We check need_resched() _before_ disabling preemption.  This opens up the
following scenario:

 swapper:	!need_resched()
 [IRQ context]
		wakes up a task
		marks idle task as need-resched

 swapper:	acpi_processor_idle(); // sleeps until next irq

instant 1msec latency introduced...

normally default_idle() is safe because it re-checks need_resched with
interrupts disabled before it truly halts the CPU.  But
acpi_processor_idle() doesnt seem to be doing this!  Your trace clearly
shows a missed preemption due to ACPI.  I'm wondering why no-one has
triggered this before, it's a really bad bug that should be fixed in
2.6.10.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: "Brown, Len" <len.brown@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>