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Date:   Thu, 23 Aug 2018 13:45:01 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Josh Triplett <josh@joshtriplett.org>
Cc:     nicolas.pitre@linaro.org, linux-kernel@vger.kernel.org
Subject: Re: Kernel-only deployments?
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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