Messages in this thread | | | Date | Mon, 9 Nov 2020 16:45:49 +0200 | From | Tony Lindgren <> | Subject | Re: [PATCH 2/3] arm: introduce IRQ stacks |
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* 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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