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

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Date:	Tue, 22 Apr 2008 12:03:04 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
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

[ 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..

			Linus
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