From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759526Ab1LPOGu (ORCPT ); Fri, 16 Dec 2011 09:06:50 -0500 Received: from mail-qy0-f174.google.com ([209.85.216.174]:45546 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755762Ab1LPOGn (ORCPT ); Fri, 16 Dec 2011 09:06:43 -0500 Date: Fri, 16 Dec 2011 15:06:38 +0100 From: Frederic Weisbecker To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, "H. Peter Anvin" , Thomas Gleixner , Peter Zijlstra , Linus Torvalds , Andrew Morton , Jan Beulich , Arjan van de Ven , Alexander van Heukelum Subject: Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size Message-ID: <20111216140635.GB22873@somewhere.redhat.com> References: <20111216081915.GA28288@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111216081915.GA28288@elte.hu> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 16, 2011 at 09:19:16AM +0100, Ingo Molnar wrote: > > This patch turns on -momit-leaf-frame-pointer on x86 builds and > thus shrinks .text noticeably. On a defconfig-ish kernel: > > text data bss dec hex filename > 9843902 1935808 3649536 15429246 eb6e7e vmlinux.before > 9813764 1935792 3649536 15399092 eaf8b4 vmlinux.after > > That's 0.3% off text size. > > The actual win is larger than this percentage suggests: many > small, hot helper functions such as find_next_bit(), > do_raw_spin_lock() or most of the list_*() functions are leaf > functions and are now shorter by 2 instructions. > > Probably a good chunk of the framepointers related runtime > overhead on common workloads is eliminated via this patch, as > small leaf functions execute more often than larger parent > functions. > > The call-chains are still intact for quality backtraces and for > call-chain profiling (perf record -g), as the backtrace walker > can deduct the full backtrace from the RIP of a leaf function > and the parent chain. In the case of linked stacks (like frame pointer linking interrupt to exception stack) we may also miss a leaf function (and not its parent). Now perhaps we can live with all that, I don't know. > > Signed-off-by: Ingo Molnar > --- > arch/x86/Makefile | 8 ++++++++ > 1 file changed, 8 insertions(+) > > Index: linux/arch/x86/Makefile > =================================================================== > --- linux.orig/arch/x86/Makefile > +++ linux/arch/x86/Makefile > @@ -72,6 +72,14 @@ else > KBUILD_CFLAGS += -maccumulate-outgoing-args > endif > > +# > +# This shrinks many small functions, we don't actually > +# need their frame pointer, in backtraces the RIP will > +# identify the function and the stack frame walker will > +# find the parent function: > +# > +KBUILD_CFLAGS += $(call cc-option,-momit-leaf-frame-pointer) > + > ifdef CONFIG_CC_STACKPROTECTOR > cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh > ifeq ($(shell $(CONFIG_SHELL) $(cc_has_sp) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)