Re: [GIT PULL rcu/next] RCU commits for 4.21/5.0

[Paul E. McKenney]

On Tue, Dec 04, 2018 at 02:38:17PM +0100, Willy Tarreau wrote:
> Hi Ingo,
> 
> On Tue, Dec 04, 2018 at 09:08:37AM +0100, Ingo Molnar wrote:
> > I noticed this bit from Willy:
> > 
> > >  tools/testing/selftests/rcutorture/bin/nolibc.h    | 2197 ++++++++++++++++++++
> > 
> > So <nolibc.h> is a rather large header and it comes with very little 
> > documentation - but once you read through the header it's obvious what it 
> > does, the code is clean and it's pretty cool all around, and in hindsight 
> > the name is a strong hint about what the header does as well. ;)
> 
> Thanks for the positive comment, as it was initially not designed to be
> merged into the kernel and was just a local home project. I figured it
> could be a perfect solution to Paul's executable size issues and offered
> some help to get it in relatively quickly, but surely we can do much better!
> 
> > Still it would be nice to at least add a top level description to the 
> > header to make people (like me) who are reading the code before the 
> > changelogs wonder less. For tooling headers we require a similar 
> > self-explanatory, feel-fuzzy structure as for kernel headers.
> 
> I'm fine with doing this. I even wrote the very small header at the last
> minute, without knowing if there was any chance it survives a review :-)
> 
> > Beyond adding a bit more documentation it would also be useful to factor 
> > nolibc.h out into tools/include/nolibc/ or so, no reason to hide it in 
> > rcutorture, I bet there's a number of other testcases and smaller 
> > utilities in tools/ that could make good use of it.
> 
> Fine as well. It's important however to keep in mind that I only covered
> the few architectures I could test (i386/x86_64/arm/arm64/mips), and even
> there the coverage is still limited. I don't want it to become too much of
> a pain to use for other utilities just by lack of initial coverage. However
> I agree that better exposure will help contributions come in.
> 
> > My long term hope would be that eventually we could even create a minimal 
> > klibc from it (a minimal libc provided by the kernel itself), giving 
> > minimalist binaries a mechanism to link against klibc.so:
> > 
> > - klibc would be an explicit opt-in mechanism, i.e. binaries that are 
> >   coupled with the kernel anyway (and initrd executables certainly are) 
> >   could use this.
> 
> In fact it's very similar to my goal. I'm using it in initramfs and initrds
> that do very little stuff and where it's acceptable to have a few #ifdef to
> adapt to this or that libc. However I found it extremely convenient *not* to
> require any external symbol, thus not to have to link against anything. But
> I'm well aware that this position cannot last forever and that at some
> point if we want to go further we'll possibly have a few layers (naked
> syscalls returning -errno, decorated syscalls making use of an explicit
> errno, libc-specific stuff like string functions). Possibly that in this
> case only the naked version would remain in the .h and that the rest will
> require linking with the .so/.a.
> 
> > - We could also add a way for the kernel to provide (non-swappable) 
> >   binaries via an automatic /klib/ mount point or so. This would allow 
> >   features like a minimal, console based rescue/debug shell that doesn't 
> >   rely on any filesystem state or external library dependencies, other 
> >   than the initial kernel+initrd image.
> 
> This could be convenient indeed, I never thought about this. I'm currently
> doing something comparable using initramfs, so maybe in the end we don't
> need the kernel to create anything beyond this, but instead just let the
> user choose in the configuration what utilities should be added to the
> initramfs sources.
> 
> (...)
> > - klibc would also eventually allow deeper integration with the vDSO 
> >   proper: for example on klibc based embedded systems we could link klibc
> >   and the vDSO into a single vDSO library, further simplifying and 
> >   optimizing it.
> 
> I already looked how to implement vDSO. I figured it was not very difficult
> but will require that I maintain variables with the AUXV, then I thought
> that it went beyond the scope of this minimalist implementaiton and
> postponed this.
> 
> > - klibc would also allow faster feature propagation from kernel to libc 
> >   as well, as we could prototype, test and benchmark new system calls and 
> >   new features on klibc - i.e. klibc integration and testcases could be a
> >   requirement for new system calls.
> 
> This actually is a good idea. There was already a discussion in another
> thread about exposing syscalls better in the kernel for better interactions
> with the libc, but it could start this way with test cases. It also increases
> the likeliness that an awkward API is detected early when the person starts
> to write his/her own part of the libc side.
> 
> > - There's no upper limit to how such a minimal kernel-shell (root only) 
> >   environment would look like, beyond a minimal shell in principle it 
> >   could include bits like:
> (...)
> 
> That's more or less what the preinit present in my initramfs provides. I
> just need a kernel and nothing else to start to manually find and mount
> my rootfs while debugging, it's quite convenient. It's very limited
> though. But I 100% agree with the benefits of such basic recovery
> utilities shipped with the kernel images!
> 
> (...)
> >     - a minimal version of 3D Tetris (just kidding)
> 
> I thought you were already kidding when talking about 3D in fact but
> apparently not! I think you really mean GPU-based acceleration rather
> than 3D since I hardly see how 3D stuff may improve my debugging
> abilities :-)
> 
> > - ... all of which would allow a fully integrated, self-contained (!) 
> >   solution with no external dependencies other than the build environment 
> >   to build the binaries, that allows the debugging of a system and the 
> >   eventual extraction of debug information, without having to interact 
> >   with the local filesystem or even the local X-window state for these 
> >   apps to run.
> 
> In my case I also ship the modules within the kernel image. It's extremely
> convenient to have the choice of a number of kernels for a given rootfs.
> You never have to wonder if the modules are present on the roofs or not,
> you never have to prepare any initrd, you just select the kernel you want
> and you're done. For development, when combined with the preinit I'm
> talking about, it's awesome, because you just want something which barely
> boots to the preinit prompt, then you can start to investigate.
> 
> > - I.e. a minimal Linux distribution done right, optimized for kernel 
> >   development, system bootstrap, rescue enviroment, etc. - far more 
> >   usable than a kdb session.
> 
> The distro I'm using was initially not made for this but to design
> reliable minimalist systems, and it turned out extremely effective for
> kernel development for these reasons. The whole rootfs is an initrd
> which contains all the tools I need, so I can only confirm the comfort
> it brings!
> 
> > Anyway, back to <nolibc.h>: wanted to ask Linus's and Arnaldo's opinion 
> > about the merge of it, we can still re-base and re-try if there's any 
> > objections.
> 
> I'm fine with this as well. I just want to be sure we don't postpone the
> coverage of Paul's rcutorture needs because it started for this initially.
> If we see the discussion going too far, we could also at least cover just
> rcutorture first which would buy us time to decide how it should be done
> better, then remove it once we can port rcutorture to the newly designed
> solution.

I of course prefer starting with what I have, but it would not be at
all difficult for me to rebase if that is what needs to happen.

							Thanx, Paul