linux-kernel - Re: CONFIG_NO_HZ: missed ticks, stall (keyb IRQ required) [2.6.18-rc4-mm1]

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Date:	Tue, 07 Nov 2006 10:28:57 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andreas Mohr <andi@...x01.fht-esslingen.de>
Cc:	Ingo Molnar <mingo@...e.hu>, Len Brown <lenb@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CONFIG_NO_HZ: missed ticks, stall (keyb IRQ required)
	[2.6.18-rc4-mm1]

On Tue, 2006-11-07 at 10:16 +0100, Andreas Mohr wrote:
> Hi,
> 
> On Tue, Nov 07, 2006 at 09:25:35AM +0100, Thomas Gleixner wrote:
> > Andreas tested with the latest -mm1-hrt-dyntick patches, so he has all
> > the checks already. The thing which worries me here is, that we detect
> > the breakage and use the fallback path already, but it still has this
> > weird effect on that system, while others just work fine. I'm cooking a
> > more brute force fallback right now.
> 
> That's what I didn't understand all that time:
> I do get the "C2 unusable, kills APIC timer" message, so I expected the code
> to not use C2, but it seems it did use it (causing hangs) and I didn't
> fully analyze the code whether it truly tried to prevent C2 here
> (handling was a bit opaque to me, should have analyzed it
> more thoroughly to get to know exactly what happens).
> 
> And like I said, brutally hard-wiring max_cstate to C1 already fixed
> dynticks things for me, so it seems as if it still touched C2 before.

Yes, it leaves the C states untouched, it uses (should use) PIT instead
of the local APIC timer. I'm a bit confused, why this does not work on
your box.

	tglx


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