genirq regression - CONFIG_AUTO_IRQ_AFFINITY

From: John Keller
Date: Fri Dec 05 2008 - 11:40:24 EST

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.

extern int select_smp_affinity(unsigned int irq);
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().


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.

* 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;
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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