linux-kernel - Re: acpi_idle: Very idle Core i7 machine never enters C3

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Message-ID: <87y6jkee1b.fsf@basil.nowhere.org>
Date:	Tue, 26 Jan 2010 22:45:20 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	jeff@...rrett.org (Jeff Garrett)
Cc:	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	linux-acpi@...r.kernel.org
Subject: Re: acpi_idle: Very idle Core i7 machine never enters C3

jeff@...rrett.org (Jeff Garrett) writes:

> Hi,
>
> I was trying to chase down a theory that my desktop machine (a core i7)
> is running warm (the fan sounds like it's at full speed all the time,
> and I think it's not always acted this way -- hence the theory).
>
> powertop is never showing it spending any time in C3...
>
> I compiled a kernel without USB/sound/radeon, and ran without X.  I was
> able to get the wakeups/sec down below 20, but no time is spent in C3.

[...]
> This may be a complete red herring, but I added some printk logic to
> acpi_idle_bm_check(), and it is getting called often, but bm_status is
> always 1.  [I infer from this that the idle logic is trying to go into
> C3, but this check is stopping it...  Unless I misread something.]

Normally a Core i7 (or any modern Intel systems) should not use
bm_check at all. That's only for older systems that didn't support
MWAIT with c-state hint, but relied on the old port based interface. 

So something is already confused there.

I think it should still work though.
Of course if you really have a lot of bus mastering in the background
then yes there will be no C3.

-Andi


-- 
ak@...ux.intel.com -- Speaking for myself only.
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