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. > 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. 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. > 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".
