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?
Date: Thu, 23 Aug 2018 14:42:45 -0400 (EDT) [thread overview]
Message-ID: <nycvar.YSQ.7.76.1808231414190.10215@knanqh.ubzr> (raw)
In-Reply-To: <20180823174359.GA13033@linux.vnet.ibm.com>
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
next prev parent reply other threads:[~2018-08-23 18:42 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-23 17:43 Kernel-only deployments? Paul E. McKenney
2018-08-23 18:16 ` Geert Uytterhoeven
2018-08-23 18:43 ` Paul E. McKenney
2018-08-23 18:42 ` Nicolas Pitre [this message]
2018-08-23 20:37 ` Paul E. McKenney
2018-08-23 18:54 ` Adam Borowski
2018-08-23 19:06 ` Willy Tarreau
2023-02-15 2:35 ` Zhangjin Wu
2023-02-15 9:47 ` Willy Tarreau
2023-02-16 13:09 ` Zhangjin Wu
2018-08-23 19:16 ` Josh Triplett
2018-08-23 20:39 ` Paul E. McKenney
2018-08-23 20:39 ` Paul E. McKenney
2018-08-23 19:12 ` Josh Triplett
2018-08-23 20:45 ` Paul E. McKenney
2018-08-23 19:22 ` Ray Clinton
2018-08-23 20:49 ` Paul E. McKenney
2018-08-23 19:52 ` Bernd Petrovitsch
2018-08-23 20:54 ` Paul E. McKenney
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