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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 E10DAC4321D for ; Thu, 23 Aug 2018 20:45:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 903AB21557 for ; Thu, 23 Aug 2018 20:45:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 903AB21557 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.vnet.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727265AbeHXAQ3 (ORCPT ); Thu, 23 Aug 2018 20:16:29 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:34670 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726740AbeHXAQ3 (ORCPT ); Thu, 23 Aug 2018 20:16:29 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w7NKdNC0042727 for ; Thu, 23 Aug 2018 16:45:05 -0400 Received: from e11.ny.us.ibm.com (e11.ny.us.ibm.com [129.33.205.201]) by mx0b-001b2d01.pphosted.com with ESMTP id 2m23q01bh9-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 23 Aug 2018 16:45:05 -0400 Received: from localhost by e11.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; Thu, 23 Aug 2018 16:45:02 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w7NKj11g26083472; Thu, 23 Aug 2018 20:45:01 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B5A97B2066; Thu, 23 Aug 2018 16:44:05 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 98507B2065; Thu, 23 Aug 2018 16:44:05 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 23 Aug 2018 16:44:05 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id E182116C6B17; Thu, 23 Aug 2018 13:45:01 -0700 (PDT) Date: Thu, 23 Aug 2018 13:45:01 -0700 From: "Paul E. McKenney" To: Josh Triplett Cc: nicolas.pitre@linaro.org, linux-kernel@vger.kernel.org Subject: Re: Kernel-only deployments? Reply-To: paulmck@linux.vnet.ibm.com References: <20180823174359.GA13033@linux.vnet.ibm.com> <20180823191235.GA3243@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180823191235.GA3243@localhost> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18082320-2213-0000-0000-000002E0FE21 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009599; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01077840; UDB=6.00555748; IPR=6.00857814; MB=3.00022893; MTD=3.00000008; XFM=3.00000015; UTC=2018-08-23 20:45:03 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18082320-2214-0000-0000-00005B4E9A8C Message-Id: <20180823204501.GX4225@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-23_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1808230213 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 23, 2018 at 12:12:35PM -0700, Josh Triplett wrote: > On Thu, Aug 23, 2018 at 10:43:59AM -0700, Paul E. McKenney wrote: > > Hello! > > > > Does anyone do kernel-only deployments, for example, setting up an > > embedded device having a Linux kernel and absolutely no userspace > > whatsoever? > > I would very much *like* to do this. One day I'd like to have a > CONFIG_USERSPACE that I can disable, and then just have the kernel call > an in-kernel main() where it would normally start init. This looks to be an easy change, though it might not seem so easy after starting to try it out. ;-) > > Those who know me will not be at all surprised to learn that I went > > overboard making the resulting initrd as small as possible. I started > > by throwing out everything not absolutely needed by the dash and sleep > > binaries, which got me down to about 2.5MB, 1.8MB of which was libc. > > This situation of course prompted me to create an initrd containing > > a statically linked binary named "init" and absolutely nothing else > > (not even /dev or /tmp directories), which weighs in at not quite 800KB. > > This is a great improvement over 10MB, to say nothing of 40MB, but 800KB > > for a C-language "for" loop containing nothing more than a single call to > > sleep()? Much of the code is there for things that I might do (dl_open(), > > for example), but don't. All I can say is that there clearly aren't many > > of us left who made heavy use of systems with naked-eye-visible bits! > > (Or naked-finger-feelable, for that matter.) > > I have definitely built initramfs images containing nothing but a single > statically linked /init before. Cool! > If you want to make it even smaller, you could avoid linking in libc at > all, and just write a short assembly stub, but I don't know any way to > do that *portably* without writing raw assembly for each target > platform. That would get you down to a few kB though. I do need portability. And even 800K isn't -that- big a deal, much though my earlier self would disbelieve this. > > This further prompted the idea of modifying kernel_init() to just loop > > forever, perhaps not even reaping orphaned zombies [*], given an appropriate > > Kconfig option and/or kernel boot parameter. I obviously cannot justify > > this to save a sub-one-megabyte initrd for rcutorture, no matter how much > > a wasted 800K might have offended my 30-years-ago self. If I take this > > next step, there have to be quite a few others benefiting significantly > > from it. > > I would *love* to have support for omitting userspace entirely. And once > we have that, we can start ripping out so many other things... ;-) > One thought, though: that won't necessarily give you a representative > rcutorture experience, given that you need to test things like the > nohz-on-non-idle support, which interacts with "am I in userspace". That is an excellent point. I should keep the initrd specifically to retain userspace execution, and should also occasionally run CPU-bound in userspace. Easy enough! And thank you! Thanx, Paul