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Date:   Thu, 23 Aug 2018 14:42:45 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@linaro.org>
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
cc:     josh@joshtriplett.org, linux-kernel@vger.kernel.org
Subject: Re: Kernel-only deployments?
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On Thu, 23 Aug 2018, 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?

Not that I know of.  For one thing, you'd lose the ability to license 
your application code the way you want.

> 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.

No surprise there.

> 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.

That is possibly still very big. You could probably get away with a 
statically linked busybox containing only the shell facilities you 
require for 100K or so.

> 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 still looks big for a custom binary, unless you do have a lot of 
code in there. It is already possible to have a kernel binary about that 
size, and even if that's a configured down kernel, quite some complex 
code remains.

The bloat might come from the C library you use. It's been a while since 
glibc stopped caring about not pulling a lot of unneeded code when all 
you want to do is printf(). It carries all those locale dependencies, 
etc. You should look at alternative C libs to get things small.

> 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.

You could easily do it from your init binary with less trouble than 
having the kernel carry such an option.

> So, does anyone in the deep embedded space already do this?

Not that I know of. Normally, if the init process dies, you typically 
want the whole system to reboot (you may force a reboot upon any kernel 
panic for example).


Nicolas