* Arnd Bergmann [off-list ref] [201021 16:07]:

On Wed, Oct 21, 2020 at 2:57 PM Russell King - ARM Linux admin
[off-list ref] wrote:

quoted

On Wed, Oct 21, 2020 at 01:45:42PM +0100, Russell King - ARM Linux admin wrote:

quoted

quoted

quoted

- 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.

quoted

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.

Out of these options a) sounds best to me.

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.

This has been working just fine for years though. In general,
removing the conditional compile ifdefferey has made things quite
a bit easier for us, so let's continue on that.

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.

And this too has been working just fine for years :)

Regards,

Tony

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