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McKenney" To: nicolas.pitre@linaro.org, josh@joshtriplett.org Cc: linux-kernel@vger.kernel.org Subject: Kernel-only deployments? Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18082317-0060-0000-0000-000002A33A9F X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009599; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01077780; UDB=6.00555712; IPR=6.00857753; MB=3.00022890; MTD=3.00000008; XFM=3.00000015; UTC=2018-08-23 17:44:01 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18082317-0061-0000-0000-00004644C39E Message-Id: <20180823174359.GA13033@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-23_07:,, 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=846 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1808230183 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello! Does anyone do kernel-only deployments, for example, setting up an embedded device having a Linux kernel and absolutely no userspace whatsoever? The reason I as is that such a mode would be mildly useful for rcutorture. You see, rcutorture runs entirely out of initrd, never mounting a real root partition. The user has been required to supply the initrd, but more people are starting to use rcutorture. This has led to confusion and complaints about the need to supply the initrd. So I am finally getting my rcutorture initrd act together, with significant dracut help from Connor Shu. I added mkinitramfs support for environments such as mine that don't support dracut, at least not without significant slashing and burning. The mkinitramfs approach results in about 40MB of initrd, and dracut about 10MB. Most of this is completely useless for rcutorture, which isn't interested in mounting filesystems, opening devices, and almost all of the other interesting things that mkinitramfs and dracut enable. 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.) 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. So, does anyone in the deep embedded space already do this? Thanx, Paul [*] What zombies??? There is no userspace!!!