From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751186AbdFANuN (ORCPT ); Thu, 1 Jun 2017 09:50:13 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:34208 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751096AbdFANuK (ORCPT ); Thu, 1 Jun 2017 09:50:10 -0400 Date: Thu, 1 Jun 2017 15:50:05 +0200 From: Ingo Molnar To: Josh Poimboeuf Cc: x86@kernel.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, Linus Torvalds , Andy Lutomirski , Jiri Slaby , "H. Peter Anvin" , Peter Zijlstra Subject: Re: [RFC PATCH 00/10] x86: undwarf unwinder Message-ID: <20170601135005.zf2lidtgslfvyihs@gmail.com> References: <20170601060824.wv2go3adbvx5ptmt@gmail.com> <20170601115819.3twoowcnvtrfzjzr@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170601115819.3twoowcnvtrfzjzr@treble> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Josh Poimboeuf wrote: > On Thu, Jun 01, 2017 at 08:08:24AM +0200, Ingo Molnar wrote: > > > > * Josh Poimboeuf wrote: > > > > > Here's the contents of the undwarf.txt file which explains the 'why' in > > > more detail: > > > > Ok, so the code quality looks pretty convincing to me - the new core 'undwarf' > > unwinder code is a _lot_ more readable than any of the Dwarf based attempts > > before. > > > > That we control the debug info generation at build time is icing on the cake to > > me. > > > > One thing I'd like to see on the list of benefits side of the equation is a size > > comparison of kernel .text, with frame pointers vs. undwarf, on 64-bit kernels. > > Ok, will do a text size comparison. The only difficulty I encountered > there is that the 'size' tool considers the .undwarf section to be text > for some reason. So the "text" size grew considerably :-) One trick I sometimes use is to only size some of the key builtin.o files. > > Being able to generate more optimal code in the hottest code paths of the kernel > > is the _real_, primary upstream kernel benefit of a different debuginfo method - > > which has to be weighed against the pain of introducing a new unwinder. But this > > submission does not talk about that aspect at all, which should be fixed I think. > > Actually I devoted an entire one-sentence paragraph to performance in > the documentation: > > The simpler debuginfo format also enables the unwinder to be relatively > fast, which is important for perf and lockdep. > > But I'll try to highlight that a little more. That's not what I meant! The speedup comes from (hopefully) being able to disable CONFIG_FRAME_POINTER, which: - creates simpler/faster function prologues and epilogues - no managing of RBP needed - gives one more generic purpose register to work from. This matters less on 64-bit kernels but it's a small effect. I've seen numbers of 1-2% of instruction count reduction in common kernel workloads, which would be pretty significant on well cached workloads. Thanks, Ingo