Arjan van de Ven wrote:
> On 9/20/2012 10:24 AM, Igor Zhbanov wrote:
>
>> Now I'm looking for solution. But I see now only four ways:
>>
>> 1) Develop another kernel mechanism for providing information about all available
>>     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 information.
>> 3) Ask the user to run PowerTOP on a heavy loaded system for the first time
>>     (or wake up all CPUs by the PowerTOP) and store collected CPU information 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 to appear.
>>     (Perhaps there could be something like D-Bus or netlink notification when 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 frequently.
> on x86, you don't win any power at all by offlining CPUs (in fact, it's very power inefficient to do so,
> idle tends to be THAT good)

Or it could be that kernel on ARM wrongly removes information from /proc/cpuinfo and /sys/
for offline cores. Or it wrongly considers them offline.

As I know many of the ARM CPUs has only two C-states. First is independent for each core:
WFI -- Wait For Interrupt. In this state the CPU just calls the halt instruction.

And the second state is ARM power down. This is the coupled state. As I understand,
the whole package could enter this state only when all cores (except current) are in WFI state.
So the last active core powers the CPU down.

But is it correct to consider ARM core to be offline when it is just in WFI state?

-- 
Best regards,
Igor Zhbanov,
Expert Software Engineer,
phone: +7 (495) 797 25 00 ext 3806
e-mail: i.zhbanov(a)samsung.com

ASWG, Moscow R&D center, Samsung Electronics
12 Dvintsev street, building 1
127018, Moscow, Russian Federation