Re: [RFC PATCH] use acpi_idle_enter_simple if bm_check && !.bm_control

[Len Brown <lenb@kernel.org>]

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