linux-kernel - Re: [BISECTED] Lots of "rescheduling IPIs" in powertop

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Date:	Wed, 14 May 2008 08:58:05 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	"S.Çag(lar Onur" 
	<caglar@...dus.org.tr>, Valdis.Kletnieks@...edu,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org,
	lenb@...nel.org
Subject: Re: [BISECTED] Lots of "rescheduling IPIs" in powertop

Arjan van de Ven wrote:

[cc Len]

> On Tue, 13 May 2008 23:19:47 +0200
> Andi Kleen <andi@...stfloor.org> wrote:
> 
>> Ok the CPU reports it doesn't support any C states in MWAIT. If that
>> is correct then it would be correct to not use MWAIT idle and might
>> actually save more power to not use it.
> 
> what does the current SVN powertop say on this cpu?
> 
>> I don't know if that's true or not. Do you have a power meter perhaps?
>> If yes can you measure if there's a difference between mwait=idle /
>> default on your box when it is idle?
>>
>> [cc Arjan he might now if that CPU is supposed to support C1 in MWAIT]
> 
> I wasn't aware that P4's supported mwait in this way; I thought it was
> core and later.

Not even C1? I generally consider MWAIT without C1 to be unusable.

Anyways if C1 doesn't work then it would be correct to not use MWAIT.
> 
>> CPU reports it supports C1/C2/C3. Are you sure there is a difference
>> on that box? The code should have kept using MWAIT because it checks
>> C1. Please double check.
> 
> The check is .. dubious I suspect... 

I don't think so.

> because the cpuid bits are not
> actually the prime source of information, the BIOS is.

Hmmm? What BIOS information are you refering to?

Normally it's my experience that CPUID is more reliable than the BIOS.

> If the bios says mwait is usable, we need to use it with the values IT
> gives us.

At least to my knowledge the ACPI FADT just says what C states are
available, not if they are implemented with MWAIT or using IO ports.

-Andi



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