linux-kernel - Re: 50 Watt idle power regression bisected to Linux-3.10

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Date:	Wed, 11 Dec 2013 12:38:39 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Mike Galbraith <bitbucket@...ine.de>, Len Brown <lenb@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Eder <jeder@...hat.com>, x86@...nel.org
Subject: Re: 50 Watt idle power regression bisected to Linux-3.10

On Wed, Dec 11, 2013 at 12:28:36PM +0100, Thomas Gleixner wrote:
> On Wed, 11 Dec 2013, Mike Galbraith wrote:
> > Alakazam..
> > Yup, magical gremlin repellent works on 8 socket DL980 too.
> 
> Now here is a less magical version of the gremlin repellent.
> 
> And just for the amusement value: The erratum for the series 7400
> says:
> 
> AAI65. MONITOR/MWAIT May Have Excessive False Wakeups
> 
> Problem:	Normally, if MWAIT is used to enter a C-state that is
> 		C1 or higher, a store to the address range armed by
> 		the MONITOR instruction will cause the processor to
> 		exit MWAIT. Due to this erratum, false wakeups may
> 		occur when the monitored address range was recently
> 		written prior to executing the MONITOR instruction.
> 
> Implication:	Due to this erratum, performance and power savings may
> 		be impacted due to excessive false wakeups.
> 
> Workaround: 	Execute a CLFLUSH Instruction immediately before every
> 		MONITOR instruction when the monitored location may
> 		have been recently written.
> 
> Now that looks like the very same issue on these westmere EX
> machines.
> 
> These false wakeups can be observed already before the idle changes
> and now they are just more prominent.
> 
> Adding that clflush() unconditionally fixes the issue at least on
> Boris machine.
> 
> Mike, can you retest on that 8 socket monstrum, please?
> 
> So it looks like the idle power regression is actually a software
> change which exhibits a hardware "regression".

Right, if it turns out that this is really the case and that this
erratum hasn't been fixed for models later than 29 - we'd need the
additional model numbers to set X86_FEATURE_CLFLUSH_MONITOR correctly.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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