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 7CD9DC43603 for ; Thu, 19 Dec 2019 14:25:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E30E24672 for ; Thu, 19 Dec 2019 14:25:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726920AbfLSOZu (ORCPT ); Thu, 19 Dec 2019 09:25:50 -0500 Received: from relay.sw.ru ([185.231.240.75]:54166 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726695AbfLSOZt (ORCPT ); Thu, 19 Dec 2019 09:25:49 -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 1ihwkH-0006cZ-4g; Thu, 19 Dec 2019 17:25:41 +0300 Subject: Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr() To: Peter Zijlstra 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> <20191219140252.GS2871@hirez.programming.kicks-ass.net> From: Kirill Tkhai Message-ID: Date: Thu, 19 Dec 2019 17:25:40 +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: <20191219140252.GS2871@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 On 19.12.2019 17:02, Peter Zijlstra wrote: > On Thu, Dec 19, 2019 at 02:12:42PM +0100, 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. > > Also, ld.lld seems a popular option. I asked on their IRC. Oh, it looks like no way is for this. About the link: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/sections.html (17:19:25) nbjoerg: but it is not guarenteed behavior (17:19:50) nbjoerg: if for some strange reason you really need to enforce relative orders of global objects, put them in consecutively named sections