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SubjectRe: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell"

* Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> > The overall policy to extend task_struct, going forward, would be to:
> >
> > - Either make simple-type or struct-pointer additions to task_struct, that
> > don't couple <linux/sched.h> to other subsystems.
> >
> > - Or, if you absolutely must - and we don't want to forbid this - use the
> > per_task() machinery to create a simple accessor to a complex embedded
> > type.
>
> I'll leave all of this up to the scheduler developers, but it still looks
> odd to me. The mess we create trying to work around issues in C :)

Yeah, so I *did* find this somewhat suboptimal too, and developed an
earlier version that used linker section tricks to gain the field offsets
more automatically.

It was an unmitigated disaster: was fragile on x86 already (which has a zoo
of linking quirks with no precedent of doing this before bounds.c
processing), but on ARM64 and probably on most of the other RISC-ish
architectures there was also a real runtime code generation cost of using
linker tricks: 2-3 extra instructions per per_task() use - clearly
unacceptable.

Found this out the hard way after making it boot & work on ARM64 and
looking at the assembly output, trying to figure out why the generated code
size increased. :-/

Anyway, the current method has the big advantage of being obviously
invariant wrt. code generation compared to the previous code, on every
architecture.

> > Do these plans sound good to you?
>
> Yes, taking the majority through the maintainer trees and then doing the
> remaining bits in a single tree seems sane, that one tree will be easier
> to review as well.

Ok. Will definitely offer it up piecemail-wise, in reviewable chunks, via
existing processes & flows.

Thanks,

Ingo

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