linux-kernel - Re: [PATCH] Option to disable AMD C1E (allows dynticks to work)

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Message-ID: <20071215021032.252455be@linux360.ro>
Date:	Sat, 15 Dec 2007 02:10:32 +0200
From:	Eduard-Gabriel Munteanu <eduard.munteanu@...ux360.ro>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Option to disable AMD C1E (allows dynticks to work)

On Fri, 14 Dec 2007 17:35:13 -0500
Chuck Ebbert <cebbert@...hat.com> wrote:

> On 12/14/2007 05:17 AM, Andi Kleen wrote:
> >> so do whatever is necessary to enable dynticks. 
> > 
> > dynticks' main purpose is to save power, but C1e saves more power.
> > Disabling C1e for dynticks would be a fairly useless default
> > trade off.
> > 
> 
> What about machines where the BIOS has disabled C1e on CPU 0 but
> left it enabled on CPU 1 ??

Do you mean Linux should enable C1E on CPU 0 if it's detected on CPU 1?
C3 + dynticks make up a better power saver than simply C1E, as far as I
know. Higher C-states should be enabled on such CPUs, as AMD docs say
firmware should either enable C1E or C2 & C3 (it must provide one of
these mutually exclusive options). I take having C1E on the second CPU
but not the first as an attempt on BIOS's part to provide higher
C-states instead of the former. How broken is it, really?

But maybe someone with access to such hardware can tell us what
happens: does he get C2/C3 power states under such circumstances?
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