Re: [Powertop] Powertop doesn't detects sleeping ARM cores

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On 9/20/2012 10:24 AM, Igor Zhbanov wrote:
> Chris Ferron wrote:
> =

>> On 09/19/2012 01:26 AM, Igor Zhbanov wrote:
>>> I mean that only online cores are shown in /proc/cpuinfo.
>>> So the PowerTOP doesn't count currently offline ones.
>>> And you must run the PowerTOP on a heavy loaded system
>>> to let the PowerTOP see all CPU cores.
>>> Therefore it is impossible to run the PowerTOP before starting
>>> of heavy application to get information about all CPU cores.
>>> Probably the PowerTOP could cache CPU information in some file,
>>> so it will be possible to register trace events for all cores.
>> Off hand I don't have a good solution to help your ARM story. Your notes
>> are interesting and I agree this is something that ARM is going to need.
>> Do you have a complete solution you would like to purpose? I assume you
>> have done some hacking to get around this, but if you have taken that
>> and tuned it i suggest purposing the change. If you find a solution that
>> doesn't digress the Architectures, and corrects the ARM story then
>> please submit patches. I would be interested in seeing what you have
>> come up with.
>> I will try and look for a general solution, are there any other ARM
>> advocates on this list who would like to chime in?
> =

> Now I'm looking for solution. But I see now only four ways:
> =

> 1) Develop another kernel mechanism for providing information about all a=
vailable
>    CPUs and cores. Or ask the kernel authors not to remove offline cores =
from
>    /proc/cpuinfo and their information files from /sys/.../cpu/.
> 2) Scan /sys/.../cpu for all CPUs and temporarily wake it up to gather in=
formation.
> 3) Ask the user to run PowerTOP on a heavy loaded system for the first ti=
me
>    (or wake up all CPUs by the PowerTOP) and store collected CPU informat=
ion somewhere
>    in /var/cache/powertop and reuse it on later runs.
> 4) Don't just sleep in a main cycle, but scan /sys/.../cpu for new CPUs t=
o appear.
>    (Perhaps there could be something like D-Bus or netlink notification w=
hen a core
>     brought online -- I don't know about it).
> =

> At your personal view, what way fits better in the PowerTOP project?

it's messy. ARM in some ways is abusing CPU Offline for idle...

> =

> P.S. This may be the case for x86 CPUs too. I have disable CPU2 by writing
> to /sys/devices/system/cpu2/online, and the PowerTOP sees only CPU0, CPU1=
, and CPU3.
> Although default PC's CPU governor as I know doesn't turn off CPUs freque=
ntly.

on x86, you don't win any power at all by offlining CPUs (in fact, it's ver=
y power inefficient to do so,
idle tends to be THAT good)
> =



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