From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 349D3C43603 for ; Thu, 19 Dec 2019 14:05:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 089A324685 for ; Thu, 19 Dec 2019 14:05:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726818AbfLSOFA (ORCPT ); Thu, 19 Dec 2019 09:05:00 -0500 Received: from relay.sw.ru ([185.231.240.75]:53412 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726695AbfLSOFA (ORCPT ); Thu, 19 Dec 2019 09:05:00 -0500 Received: from dhcp-172-16-24-104.sw.ru ([172.16.24.104]) by relay.sw.ru with esmtp (Exim 4.92.3) (envelope-from ) id 1ihwQ6-0006RQ-DH; Thu, 19 Dec 2019 17:04:50 +0300 Subject: [Q] ld: Does LTO reorder ro variables in two files? To: Peter Zijlstra , gcc-help@gcc.gnu.org Cc: mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, linux-kernel@vger.kernel.org References: <157675913272.349305.8936736338884044103.stgit@localhost.localdomain> <20191219131242.GK2827@hirez.programming.kicks-ass.net> From: Kirill Tkhai Message-ID: <3db1b1c8-0228-56e4-a04f-e8d24cd1dd51@virtuozzo.com> Date: Thu, 19 Dec 2019 17:04:49 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20191219131242.GK2827@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org CC: gcc-help@gcc.gnu.org Hi, gcc guys, this thread starts here: https://lkml.org/lkml/2019/12/19/403 There are two const variables: struct sched_class idle_sched_class and struct sched_class fair_sched_class, which are declared in two files idle.c and fair.c. 1)In Makefile the order is: idle.o fair.o 2)the variables go to the same ro section 3)there is no SORT(.*) keyword in linker script. Is it always true, that after linkage &idle_sched_class < &fair_sched_class? Thanks! Kirill On 19.12.2019 16:12, Peter Zijlstra wrote: > On Thu, Dec 19, 2019 at 03:39:14PM +0300, Kirill Tkhai wrote: >> In kernel/sched/Makefile files, describing different sched classes, already >> go in the order from the lowest priority class to the highest priority class: >> >> idle.o fair.o rt.o deadline.o stop_task.o >> >> The documentation of GNU linker says, that section appears in the order >> they are seen during link time (see [1]): >> >>> Normally, the linker will place files and sections matched by wildcards >>> in the order in which they are seen during the link. You can change this >>> by using the SORT keyword, which appears before a wildcard pattern >>> in parentheses (e.g., SORT(.text*)). >> >> So, we may expect const variables from idle.o will go before ro variables >> from fair.o in RO_DATA section, while ro variables from fair.o will go >> before ro variables from rt.o, etc. >> >> (Also, it looks like the linking order is already used in kernel, e.g. >> in drivers/md/Makefile) >> >> Thus, we may introduce an optimization based on xxx_sched_class addresses >> in these two hot scheduler functions: pick_next_task() and check_preempt_curr(). >> >> One more result of the patch is that size of object file becomes a little >> less (excluding added BUG_ON(), which goes in __init section): >> >> $size kernel/sched/core.o >> text data bss dec hex filename >> before: 66446 18957 676 86079 1503f kernel/sched/core.o >> after: 66398 18957 676 86031 1500f kernel/sched/core.o > > Does LTO preserve this behaviour? I've never quite dared do this exact > optimization.