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Subject[patch 25/52] x86/apic: Get rid of multi CPU affinity
Setting the interrupt affinity of a single interrupt to multiple CPUs has a
dubious value.

1) This only works on machines where the APIC uses logical destination
mode. If the APIC uses physical destination mode then it is already
restricted to a single CPU

2) Experiments have shown, that the benefit of multi CPU affinity is close
to zero and in some test even worse than setting the affinity to a
single CPU.

The reason for this is that the delivery targets the APIC with the
lowest ID first and only if that APIC is busy (servicing an interrupt,
i.e. ISR is not empty) it hands it over to the next APIC. In the
conducted tests the vast majority of interrupts ends up on the APIC
with the lowest ID anyway, so there is no natural spreading of the
interrupts possible.

Supporting multi CPU affinities adds a lot of complexity to the code, which
can turn the allocation search into a worst case of

nr_vectors * nr_online_cpus * nr_bits_in_target_mask

As a first step disable it by restricting the vector search to a single

Signed-off-by: Thomas Gleixner <>
arch/x86/kernel/apic/vector.c | 13 +++----------
1 file changed, 3 insertions(+), 10 deletions(-)

--- a/arch/x86/kernel/apic/vector.c
+++ b/arch/x86/kernel/apic/vector.c
@@ -136,8 +136,7 @@ static int __assign_irq_vector(int irq,
while (cpu < nr_cpu_ids) {
int new_cpu, offset;

- /* Get the possible target cpus for @mask/@cpu from the apic */
- apic->vector_allocation_domain(cpu, vector_cpumask, mask);
+ cpumask_copy(vector_cpumask, cpumask_of(cpu));

* Clear the offline cpus from @vector_cpumask for searching
@@ -367,17 +366,11 @@ static int x86_vector_alloc_irqs(struct
irq_data->chip = &lapic_controller;
irq_data->chip_data = data;
irq_data->hwirq = virq + i;
+ irqd_set_single_target(irq_data);
err = assign_irq_vector_policy(virq + i, node, data, info,
if (err)
goto error;
- /*
- * If the apic destination mode is physical, then the
- * effective affinity is restricted to a single target
- * CPU. Mark the interrupt accordingly.
- */
- if (!apic->irq_dest_mode)
- irqd_set_single_target(irq_data);

return 0;
@@ -434,7 +427,7 @@ static void __init init_legacy_irqs(void

data->cfg.vector = ISA_IRQ_VECTOR(i);
- cpumask_setall(data->domain);
+ cpumask_copy(data->domain, cpumask_of(0));
irq_set_chip_data(i, data);

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