linux-kernel - genirq regression - CONFIG_AUTO_IRQ

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Message-Id: <200812051640.mB5Ge2U1122172@fcbayern.americas.sgi.com>
Date:	Fri, 5 Dec 2008 10:40:01 -0600 (CST)
From:	John Keller <jpk@....com>
To:	linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org
Cc:	akpm@...ux-foundation.org, tglx@...utronix.de
Subject: genirq regression - CONFIG_AUTO_IRQ_AFFINITY

The addition of the generic affinity autoselector has introduced
a regression in SN2 Altix.

In the past, request_irq()'s call to select_smp_affinity() was
essentially a noop. 

#ifdef CONFIG_AUTO_IRQ_AFFINITY
extern int select_smp_affinity(unsigned int irq);
#else
static inline int select_smp_affinity(unsigned int irq)
{
        return 1;
}

Now changes have created do_irq_select_affinity(), which is
executed by a call to request_irq().

  request_irq()
   __setup_irq()
    do_irq_select_affinity()

do_irq_select_affinity() will call the set_affinity() routine,
with a default mask. On SN2 (and possibly all ia64 platforms),
this will retarget all device interrupts to the same CPU, most
likely CPU0.

#ifndef CONFIG_AUTO_IRQ_AFFINITY
/*
 * Generic version of the affinity autoselector.
 */
int do_irq_select_affinity(unsigned int irq, struct irq_desc *desc)
{
        cpumask_t mask;

        if (!irq_can_set_affinity(irq))
                return 0;

        cpus_and(mask, cpu_online_map, irq_default_affinity);

        /*
         * Preserve an userspace affinity setup, but make sure that
         * one of the targets is online.
         */
        if (desc->status & (IRQ_AFFINITY_SET | IRQ_NO_BALANCING)) {
                if (cpus_intersects(desc->affinity, cpu_online_map))
                        mask = desc->affinity;
                else
                        desc->status &= ~IRQ_AFFINITY_SET;
        }

        desc->affinity = mask;
        desc->chip->set_affinity(irq, mask);

        return 0;
}

This set_affinity call undoes all the performance related interrupt
targetting that the SN2 PROM did at boot.

I'm looking at a patch that makes use of the new IRQ_AFFINITY_SET
flag in the irq_desc to get around this.

Is this an issue for all ia64 platforms? Or does irq balancing
factor in here? IRQ balancing is disabled on SN2.

John Keller

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