On Fri, 3 Mar 2000 yodaiken@chelm.cs.nmt.edu wrote:
> On Sun, Feb 27, 2000 at 04:04:13PM +0100, Ingo Molnar wrote:
> > - per-IRQ-source spinlocks and per-IRQ-controller spinlocks
> > increasing scalability: now two IRQ handlers on two CPUs
> > can run do_IRQ in parallel. Note that level-triggered PCI IRQ
> > handlers never actually take the IRQ-controller spinlock in the
> > 'IRQ handling fast path'.
>
> This change puts spinlock back into the low level irq code -- and I took
> them out specifically to make RTLinux work and to make Linus happy since
> he was concerned about minimizing the number of spinlocks. The
> "optimization" means that you can, in parallel run a tiny section of code
> that is called infrequently -- [...]
it actually makes a huge difference as the _real_ optimization you missed
is the lack of cacheline ping-pongs, which pingpong inevitably happens if
globally shared spinlocks are used.
> [...] if there is any performance gain at all, it
> comes at the expense of introducing extra spin locks, making the low
> level code bigger and therefore less cache friendly, and causing an
> additional check for irq status which is quite expensive on some hardware
> (including x86).
you have not actually checked the fast IRQ handling path, have you? The
IO-APIC level-triggered IRQ code does not use the lowlevel spinlock, at
all. This was one of the goals of the optimization.
> I don't see the "fast path" in a first look at the code. Are you now
> skiping "do_irq" for some interrupt handlers?
check out the fast path of level-triggered PCI IRQs:
do_IRQ()
mask_and_ack_level_ioapic_irq: empty!
->handler()
end_level_ioapic_irq: a fast local-APIC write
no lowlevel spinlock taken. This is actually the case where the IOAPIC IRQ
hardware turned out to be very sane. The borken edge-triggered case is
nicely isolated.
-- mingo
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