[PATCH] drop note_interrupt() for per-CPU for proper scaling

[hawkes@sgi.com (John Hawkes)]

The "unhandled interrupts" catcher, note_interrupt(), increments a global
desc->irq_count and grossly damages scaling of very large systems, e.g.,
>192p ia64 Altix, because of this highly contented cacheline, especially
for timer interrupts.  384p is severely crippled, and 512p is unuseable.

All calls to note_interrupt() can be disabled by booting with "noirqdebug",
but this disables the useful interrupt checking for all interrupts.

I propose eliminating note_interrupt() for all per-CPU interrupts.
This was the behavior of linux-2.6.10 and earlier, but in 2.6.11 a
code restructuring added a call to note_interrupt() for per-CPU interrupts.
Besides, note_interrupt() is a bit racy for concurrent CPU calls anyway,
as the desc->irq_count++ increment isn't atomic (which, if done, would make
scaling even worse).

Signed-off-by: John Hawkes <hawkes@sgi.com>
---

Index: linux/kernel/irq/handle.c
===================================================================
--- linux.orig/kernel/irq/handle.c	2005-03-01 23:38:37.000000000 -0800
+++ linux/kernel/irq/handle.c	2005-05-18 11:02:18.000000000 -0700
@@ -118,8 +118,6 @@
 		 */
 		desc->handler->ack(irq);
 		action_ret = handle_IRQ_event(irq, regs, desc->action);
-		if (!noirqdebug)
-			note_interrupt(irq, desc, action_ret);
 		desc->handler->end(irq);
 		return 1;
 	}