linux-kernel - Re: [PATCH V2 0/3] x86,idle: Enhance cpuidle prediction to handle its failure

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Date:	Mon, 17 Sep 2012 23:30:20 -0400
From:	Youquan Song <youquan.song@...ux.intel.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Youquan Song <youquan.song@...el.com>,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	arjan@...ux.intel.com, lenb@...nel.org,
	Rik van Riel <riel@...hat.com>,
	Youquan Song <youquan.song@...ux.intel.com>
Subject: Re: [PATCH V2 0/3] x86,idle: Enhance cpuidle prediction to handle
	its failure

> > One case is turbostat utility (tools/power/x86/turbostat) at kernel 3.3 or early
> > . turbostat utility will read 10 registers one by one at Sandybridge, so it will
> > generate 10 IPIs to wake up idle CPUs. So cpuidle menu governor will predict it
> >  is repeat mode and there is another IPI wake up idle CPU soon, so it keeps idle
> >  CPU stay at C1 state even though CPU is totally idle. However, in the turbostat
> > , following 10 registers reading is sleep 5 seconds by default, so the idle CPU
> >  will keep at C1 for a long time though it is idle until break event occurs.
> > In a idle Sandybridge system, run "./turbostat -v", we will notice that deep 
> > C-state dangles between "70% ~ 99%". After patched the kernel, we will notice
> > deep C-state stays at >99.98%.
> 
> Is there an impact on performances ?

In this case, turbostat is utility to measure cpu idle status and itself
also is a workload to system. Its purpose is that show cpu C-state
information every 5 seconds. After patched the kernel, it also does
the same thing as usual. So I think the performance has no/little impact.

I do not find performance impact in my tests. If you performance impact cases or
suggestions, I will be very glad to try. 

Thanks
-Youquan
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