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Date: Wed, 1 Dec 2004 17:20:34 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Rui Nuno Capela <rncbc@rncbc.org>
Cc: linux-kernel@vger.kernel.org, Lee Revell <rlrevell@joe-job.com>,
       mark_h_johnson@raytheon.com, "K.R. Foley" <kr@cybsft.com>,
       Bill Huey <bhuey@lnxw.com>, Adam Heath <doogie@debian.org>,
       Florian Schmidt <mista.tapas@gmx.net>,
       Thomas Gleixner <tglx@linutronix.de>,
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       Shane Shrybman <shrybman@aei.ca>, Amit Shah <amit.shah@codito.com>,
       Esben Nielsen <simlo@phys.au.dk>, Andrew Morton <akpm@osdl.org>
Subject: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.31-19
Message-ID: <20041201162034.GA8098@elte.hu>
References: <41358.195.245.190.93.1101734020.squirrel@195.245.190.93> <20041129143316.GA3746@elte.hu> <20041129152344.GA9938@elte.hu> <48590.195.245.190.94.1101810584.squirrel@195.245.190.94> <20041130131956.GA23451@elte.hu> <17532.195.245.190.94.1101829198.squirrel@195.245.190.94> <20041201103251.GA18838@elte.hu> <32831.192.168.1.5.1101905229.squirrel@192.168.1.5> <20041201154046.GA15244@elte.hu> <20041201160632.GA3018@elte.hu>
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* 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]);
 
 	/*