Re: [PATCH 2/3] arm: introduce IRQ stacks

[Tony Lindgren <tony@atomide.com>]

* Arnd Bergmann <arnd@kernel.org> [201021 16:07]:
> On Wed, Oct 21, 2020 at 2:57 PM Russell King - ARM Linux admin
> <linux@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.

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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