Messages in this thread | | | Date | Wed, 23 Apr 2008 14:54:46 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH 2/11] x86: convert to generic helpers for IPI function calls |
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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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