linux-kernel - Re: [PATCH] irq: sparse irqs, fix #2

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Message-Id: <20080814093326.1d8d0a88.akpm@linux-foundation.org>
Date:	Thu, 14 Aug 2008 09:33:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] irq: sparse irqs, fix #2

On Thu, 14 Aug 2008 15:36:52 +0200 Ingo Molnar <mingo@...e.hu> wrote:

> +static inline cpumask_t vector_allocation_domain(int cpu)
> +{
> +        /* Careful. Some cpus do not strictly honor the set of cpus
> +         * specified in the interrupt destination when using lowest
> +         * priority interrupt delivery mode.
> +         *
> +         * In particular there was a hyperthreading cpu observed to
> +         * deliver interrupts to the wrong hyperthread when only one
> +         * hyperthread was specified in the interrupt desitination.
> +         */
> +        cpumask_t domain = { { [0] = APIC_ALL_CPUS, } };
> +        return domain;
> +}

I haven't looked at callers of this, but...

Does it need to be allocated on the stack?  Local cpumask_t's are a
size problem.  Can we build this in .rodata at compile time instead?

Is this the caller?

+	for_each_cpu_mask(cpu, mask) {
+		cpumask_t domain, new_mask;
+		int new_cpu;
+		int vector;
+
+		domain = vector_allocation_domain(cpu);
+		cpus_and(new_mask, domain, cpu_online_map);

If so we could perhaps do


static noinline const cpumask_t *vector_allocation_domain(int cpu)
{
        /* Careful. Some cpus do not strictly honor the set of cpus
         * specified in the interrupt destination when using lowest
         * priority interrupt delivery mode.
         *
         * In particular there was a hyperthreading cpu observed to
         * deliver interrupts to the wrong hyperthread when only one
         * hyperthread was specified in the interrupt desitination.
         */
        static const cpumask_t domain = { { [0] = APIC_ALL_CPUS, } };
        return &domain;
}


...

+	for_each_cpu_mask(cpu, mask) {
+		cpumask_t domain, new_mask;
+		int new_cpu;
+		int vector;
+
+		__cpus_and(new_mask, vector_allocation_domain(cpu),
+				&cpu_online_map);

otoh, perhaps this new function is one implementation of
genapic.vector_allocation_domain(), in which case the inlining was
unneeded and misleading.

I give up.  Have a little think about the stack bloat, please.

btw, whoever wrote that function is in need of a tab key.
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