linux-kernel - Re: [patch 6/6] x86: add c1e aware idle function

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Message-Id: <200806182226.07590.rjw@sisk.pl>
Date:	Wed, 18 Jun 2008 22:26:06 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@...e.cz>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Arjan van de Veen <arjan@...radead.org>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [patch 6/6] x86: add c1e aware idle function

On Wednesday, 18 of June 2008, Pavel Machek wrote:
> On Thu 2008-06-12 10:29:00, Thomas Gleixner wrote:
> > C1E on AMD machines is like C3 but without control from the OS. Up to
> > now we disabled the local apic timer for those machines as it stops
> > when the CPU goes into C1E. This excludes those machines from high
> > resolution timers / dynamic ticks, which hurts especially the X2 based
> > laptops.
> > 
> > The current boot time C1E detection has another more serious flaw:
> > some BIOSes do not enable C1E until the ACPI processor module is
> > loaded. This causes systems to stop working after that point.
> > 
> > To work nicely with C1E enabled machines we use a separate idle
> > function, which checks on idle entry whether C1E was enabled in the
> > Interrupt Pending Message MSR. This allows us to do timer broadcasting
> 
> Entering idle is quite a common operation, and reading MSR is quite
> slow. Is it possible to do better here?
> 
> What happens if ACPI BIOS toggles MSR on all cpus *while* we are
> entering idle? This seems inherently racy...

Yes, and that fits the picture I'm observing on the nx6325 (see the
"linux-next: Tree for June 13: IO APIC breakage on HP nx6325" thread).

Thanks,
Rafael
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