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

[Youquan Song <youquan.song@linux.intel.com>]

> > 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