On Mon, Jun 07 2021 at 15:33, Shung-Hsi Yu wrote:

Most driver's IRQ spreading scheme is naive compared to the IRQ spreading
scheme introduced since IRQ subsystem rework, so it better to rely
request_irq() to spread IRQ out.

However, drivers that care about performance enough also tends to try
allocating memory on the same NUMA node on which the IRQ handler will run.
For such driver to rely on request_irq() for IRQ spreading, we also need to
provide an interface to retrieve the CPU index after calling
request_irq().

So if you are interested in the resulting NUMA node, then why exposing a
random CPU out of the affinity mask instead of exposing a function to
retrieve the NUMA node?
  

+/**
+ * irq_get_effective_cpu - Retrieve the effective CPU index
+ * @irq:	Target interrupt to retrieve effective CPU index
+ *
+ * When the effective affinity cpumask has multiple CPU toggled, it just
+ * returns the first CPU in the cpumask.
+ */
+int irq_get_effective_cpu(unsigned int irq)
+{
+	struct irq_data *data = irq_get_irq_data(irq);

This can be NULL.

+	struct cpumask *m;
+
+	m = irq_data_get_effective_affinity_mask(data);
+	return cpumask_first(m);
+}

Thanks,

        tglx