linux-kernel - Re: [PATCH 2/11] x86: convert to generic helpers for IPI function calls

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Date:	Wed, 23 Apr 2008 14:54:46 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-arch@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	npiggin@...e.de, peterz@...radead.org, sam@...nborg.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/11] x86: convert to generic helpers for IPI function  calls

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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