genirq/matrix: Exclude managed interrupts in irq_matrix_allocated()

When a CPU is about to be offlined, x86 validates that all active
interrupts which are targeted to this CPU can be migrated to the remaining
online CPUs. If not, the offline operation is aborted.

The validation uses irq_matrix_allocated() to retrieve the number of
vectors which are allocated on the outgoing CPU. The returned number of
allocated vectors includes also vectors which are associated to managed
interrupts.

That's overaccounting because managed interrupts are:

  - not migrated when the affinity mask of the interrupt targets only
    the outgoing CPU

  - migrated to another CPU, but in that case the vector is already
    pre-allocated on the potential target CPUs and must not be taken into
    account.

As a consequence the check whether the remaining online CPUs have enough
capacity for migrating the allocated vectors from the outgoing CPU might
fail incorrectly.

Let irq_matrix_allocated() return only the number of allocated non-managed
interrupts to make this validation check correct.

[ tglx: Amend changelog and fixup kernel-doc comment ]

Fixes: 2f75d9e ("genirq: Implement bitmap matrix allocator")
Reported-by: Wendy Wang <wendy.wang@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231020072522.557846-1-yu.c.chen@intel.com

kernel/irq/matrix.c

@@ -466,16 +466,16 @@ unsigned int irq_matrix_reserved(struct irq_matrix *m)
 }
 
 /**
- * irq_matrix_allocated - Get the number of allocated irqs on the local cpu
+ * irq_matrix_allocated - Get the number of allocated non-managed irqs on the local CPU
  * @m:		Pointer to the matrix to search
  *
- * This returns number of allocated irqs
+ * This returns number of allocated non-managed interrupts.
  */
 unsigned int irq_matrix_allocated(struct irq_matrix *m)
 {
 	struct cpumap *cm = this_cpu_ptr(m->maps);
 
-	return cm->allocated;
+	return cm->allocated - cm->managed_allocated;
 }
 
 #ifdef CONFIG_GENERIC_IRQ_DEBUGFS