno-stack-protector)
> + $(filter -g%,$(KBUILD_CFLAGS)) $(call cc-option, -fno-stack-protector) \
> + -fno-omit-frame-pointer -foptimize-sibling-calls
>
> $(vobjs): KBUILD_CFLAGS += $(CFL)
>
> +#
> +# vDSO code runs in userspace and -pg doesn't help with profiling anyway.
> +#
> +CFLAGS_REMOVE_vdso-note.o = -pg
> +CFLAGS_REMOVE_vclock_gettime.o = -pg
> +CFLAGS_REMOVE_vgetcpu.o = -pg
> +CFLAGS_REMOVE_vvar.o = -pg

The comment makes no sense in this form, -pg is not used for profiling but
in-kernel tracing.

Also, please do not do multiple changes in a single patch, split out the -pg,
the -fomit-frame-pointers and the -foptimize-sibling-calls change into two
patches. This will also help bisection, should some GCC version do something
silly with -foptimize-sibling-calls ...

Thanks,

Ingo