Re: [PATCH] cpu-freq: add troubleshooting section for FSB changes

[Corrado Zoccolo <czoccolo@gmail.com>]

Hi,
On Mon, Nov 9, 2009 at 5:32 PM, Luis R. Rodriguez <mcgrof@gmail.com> wrote:
>
> And we tested this (reducing the min cpu freq to one less than the
> highest supported P state to avoid an FSB speed change) and it seems
> doing the steps described here did not fix the issue. But at least now
> if anyone else wants to verify this they can with some sort of
> documentaiton.
>
> So to confirm though -- we are seeing a huge performance depredation
> mainly on RX on an Intel Pine Trail platform with SpeedStep enabled on
> the BIOS.
>
> Let me get into the specifics in case anyone is able to help. The
> issue is with ath9k on RX and the CPU on C3 state requesting DMA over
> PCI-E. We typically would get about 110 Mbps with an AR9285 (single
> stream) but when SpeedStep is enabled it goes down to 25 Mbps. At the
> PCI-E level we are seeing huge latencies introduced when SpeedStep is
> used for DMA requests to the Intel root complex on the Intel Pine
> trail platform. Latencies are about 20-60 us.

You mentioned speedstep and cpufreq, but the problem is with C3 state
and cpuidle (probably the BIOS mixes the two concepts, but we should
keep them separated).
C3 is not related to the core or FSB frequency, it is an idle state.
When in C3, the CPU is not ready to perform any operation, not just
slower, and depending on the CPU hw, it may take several us to wake up
(even 85us, on an Atom).

>
> Is there a timeout threshold change that will cause the Intel chipset
> wait for some time after completion before going into a C3 state? Are
> there any other explanations for seeing such huge latencies on C3
> state?
There is a patch from Arjan for the cpuidle menu governor, that may fix it.
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-09/msg05276.html
It is already present in 2.6.32.

Thanks,
Corrado

>
> Thanks for your review and help with this. Any suggestions are greatly
> appreciated.
>
>  Luis
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-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
PhD - Department of Computer Science - University of Pisa, Italy
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