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

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Message-ID: <20080516084059.GA31574@elte.hu>
Date:	Fri, 16 May 2008 10:40:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andreas Herrmann <andreas.herrmann3@....com>
Cc:	Vegard Nossum <vegard.nossum@...il.com>, Andi Kleen <ak@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"S.??a??lar Onur" <caglar@...dus.org.tr>, Valdis.Kletnieks@...edu,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org
Subject: Re: [BISECTED] Lots of "rescheduling IPIs" in powertop


* Andreas Herrmann <andreas.herrmann3@....com> wrote:

> > > Recap: powertop shows between 200-400 wakeups/second with the 
> > > description "<kernel IPI>: Rescheduling interrupts" when all 
> > > processors have load (e.g. I need to run two busy-loops on my 
> > > 2-CPU system for this to show up).
> > 
> > ok, could you try the fix below? It was a mistake to make mwait use 
> > dependent on power considerations - on a desktop CPU it is unlikely 
> > to use more power than a simple HLT - and the IPIs are extra 
> > scheduling latency and extra power used.
> 
> It depends on the CPU. For AMD CPUs that support MWAIT this is wrong. 
> Family 0x10 and 0x11 CPUs will enter C1 on HLT. Powersavings then 
> depend on a clock divisor and current Pstate of the core.
> 
> If all cores of a processor are in halt state (C1) the processor can 
> enter the C1E (C1 enhanced) state. If mwait is used this will never 
> happen.
> 
> Thus HLT saves more power than MWAIT here.
> 
> It might be best to switch off the mwait flag for these AMD CPU 
> families like it was introduced with commit 
> f039b754714a422959027cb18bb33760eb8153f0 (x86: Don't use MWAIT on AMD 
> Family 10)

agreed, that looks like the cleanest solution. Could you please send a 
patch for that, ontop of my patch?

	Ingo
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