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From:   Ray Clinton <mr.ray.clinton@gmail.com>
Date:   Thu, 23 Aug 2018 15:22:48 -0400
Message-ID: <CAPMnawvzuAaXmuMg_Z4_Av59zHmxeWzdbfZv3GaX6Vmw48t+Aw@mail.gmail.com>
Subject: Re: Kernel-only deployments?
To:     paulmck@linux.vnet.ibm.com
Cc:     nicolas.pitre@linaro.org, josh@joshtriplett.org,
        linux-kernel@vger.kernel.org
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On Thu, Aug 23, 2018 at 7:44 PM Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> Does anyone do kernel-only deployments, for example, setting up an
> embedded device having a Linux kernel and absolutely no userspace
> whatsoever?

To be honest I'm a total newb to kernel dev, so much so that I copied and
pasted the above quote in the hopes that I did the formatting right. I'm such
 a newb that I realize I might not even understand your question.

That beingsaid, wouldn't building a uImage of the kernel and loading it onto
your device using tftpboot accomplish this?

Ray On Thu, Aug 23, 2018 at 1:46 PM Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> 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!!!
>