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 5C445C43603 for ; Thu, 19 Dec 2019 14:31:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 30A7A24672 for ; Thu, 19 Dec 2019 14:31:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726906AbfLSObZ (ORCPT ); Thu, 19 Dec 2019 09:31:25 -0500 Received: from relay.sw.ru ([185.231.240.75]:54454 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726757AbfLSObY (ORCPT ); Thu, 19 Dec 2019 09:31:24 -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 1ihwpc-0006gx-Ov; Thu, 19 Dec 2019 17:31:12 +0300 Subject: Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr() From: Kirill Tkhai 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> Message-ID: <423860b3-b35a-ff25-6abf-845ef4be6e8e@virtuozzo.com> Date: Thu, 19 Dec 2019 17:31:12 +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: 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 17:25, Kirill Tkhai wrote: > 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 Introduction of sched_class::id instead of this patch's approach does not have a big sense, since this will help in check_preempt_curr() only. And this requires too many new lines of code.