Improve behaviour of spurious IRQ detect

Currently we handle spurious IRQ activity based upon seeing a lot of
invalid interrupts, and we clear things back on the base of lots of valid
interrupts.

Unfortunately in some cases you get legitimate invalid interrupts caused by
timing asynchronicity between the PCI bus and the APIC bus when disabling
interrupts and pulling other tricks.  In this case although the spurious
IRQs are not a problem our unhandled counters didn't clear and they act as
a slow running timebomb.  (This is effectively what the serial port/tty
problem that was fixed by clearing counters when registering a handler
showed up)

It's easy enough to add a second parameter - time.  This means that if we
see a regular stream of harmless spurious interrupts which are not harming
processing we don't go off and do something stupid like disable the IRQ
after a month of running.  OTOH lockups and performance killers show up a
lot more than 10/second

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/irq.h

@@ -161,6 +161,7 @@ struct irq_desc {
 	unsigned int		wake_depth;	/* nested wake enables */
 	unsigned int		irq_count;	/* For detecting broken IRQs */
 	unsigned int		irqs_unhandled;
+	unsigned long		last_unhandled;	/* Aging timer for unhandled count */
 	spinlock_t		lock;
 #ifdef CONFIG_SMP
 	cpumask_t		affinity;

kernel/irq/spurious.c

@@ -172,7 +172,17 @@ void note_interrupt(unsigned int irq, struct irq_desc *desc,
 		    irqreturn_t action_ret)
 {
 	if (unlikely(action_ret != IRQ_HANDLED)) {
+		/*
+		 * If we are seeing only the odd spurious IRQ caused by
+		 * bus asynchronicity then don't eventually trigger an error,
+		 * otherwise the couter becomes a doomsday timer for otherwise
+		 * working systems
+		 */
+		if (jiffies - desc->last_unhandled > HZ/10)
+			desc->irqs_unhandled = 1;
+		else
 			desc->irqs_unhandled++;
+		desc->last_unhandled = jiffies;
 		if (unlikely(action_ret != IRQ_NONE))
 			report_bad_irq(irq, desc, action_ret);
 	}