[patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.31-19

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> ok, this could be ACPI CPU-sleep related. Could you disable all ACPI
> options in your .config (as a workaround), and re-check whether the
> xruns still occur?

i think i found the bug - it's an upstream ACPI bug. Does the patch
below (or the -31-19 kernel, which i've just uploaded) fix the xruns?

the upstream ACPI bug is this: 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.

	Ingo

Signed-off-by: Ingo Molnar <mingo@elte.hu>

--- linux/drivers/acpi/processor.c.orig
+++ linux/drivers/acpi/processor.c
@@ -370,6 +370,15 @@ acpi_processor_idle (void)
 	 */
 	local_irq_disable();
 
+	/*
+	 * Check whether we truly need to go idle, or should
+	 * reschedule:
+	 */
+	if (unlikely(need_resched())) {
+		local_irq_enable();
+		return;
+	}
+
 	cx = &(pr->power.states[pr->power.state]);
 
 	/*