[PATCH 2/3] arm: introduce IRQ stacks

[Arnd Bergmann]

On Wed, Oct 21, 2020 at 2:57 PM Russell King - ARM Linux admin
<linux at armlinux.org.uk> wrote:
> On Wed, Oct 21, 2020 at 01:45:42PM +0100, Russell King - ARM Linux admin wrote:
> > > > - define 'current' as 'this_cpu_read_stable(current_task);'
> > > > - convert to CONFIG_THREAD_INFO_IN_TASK
> >
> > That means we need to also code that up in assembly - remember, we
> > need to access thread_info from assembly code.
>
> Note also that there is a circular dependency involved. If you make
> thread_info accessible via per-cpu, then:
>
> #ifndef __my_cpu_offset
> #define __my_cpu_offset per_cpu_offset(raw_smp_processor_id())
> #endif
> #ifdef CONFIG_DEBUG_PREEMPT
> #define my_cpu_offset per_cpu_offset(smp_processor_id())
> #else
> #define my_cpu_offset __my_cpu_offset
> #endif

Right, I had missed the fallback path using asm-generic/percpu.h
that is used with CONFIG_SMP && CONFIG_CPU_V6
Almost everything either uses fixed percpu data (on UP builds)
or TPIDRPRW when building a v7-only or v6k/v7 kernel without
v6 support.

> smp_processor_id() ultimately ends up as raw_smp_processor_id() which
> is:
>
> #define raw_smp_processor_id() (current_thread_info()->cpu)
>
> and if current_thread_info() itself involves reading from per-cpu data,
> we end up recursing... infinitely.
>
> This is why I said in the other thread:
>
> "We don't do it because we don't have a separate register to be able
> to store the thread_info pointer, and copying that lump between the
> SVC and IRQ stack will add massively to IRQ latency, especially for
> older machines."

As discussed on IRC, I think it can still be done in one of these
ways, though admittedly none of them are perfect:

a) add runtime patching for __my_cpu_offset() when
  CONFIG_SMP_ON_UP is set. This adds complexity but avoids the
  fallback for for SMP&&CPU_V6. It possibly also speeds up
  running on single-cpu systems if the TPIDRPRW access adds
  any measurable runtime overhead compared to patching it out.

b) If irq stacks are left as a compile-time option, that could be
  made conditional on "!(SMP&&CPU_V6)". Presumably very
  few people still run kernels built that way any more. The only
  supported platforms are i.MX3, OMAP2 and Realview-eb, all of
  which are fairly uncommon these days and would usually
  run v6-only non-SMP kernels.

c) If we decide that we no longer care about that configuration
  at all, we could decide to just make SMP depend on !CPU_V6,
  and possibly kill off the entire SMP_ON_UP patching logic.
  I suspect we still want to keep SMP_ON_UP for performance
  reasons, but I don't know how significant they are to start with.

       Arnd