x86/apic: Try to spread IRQ vectors to different priority levels

When assigning a new vector it is primarially done by adding 8
to the previously given out vector number. Hence, two
consequently allocated vector numbers would likely fall into the
same priority level. Try to spread vector numbers to different
priority levels better by changing the step from 8 to 16.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/20120607131514.GD4759@dhcp-26-207.brq.redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/apic/io_apic.c

@@ -1112,7 +1112,7 @@ __assign_irq_vector(int irq, struct irq_cfg *cfg, const struct cpumask *mask)
 	 * 0x80, because int 0x80 is hm, kind of importantish. ;)
 	 */
 	static int current_vector = FIRST_EXTERNAL_VECTOR + VECTOR_OFFSET_START;
-	static int current_offset = VECTOR_OFFSET_START % 8;
+	static int current_offset = VECTOR_OFFSET_START % 16;
 	unsigned int old_vector;
 	int cpu, err;
 	cpumask_var_t tmp_mask;
@@ -1148,10 +1148,9 @@ __assign_irq_vector(int irq, struct irq_cfg *cfg, const struct cpumask *mask)
 		vector = current_vector;
 		offset = current_offset;
 next:
-		vector += 8;
+		vector += 16;
 		if (vector >= first_system_vector) {
-			/* If out of vectors on large boxen, must share them. */
-			offset = (offset + 1) % 8;
+			offset = (offset + 1) % 16;
 			vector = FIRST_EXTERNAL_VECTOR + offset;
 		}
 		if (unlikely(current_vector == vector))