On Tue, Apr 22 2008, Linus Torvalds wrote:
>
> [ Ingo added to cc, since this is x86-specific ]
>
> On Tue, 22 Apr 2008, Jens Axboe wrote:
> > +++ b/arch/x86/kernel/apic_32.c
> > @@ -1357,6 +1357,10 @@ void __init smp_intr_init(void)
> >
> > /* IPI for generic function call */
> > set_intr_gate(CALL_FUNCTION_VECTOR, call_function_interrupt);
> > +
> > + /* IPI for single call function */
> > + set_intr_gate(CALL_FUNCTION_SINGLE_VECTOR,
> > + call_function_single_interrupt);
>
> Ok, one more comment..
>
> Why bother with separate vectors for this?
>
> Why not just make the single vector do
>
> void smp_call_function_interrupt(void)
> {
> ack_APIC_irq();
> irq_enter();
> generic_smp_call_function_single_interrupt();
> generic_smp_call_function_interrupt();
> #ifdef CONFIG_X86_32
> __get_cpu_var(irq_stat).irq_call_count++;
> #else
> add_pda(irq_call_count, 1);
> #endif
> irq_exit();
> }
>
> since they are both doing the exact same thing anyway?
>
> Do we really require us to be able to handle the "single" case _while_ a
> "multiple" case is busy? Aren't we running all of these things with
> interrupts disabled anyway, so that it cannot happen?
>
> Or is it just a performance optimization? Do we expect to really have so
> many of the multiple interrupts that it's expensive to walk the list just
> because we also had a single interrupt to another CPU? That sounds a bit
> unlikely, but if true, very interesting..
>
> Inquiring minds want to know..
Regarding that last comment... The reason why I'm doing this work is
because I want to use smp_call_function_single() to redirect IO
completions. So there WILL be lots of
smp_call_function_single_interrupt() interrupts, they will be a lot more
prevalent than smp_call_function() interrupts. I don't have any numbers
on this since I haven't tried collapsing them all, but I'd be surprised
if it wasn't noticable.
That said, some archs do use a single IPI for multiple actions and just
keep a bitmask of what to do in that IPI. So it would still be possible
to use a single hardware IPI to do various things, without resorting to
calling into the interrupt handler for each of them. The _single()
interrupt handler is a cheap check though, an smp memory barrier and a
list_empty() check is enough (like it currently does).
--
Jens Axboe
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