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Date: Wed, 21 Jul 2010 17:29:00 -0400 (EDT)
From: Len Brown <lenb@kernel.org>
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To: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Luming Yu <luming.yu@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>,
        "Siddha, Suresh B" <suresh.b.siddha@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [RFC PATCH] use acpi_idle_enter_simple if bm_check && !.bm_control
In-reply-to: <20100419174737.GA13331@srcf.ucam.org>
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 <20100419174737.GA13331@srcf.ucam.org>
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On Mon, 19 Apr 2010, Matthew Garrett wrote:

> On Tue, Nov 10, 2009 at 02:12:41PM +0800, Luming Yu wrote:
> 
> > I came across acpi processor idle driver, noticed that we can cut a
> > bit overhead at C3 entry that can improve C3 residency a bit,
> > especially when it has similar kernel config as old RHEL 5 kernel
> > (2.6.18) on systems with as many as 64 logical CPUs. The point of this
> > patch is bm_sts is an optional bit. It never returns 1 on systems with
> > bm_check && !bm_control I tested. Instead, I have observed lower C3
> > residency due to accessing bm_sts and relevant code on a system with
> > 64 logical CPUs with HZ=1000. Please review. If make sense, please
> > apply.
> 
> Saves 40W or so on a dual-socket Nehalem system here. Is there a reason 
> it wasn't picked up?

The reason it wasn't picked up is because it assumed that the need
for checking BM_STS is identical to the ability to use ARB_DIS.

However, the two are entirely independent.  The result is that
with the patch we would stop checking BM_STS for every Core2 Duo
system -- which would be incorrect.

thanks,
Len Brown, Intel Open Source Technology Center